TypeScript errors and how to fix them

Common Errors

Below you find a list of common TypeScript errors along with the buggy code and its fixed version.

1. 1. TS1002
2. 2. TS1005
3. 3. TS1006
4. 4. TS1016
5. 5. TS1029
6. 6. TS1036
7. 7. TS1046
8. 8. TS1055
9. 9. TS1066
10. 10. TS1070
11. 11. TS1109
12. 12. TS1117
13. 13. TS1127
14. 14. TS1149
15. 15. TS1155
16. 16. TS1160
17. 17. TS1192
18. 18. TS1196
19. 19. TS1202
20. 20. TS1208
21. 21. TS1218
22. 22. TS1219
23. 23. TS1228
24. 24. TS1243
25. 25. TS1244
26. 26. TS1259
27. 27. TS1308
28. 28. TS1337
29. 29. TS1375
30. 30. TS1378
31. 31. TS1434
32. 32. TS17000
33. 33. TS17009
34. 34. TS18004
35. 35. TS2300
36. 36. TS2304
37. 37. TS2305
38. 38. TS2306
39. 39. TS2307
40. 40. TS2314
41. 41. TS2315
42. 42. TS2322
43. 43. TS2335
44. 44. TS2339
45. 45. TS2345
46. 46. TS2348
47. 47. TS2349
48. 48. TS2351
49. 49. TS2352
50. 50. TS2355
51. 51. TS2365
52. 52. TS2366
53. 53. TS2368
54. 54. TS2370
55. 55. TS2371
56. 56. TS2377
57. 57. TS2378
58. 58. TS2391
59. 59. TS2394
60. 60. TS2411
61. 61. TS2420
62. 62. TS2428
63. 63. TS2445
64. 64. TS2448
65. 65. TS2454
66. 66. TS2475
67. 67. TS2488
68. 68. TS2497
69. 69. TS2503
70. 70. TS2507
71. 71. TS2511
72. 72. TS2515
73. 73. TS2532
74. 74. TS2538
75. 75. TS2554
76. 76. TS2556
77. 77. TS2564
78. 78. TS2571
79. 79. TS2580
80. 80. TS2582
81. 81. TS2584
82. 82. TS2588
83. 83. TS2613
84. 84. TS2616
85. 85. TS2654
86. 86. TS2656
87. 87. TS2668
88. 88. TS2669
89. 89. TS2680
90. 90. TS2683
91. 91. TS2684
92. 92. TS2686
93. 93. TS2689
94. 94. TS2691
95. 95. TS2693
96. 96. TS2695
97. 97. TS2715
98. 98. TS2720
99. 99. TS2730
100. 100. TS2732
101. 101. TS2739
102. 102. TS2741
103. 103. TS2769
104. 104. TS2794
105. 105. TS2813
106. 106. TS2814
107. 107. TS4020
108. 108. TS4025
109. 109. TS4063
110. 110. TS4075
111. 111. TS4112
112. 112. TS4113
113. 113. TS4114
114. 114. TS5025
115. 115. TS5055
116. 116. TS6053
117. 117. TS6059
118. 118. TS6133
119. 119. TS6196
120. 120. TS6198
121. 121. TS6504
122. 122. TS7006
123. 123. TS7016
124. 124. TS7017
125. 125. TS7027
126. 126. TS7030
127. 127. TS7031
128. 128. TS7034
129. 129. TS7041
130. 130. TS7053
131. 131. TS8020

TS1002

error TS1002: Unterminated string literal.

Broken Code ❌

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const text = 'Lorem ipsum dolor sit amet, consetetur sadipscing elitr

Fixed Code ✔️

You have to close the string literal with an ending ':

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const text = 'Lorem ipsum dolor sit amet, consetetur sadipscing elitr';

If you want to support multiline text, then you would have to use string concatenation:

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const text = 'Lorem ipsum dolor sit amet, consetetur sadipscing elitr, ' +
  'sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.';

Another solution would be using a template literal:

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const text = `Lorem ipsum dolor sit amet, consetetur sadipscing elitr, 
  sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.`;

TS1005

error TS1005: ‘=’ expected.

Broken Code ❌

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type Person {
  age: number;
  name: string;
}

Fixed Code ✔️

You need to assign your type declaration using the = character:

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type Person = {
  age: number;
  name: string;
}

Alternatively you can declare an interface:

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interface Person {
  age: number;
  name: string;
}

TS1006

error TS1006: A file cannot have a reference to itself.

Broken Code ❌

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/// <reference path='index.d.ts' />

Fixed Code ✔️

You cannot reference a file to itself (causes recursive loop). To fix the problem you have to update the reference path to point to another declaration file:

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/// <reference path='some-other-file.d.ts' />

TS1016

error TS1016: A required parameter cannot follow an optional parameter.

Broken Code ❌

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function createUser(firstName: string, lastName: string, middleName?: string, age: number) {
  // ...
}

Fixed Code ✔️

The easiest way to fix the error is to make age optional as well:

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function createUser(firstName: string, lastName: string, middleName?: string, age?: number) {
  // ...
}

Alternatively, you can flip the order of middleName and age. Be aware that this breaks the contract (signature) of the function and is considered a “breaking change”:

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function createUser(firstName: string, lastName: string, age: number, middleName?: string) {
  // ...
}

You could also make middleName non-optional:

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function createUser(firstName: string, lastName: string, middleName: string, age?: number) {
  // ...
}

Yet another solution would be to assign a default value to middleName so it won’t be optional by default. This allows age to be optional then:

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function createUser(firstName: string, lastName: string, middleName: string = '', age?: number) {
  // ...
}

TS1029

error TS1029: ‘public’ modifier must precede ‘abstract’ modifier.

Broken Code ❌

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abstract class Animal {
  abstract public makeNoise(): string;
}

Fixed Code ✔️

They keywords public, private, and protected define the access to a class member. Access modifiers have to be defined first in TypeScript.

Solution 1:

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abstract class Animal {
  public abstract makeNoise(): string;
}

Solution 2:

The visibility is public by default, so you don’t have to explicitly declare it:

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abstract class Animal {
  abstract makeNoise(): string;
}

Video Tutorial

TS1036

error TS1036: Statements are not allowed in ambient contexts.

Broken Code ❌

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import { APIClient } from "../../APIClient";

declare global {
  client: APIClient;
}

Fixed Code ✔️

With declare global an ambient context is created. TypeScript does not allow statements in such ambient context declaration which is why we have to change the statement into a declaration:

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declare global {
  function client(): APIClient;
}

If you don’t want client to be a function, you have to use thevar` keyword:

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declare global {
  var client: APIClient;
}

TS1046

error TS1046: Top-level declarations in .d.ts files must start with either a ‘declare’ or ‘export’ modifier.

Broken Code ❌

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const MAGIC_NUMBER = 1337;

Fixed Code ✔️

If you want to export a constant from a definition file (d.ts), then you have to use the export modifier:

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export const MAGIC_NUMBER = 1337;

TS1055

error TS1055: Type ‘AxiosPromise‘ is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

Broken Code ❌

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export const sendRequestWithCookie = async (
  client: HttpClient,
  config: AxiosRequestConfig,
  engine: CRUDEngine
): AxiosPromise => {
  const cookie: Cookie = await loadExistingCookie(engine);

  if (!cookie.isExpired) {
    config.headers = config.headers || {};
    config.headers['Cookie'] = `zuid=${cookie.zuid}`;
    config.withCredentials = true;
  }

  return client._sendRequest(config);
};

Fixed Code ✔️

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export const sendRequestWithCookie = (
  client: HttpClient,
  config: AxiosRequestConfig,
  engine: CRUDEngine
): AxiosPromise => {
  return loadExistingCookie(engine).then((cookie: Cookie) => {
    if (!cookie.isExpired) {
      config.headers = config.headers || {};
      config.headers['Cookie'] = `zuid=${cookie.zuid}`;
      config.withCredentials = true;
    }

    return client._sendRequest(config);
  });
};

TS1066

error TS1066: In ambient enum declarations member initializer must be constant expression.

Broken Code ❌

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enum PreKeyAuth {
  INVALID = 'Invalid',
  UNKNOWN = 'Unknown',
  VALID = 'Valid'
}

Fixed Code ✔️

Try to replace your enum declaration with a type:

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type PreKeyAuth = 'Invalid' | 'Unknown' | 'Valid';

TS1070

error TS1070: ‘private’ modifier cannot appear on a type member.

Broken Code ❌

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interface Animal {
  private name: string;
}

Fixed Code ✔️

Interfaces are structures that define the public contract. This prohibits you from using private modifiers. Only public and protected can be used. To solve the problem, the private keyword must be removed from the name property of the Animal interface:

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interface Animal {
  name: string;
}

Video Tutorial

TS1109

error TS1109: Expression expected.

Broken Code ❌

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const lastName = throw new Error('Missing last name');

Fixed Code ✔️

Any snippet of code that evaluates to a value is an expression. Any snippet of code that performs an action is a statement. We need a statement to throw an error inside:

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const lastName = undefined;
if (!lastName) {
  throw new Error('Missing last name');
}

TS1117

error TS1117: An object literal cannot have multiple properties with the same name in strict mode.

Broken Code ❌

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const objectLiteral = {
  name: 'Benny',
  name: 'Sofia'
};

Fixed Code ✔️

We can only have one value per property:

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const objectLiteral = {
  name: 'Benny'
};

TS1127

error TS1127: Invalid character.

Broken Code ❌

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# My comment

Fixed Code ✔️

Unlike in Python, inline comments cannot begin with a single hash sign (#) in TypeScript. You must use 3 slashes:

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// My comment

TS1149

TS1149: File name differs from already included file name only in casing.

Broken Code ❌

This error occurs when you import the same file in two different files using two different casing styles (ex. camelCase and UpperCamelCase):

File A:

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import { BaseTestPage } from './baseTestPage';

File B:

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import { BaseTestPage } from './BaseTestPage';

Fixed Code ✔️

The error can be fixed by using the same casing style:

File A:

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import { BaseTestPage } from './BaseTestPage';

File B:

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import { BaseTestPage } from './BaseTestPage';

Alternatively, you can set forceConsistentCasingInFileNames to false in your “tsconfig.json” file:

tsconfig.json

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{
  "compilerOptions": {
    "forceConsistentCasingInFileNames": false
  }
}

TS1155

error TS1155: ‘const’ declarations must be initialized.

Broken Code ❌

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const name: string;

Fixed Code ✔️

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const name: string = 'Benny';

Alternatively you can define a block-scoped local variable:

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let name: string;

TS1160

error TS1160: Unterminated template literal.

Broken Code ❌

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const text = `Lorem ipsum dolor sit amet, consetetur sadipscing elitr, 
  sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.

Fixed Code ✔️

This error is similar to TS1002 but refers to the ending of a template literal. To fix it, we have to close the template literal with an ending `:

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const text = `Lorem ipsum dolor sit amet, consetetur sadipscing elitr, 
  sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.`;

TS1192

error TS1192: Module ‘.../logdown‘ has no default export.

Broken Code ❌

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declare class Logdown {
  // ...
}

export = Logdown;

Fixed Code ✔️

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declare class Logdown {
  // ...
}

export default Logdown;

error TS1192: ‘export *’ does not re-export a default.

Broken Code ❌

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export * from './parseUrls';
export * from './runWhenReady';

You have to re-export a default (in this case coming from runWhenReady.ts):

Fixed Code ✔️

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export * from './parseUrls';
export * from './runWhenReady';
export { default as default } from './runWhenReady';

TS1196

error TS1196: Catch clause variable type annotation must be ‘any’ or ‘unknown’ if specified.

Broken Code ❌

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type MyError = {
  code: number
}

try {
  // ...
} catch (error: MyError) {
  console.log(error.code);
}

Fixed Code ✔️

Errors in catch clauses can only be typed with any or unknown. If you need a more precise error typing, you can use a type guard as follows:

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// Type Guard
function isMyError(error: any): error is MyError {
  return typeof error.code === 'number';
}

type MyError = {
  code: number
}

try {
  // ...
} catch (error: unknown) {
  if (isMyError(error)) {
    console.log(error.code);
  }
}

Video Tutorial

TS1202

error TS1202: Import assignment cannot be used when targeting ECMAScript modules. Consider using ‘import * as ns from “mod”‘, ‘import {a} from “mod”‘, ‘import d from “mod”‘, or another module format instead.

Broken Code ❌

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import sinon = require('sinon');

Fixed Code ✔️

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import * as sinon from 'sinon';

TS1208

error TS1208: ‘index.ts’ cannot be compiled under ‘–isolatedModules’ because it is considered a global script file. Add an import, export, or an empty ‘export {}’ statement to make it a module.

Broken Code ❌

tsconfig.json

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{
  "compilerOptions": {
    "isolatedModules": true
  }
}

index.ts

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require(`dotenv-defaults`).config({
  path: '.env',
});

Fixed Code ✔️

To solve the issue you can turn off “isolatedModules” in your “tsconfig.json”. If you want to keep going with isolated modules, then you have to add an import or export to your code:

index.ts

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require(`dotenv-defaults`).config({
  path: '.env',
});

export default {};

TS1218

error TS1218: Export assignment is not supported when ‘–module’ flag is ‘system’.

Broken Code ❌

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class LRUCache {
  // ...
}

export = LRUCache;

Fixed Code ✔️

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export class LRUCache {
  // ...
}

TS1219

error TS1219: Experimental support for decorators is a feature that is subject to change in a future release. Set the ‘experimentalDecorators’ option in your ‘tsconfig’ or ‘jsconfig’ to remove this warning.

Broken Code ❌

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import { Injectable } from '@nestjs/common';

@Injectable()
export class AppService {
  getHello(): string {
    return 'Hello World!';
  }
}

Fixed Code ✔️

Simply set “experimentalDecorators” to true in your “tsconfig.json” file. As long as decorators are experimental you will also have to install the reflect-metadata package to shim the upcoming Metadata Reflection API for ECMAScript. For proper functionality the “emitDecoratorMetadata” option should also be set to true.

TS1228

error TS1228: A type predicate is only allowed in return type position for functions and methods.

Broken Code ❌

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function hasErrorCode(error: any) error is {code: string} {
  return typeof (error && error.code) === 'string'
}

Fixed Code ✔️

You have to separate the argument list from the return type definition by a ::

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function hasErrorCode(error: any): error is {code: string} {
  return typeof (error && error.code) === 'string'
}

TS1243

error TS1243: ‘static’ modifier cannot be used with ‘abstract’ modifier.

Solution

Broken Code ❌

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abstract class CustomNumber {
  abstract static getNumber(): number;
}

Your abstract class cannot define an abstract static function. You have to keep it static:

Fixed Code ✔️

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abstract class CustomNumber {
  static getNumber(): number {
    return 1337;
  };
}

error TS1243: ‘async’ modifier cannot be used with ‘abstract’ modifier.

Broken Code ❌

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abstract async goto(): Promise<void>;

Fixed Code ✔️

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abstract goto(): Promise<void>;

TS1244

error TS1244: Abstract methods can only appear within an abstract class.

Broken Code ❌

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class Animal {
  abstract makeNoise(): string;
}

Fixed Code ✔️

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abstract class Animal {
  abstract makeNoise(): string;
}

TS1259

error TS1259: Module can only be default-imported using the ‘esModuleInterop’ flag

Broken Code ❌

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tsc mycode.js --allowJs --noEmit 

Fixed Code ✔️

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tsc mycode.js --allowJs --noEmit --esModuleInterop

TS1308

error TS1308: ‘await’ expressions are only allowed within async functions and at the top levels of modules.ts.

Broken Code ❌

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React.useEffect(() => {
  const myPromise = Promise.resolve;
  await myPromise();
  console.log('Promise is resolved.');
}, []);

Fixed Code ✔️

You cannot use an await expression in an useEffect hook, but you can use legacy Promise calls:

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React.useEffect(() => {
  const myPromise = Promise.resolve;
  myPromise().then(() => {
    console.log('Promise is resolved.');
  });
}, []);

Alternatively, you can use an IIFE (Immediately-invoked Function Expression) in your useEffect hook:

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React.useEffect(() => {
  (async () => {
    await Promise.resolve();
  })();
}, []);

TS1337

error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.

Broken Code ❌

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export interface StreetMap {
  [city: 'New York']: ['Broadway', 'Park Avenue'];
}

Fixed Code ✔️

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export type StreetMap = {
  'New York': ['Broadway', 'Park Avenue'];
};

Alternative:

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export interface StreetMap {
  [city: string]: ['Broadway', 'Park Avenue'];
}

error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

Broken Code ❌

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interface CustomError {
  [key: string | number]: string | number;
}

const error: CustomError = {
  401: 'Unauthorized',
};

Fixed Code ✔️

Solution with mapped object type:

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type CustomError = {
  [key in number | string]: string | number;
};

const error: CustomError = {
  401: 'Unauthorized',
};

Alternative:

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interface ErrorNumber {
  [key: number]: string | number;
}

interface ErrorString {
  [key: string]: string | number;
}

const error: ErrorNumber | ErrorString = {
  401: 'Unauthorized',
};

TS1375

error TS1375: ‘await’ expressions are only allowed at the top level of a file when that file is a module, but this file has no imports or exports. Consider adding an empty ‘export {}’ to make this file a module.

Broken Code ❌

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const response = await fetch('https://jsonplaceholder.typicode.com/todos/1');

Fixed Code ✔️

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export {};

const response = await fetch('https://jsonplaceholder.typicode.com/todos/1');

TS1378

error TS1378: Top-level ‘await’ expressions are only allowed when the ‘module’ option is set to ‘es2022’, ‘esnext’, ‘system’, or ‘nodenext’, and the ‘target’ option is set to ‘es2017’ or higher.

Broken Code ❌

tsconfig.json

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{
  "compilerOptions": {
    "lib": ["es2017"],
    "module": "commonjs",
    "outDir": "dist",
    "rootDir": "src",
    "target": "es6"
  }
}

Fixed Code ✔️

tsconfig.json

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{
  "compilerOptions": {
    "lib": ["es2017"],
    "module": "esnext",
    "outDir": "dist",
    "rootDir": "src",
    "target": "es2017"
  }
}

TS1434

error TS1434: Unexpected keyword or identifier.

Broken Code ❌

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class MyClass {
  static static ID: number = 1337;
}

Fixed Code ✔️

You have to remove the duplicate static keyword:

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class MyClass {
  static ID: number = 1337;
}

TS17000

error TS17000: JSX attributes must only be assigned a non-empty ‘expression’.

Broken Code ❌

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<Typography variant={}>Title</Typography>

Fixed Code ✔️

You can’t use an empty expression ({}) in JSX attributes:

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<Typography variant={'h2'}>Title</Typography>

TS17009

error TS17009: ‘super’ must be called before accessing ‘this’ in the constructor of a derived class.

Broken Code ❌

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abstract class Animal {
  abstract name: string;
}

class Dog extends Animal {
  public name;

  constructor(name: string) {
    this.name = name;
    super();
  }
}

Fixed Code ✔️

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abstract class Animal {
  abstract name: string;
}

class Dog extends Animal {
  public name;

  constructor(name: string) {
    super();
    this.name = name;
  }
} 

TS18004

error TS18004: No value exists in scope for the shorthand property ‘age’. Either declare one or provide an initializer.

Broken Code ❌

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export function getPerson() {
  return {
    age,
    fistName: 'Benny',
    lastName: 'Neugebauer'
  }
}

Fixed Code ✔️

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export function getPerson() {
  const age = 34;
  return {
    age,
    fistName: 'Benny',
    lastName: 'Neugebauer',
  };
}

Alternative:

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export function getPerson() {
  return {
    age: 34,
    fistName: 'Benny',
    lastName: 'Neugebauer',
  };
}

TS2300

error TS2300: Duplicate identifier ‘name’.

Broken Code ❌

Objects don’t support multiple properties with the same name:

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const objectLiteral = {
  name: 'Benny',
  name: 'Sofia'
};

Fixed Code ✔️

To fix the error we have to remove the duplicated property:

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const objectLiteral = {
  name: 'Sofia'
};

TS2304

error TS2304: Cannot find name ‘world’.

Broken Code ❌

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console.log(world.name);

Fixed Code ✔️

It can happen that TypeScript does not know about your global objects because those may be injected from an unknown runtime environment or third-party JavaScript library. The easiest way to let TypeScript know about this is to declare the ambience (ambient context):

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declare var world: {
  name: string;
};

console.log(world.name);

error TS2304: Cannot find name ‘Promise’

Broken Code ❌

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// ...

public load_prekey(prekey_id: number): Promise<Proteus.keys.PreKey> {
  return new Promise((resolve) => {
    resolve(42);
  });
}

// ...

Fixed Code ✔️

Install es6-promise type definitions with the typings tool.

1

typings install dt~es6-promise --global --save

Adding the following line to the beginning of every file using definitions from es6-promise.

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/// <reference path='es6-promise.d.ts' />

...

public load_prekey(prekey_id: number): Promise<Proteus.keys.PreKey> {
  return new Promise((resolve) => {
    resolve(42);
  });
}

...

error TS2304: Cannot find name ‘Promise’

Broken Code ❌

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const UUID = require('uuidjs');

Fixed Code ✔️

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npm install @types/node --save-dev

error TS2304: Cannot find name ‘FC’.

Broken Code ❌

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import React from 'react';

const App: FC = (): JSX.Element => {
  return <></>;
};

Fixed Code ✔️

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import React, {FC} from 'react';

const App: FC = (): JSX.Element => {
  return <></>;
};

TS2305

error TS2305: Module ‘.../proteus‘ has no exported member ‘keys’.

Broken Code ❌

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declare module keys {
}

Fixed Code ✔️

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export module keys {
}

error TS2305: Module ‘.../proteus.keys‘ has no exported member ‘PreKey’.

Broken Code ❌

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export module keys {
}

Fixed Code ✔️

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export module keys {
  class PreKey {
  }
}

TS2306

error TS2306: File ‘add.ts‘ is not a module.

Broken Code ❌

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function add(a: number, b: number): number {
  return a + b;
}

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import { add } from './add';

console.log(add(1000, 337));

Fixed Code ✔️

The error TS2306 signals that the file (add.ts) can be found (otherwise it would throw error TS2307) but does not provide the necessary exports. We can solve this with a named export:

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export function add(a: number, b: number): number {
  return a + b;
}

Alternatively we can use a default export:

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export default function add(a: number, b: number): number {
  return a + b;
}

Using a default export requires that we also adjust our import statement in main.ts (otherwise we would end up with error TS2614):

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import add from './add';

console.log(add(1000, 337));

TS2307

error TS2307: Cannot find module ‘events’ or its corresponding type declarations.

Broken Code ❌

You are importing from a core Node.js module (e.g. event) without having Node.js type definitions installed:

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import {EventEmitter} from 'events';

Fixed Code ✔️

Import Node.js type definitions first in order to use Node.js core modules:

1

npm install @types/node

More: Error TS2307: Cannot find module events

TS2314

error TS2314: Generic type ‘Omit’ requires 2 type argument(s).

Broken Code ❌

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export interface SerializedBatchedCandle extends Omit<BatchedCandle, 'close', 'open'> {
  open: string;
  close: string;
}

Fixed Code ✔️

When using the Omit utility type, you have to list property overwrites with a pipe (|):

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export interface SerializedBatchedCandle extends Omit<BatchedCandle, 'close' | 'closeAsk'> {
  close: string;
  closeAsk: string;
}

TS2315

error TS2315: Type ‘CustomRequest‘ is not generic.

Broken Code ❌

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type CustomRequest = {
  url: string;
  data: string;
}

const request: CustomRequest<string> = {
  url: 'https://typescript.tv/',
  data: 'example'
};

Fixed Code ✔️

When supplying a type (recognizable by the use of the diamond operator <>), then we have to make sure that our type actually supports generics to capture the type that we provide:

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type CustomRequest<CustomType> = {
  url: string;
  data: CustomType;
}

const request: CustomRequest<string> = {
  url: 'https://typescript.tv/',
  data: 'example'
};

TS2322

error TS2322: Type ‘string’ is not assignable to type ‘number’.

Broken Code ❌

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export function add(a: number, b: number): number {
  return `${a + b}`;
}

Fixed Code ✔️

The type of the returned value must match the return type specified in the function signature:

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export function add(a: number, b: number): number {
  return parseInt(`${a + b}`, 10);
}

Video Tutorial

TS2335

error TS2335: ‘super’ can only be referenced in a derived class.

Broken Code ❌

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abstract class Animal {
  abstract makeNoise(): string;
}

class Cat {
  constructor() {
    super();
  }

  makeNoise(): string {
    return 'Meow!';
  }
}

Fixed Code ✔️

Your derived class has to “extend” the base class:

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abstract class Animal {
  abstract makeNoise(): string;
}

class Cat extends Animal {
  constructor() {
    super();
  }

  makeNoise(): string {
    return 'Meow!';
  }
}

TS2339

error TS2339: Property ‘width‘ does not exist on type ‘Shape‘.

Broken Code ❌

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type Shape = {
  kind: 'rectangle' | 'square';
}

type Rectangle = {
  kind: 'rectangle';
  width: number;
  height: number;
} & Shape

type Square = {
  kind: 'square';
  size: number;
} & Shape;

function handleShape(shape: Shape): void {
  switch (shape.kind) {
    case 'rectangle':
      console.log(shape.width);
      break;
    case 'square':
      console.log(shape.size);
      break;
  }
}

Fixed Code ✔️

You can create discriminated unions by sharing a single field (e.g. kind) in your type definitions and using a union type in connection with a switch-case statement that helps the TypeScript compiler to distinguish the different types:

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type Rectangle = {
  kind: 'rectangle';
  width: number;
  height: number;
}

type Square = {
  kind: 'square';
  size: number;
}

type Shape = Rectangle | Square;

function handleShape(shape: Shape): void {
  switch (shape.kind) {
    case 'rectangle':
      console.log(shape.width);
      break;
    case 'square':
      console.log(shape.size);
      break;
  }
}

TS2345

error TS2345: Argument of type ‘number‘ is not assignable to parameter of type ‘TimerHandler‘.

Broken Code ❌

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function add(a: number, b: number): number {
  return a + b;
}

setTimeout(add(1000, 337), 5000);

Fixed Code ✔️

There is a mismatch in the expected arguments of a function. The setTimeout function expects the first argument to be a callback function and not the returned value (in this case a number) of a function call:

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function add(a: number, b: number): number {
  return a + b;
}

setTimeout(() => add(1000, 337), 5000);

Video Tutorial

TS2348

error TS2348: Value of type ‘typeof BaseN’ is not callable. Did you mean to include ‘new’?

Broken Code ❌

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import {BaseN} from 'js-combinatorics';

const iterator = BaseN([1, 2, 3], 2);

Fixed Code ✔️

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import {BaseN} from 'js-combinatorics';

const iterator = new BaseN([1, 2, 3], 2);

TS2349

error TS2349: Cannot invoke an expression whose type lacks a call signature. Type ‘Promise‘ has no compatible call signatures.

Broken Code ❌

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function bugged(param: Promise<Object>): void {
  param().then(() => console.log('error TS2349'));
}

Fixed Code ✔️

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function bugged(param: Promise<Object>): void {
  param.then(() => console.log('error TS2349'));
}

TS2351

error TS2351: This expression is not constructable. Type ‘EMA‘ has no construct signatures.

Broken Code ❌

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export class RSI {
  private readonly avgGain: MovingAverage;

  constructor(private readonly interval: number, Indicator: EMA) {
    this.avgGain = new Indicator(this.interval);
  }
}

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export class EMA {}

Fixed Code ✔️

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export class RSI {
  private readonly avgGain: MovingAverage;

  constructor(private readonly interval: number, Indicator: typeof EMA) {
    this.avgGain = new Indicator(this.interval);
  }
}

TS2352

error TS2352: Conversion of type ‘{ name: string; age: number; }‘ to type ‘Person‘ may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to ‘unknown‘ first.

Broken Code ❌

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type Person = {
  firstName: string;
  age: number;
}

const myObject = {
  name: 'Benny',
  age: 34,
} as Person;

Fixed Code ✔️

Make sure all properties of your object match the properties of your declared type:

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type Person = {
  firstName: string;
  age: number;
}

const myObject = {
  firstName: 'Benny',
  age: 34,
} as Person;

Alternative but not recommended: Convert your object to unknown first:

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type Person = {
  firstName: string;
  age: number;
}

const myObject = {
  name: 'Benny',
  age: 34,
} as unknown as Person;

TS2355

error TS2355: A function whose declared type is neither ‘void’ nor ‘any’ must return a value.

Broken Code ❌

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getName(): string {

}

Fixed Code ✔️

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getName(): string {
  return 'Benny';
}

TS2365

error TS2365: Operator ‘+’ cannot be applied to types ‘number’ and ‘object’.

Broken Code ❌

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export function add(a: number, b: object): number {
  return a + b;
}

Fixed Code ✔️

You can use the + operator only with equivalent data types (strings + strings or numbers + numbers):

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export function add(a: number, b: number): number {
  return a + b;
}

TS2366

error TS2366: Function lacks ending return statement and return type does not include ‘undefined’.

Broken Code ❌

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export function getInterestRate(years: 1 | 2 | 3): number {
  switch (years) {
    case 1:
      return 1.75;
    case 2:
      return 2.96;
  }
}

Fixed Code ✔️

The switch-case statement isn’t handling all cases from every possible input. We can solve that by defining a default case:

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9
10

export function getInterestRate(years: 1 | 2 | 3): number {
  switch (years) {
    case 1:
      return 1.75;
    case 2:
      return 2.96;
    default:
      return 3;
  }
}

Another solution would be to implement the missing case for 3:

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export function getInterestRate(years: 1 | 2 | 3): number {
  switch (years) {
    case 1:
      return 1.75;
    case 2:
      return 2.96;
    case 3:
      return 3;
  }
}

Video Tutorial

TS2368

error TS2368: Type parameter name cannot be ‘number’.

Broken Code ❌

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export interface SimpleNumberIndicator<number> {
  update(price: T): T;
}

Fixed Code ✔️

The easiest way to fix the error is to make age optional as well:

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export interface SimpleNumberIndicator<T = number> {
  update(price: T): T;
}

TS2370

error TS2370: A rest parameter must be of an array type.

Broken Code ❌

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function sum(...array: number): number {
  return array.reduce((a, b) => a + b);
}

console.log(sum(...[1, 2, 3, 4, 5]));

Fixed Code ✔️

A rest parameter allows a function to accept an indefinite number of parameters. To signal that it can be multiple values, we have to use an array type for our rest parameter:

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function sum(...array: number[]): number {
  return array.reduce((a, b) => a + b);
}

console.log(sum(...[1, 2, 3, 4, 5]));

TS2371

error TS2371: A parameter initializer is only allowed in a function or constructor implementation.

Broken Code ❌

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function add(a: number, b: number, c: number = 0): number;
function add(a: number | string, b: number | string, c: number | string = 0): number {
  return parseInt(`${a}`, 10) + parseInt(`${b}`, 10);
}

Fixed Code ✔️

Remove parameter initializer from function overload:

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function add(a: number, b: number, c: number): number;
function add(a: number | string, b: number | string, c: number | string = 0): number {
  return parseInt(`${a}`, 10) + parseInt(`${b}`, 10);
}

TS2377

error TS2377: Constructors for derived classes must contain a ‘super’ call.

Broken Code ❌

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abstract class Animal {
  abstract name: string;
}

class Dog extends Animal {
  public name;

  constructor(name: string) {
    this.name = name;
  }
}

Fixed Code ✔️

Every constructor in a derived class has to call the super method to invoke the constructor of the base class. It has to be the very first call:

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abstract class Animal {
  abstract name: string;
}

class Dog extends Animal {
  public name;

  constructor(name: string) {
    super();
    this.name = name;
  }
}

Video Tutorial

TS2378

error TS2378: A ‘get’ accessor must return a value.

Broken Code ❌

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get name() {

}

Fixed Code ✔️

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get name(): string {
  return 'Benny';
}

TS2391

error TS2391: Function implementation is missing or not immediately following the declaration.

Broken Code ❌

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abstract class Animal {
  abstract name: string;
  makeNoise(): string;
}

Fixed Code ✔️

An abstract class is different from an interface. You have to use the abstract modifier if you want to define a contract in an abstract class. If there is no abstract modifier you will have to provide a implementation.

Solution 1:

To solve the problem, we can mark makeNoise with the abstract keyword. That will enforce derived classes to implement this method on their own:

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abstract class Animal {
  abstract name: string;
  abstract makeNoise(): string;
}

Solution 2:

Another solution is to provide a base implementation for makeNoise:

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abstract class Animal {
  abstract name: string;
  makeNoise(): string {
    return 'Woof';
  };
}

Video Tutorial

TS2394

error TS2394: This overload signature is not compatible with its implementation signature.

Broken Code ❌

The implementation does not match all signatures:

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5

function sum(a: number, b: number): number;
function sum(a: string, b: string): string;
function sum(a: number | string, b: number | string) {
  return `${parseInt(a + '', 10) + parseInt(b + '', 10)}`;
}

Fixed Code ✔️

To match the first function overload, we have to add code to our function body which can also return a number:

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8

function sum(a: number, b: number): number;
function sum(a: string, b: string): string;
function sum(a: number | string, b: number | string) {
  if (typeof a === 'number' && typeof b === 'number') {
    return a + b;
  }
  return `${parseInt(a + '', 10) + parseInt(b + '', 10)}`;
}

Video Tutorial

TS2411

error TS2411: Property ‘age’ of type ‘number’ is not assignable to ‘string’ index type ‘string’.

Broken Code ❌

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interface Person {
  [key: string]: string;
  age: number;
  name: string;
}

Fixed Code ✔️

We defined an interface where every key has a value of type string. This doesn’t work for age which is why we have to extend the possible value types using a union type:

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interface Person {
  [key: string]: number | string;
  age: number;
  name: string;
}

TS2420

error TS2420: Class ‘Dog’ incorrectly implements interface ‘Animal’. Property ‘name’ is private in type ‘Dog’ but not in type ‘Animal’.

Broken Code ❌

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interface Animal {
  name: string;
}

class Dog implements Animal {
  constructor(private name: string = 'Bobby') {
  }
}

Fixed Code ✔️

The Animal interface defines a public name member. The name property in our Dog class must therefore also be public:

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interface Animal {
  name: string;
}

class Dog implements Animal {
  constructor(public name: string = 'Bobby') {
  }
}

Video Tutorial

TS2428

error TS2428: All declarations of ‘Strategy’ must have identical type parameters.

Broken Code ❌

Strategy.ts

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enum TOPIC {
  ON_LOGOUT = 'ON_LOGOUT',
}

export interface Strategy {
  on(event: TOPIC.ON_LOGOUT, listener: (reason: string) => void): this;
}

export abstract class Strategy<SpecificConfig extends StrategyConfig> extends EventEmitter {
  // ...
}

Fixed Code ✔️

Solution: The generic abstract class Strategy has a generic type parameter list in angle brackets (diamond notation). This generic type parameter list must also be added to the interface definition of Strategy.

Strategy.ts

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enum TOPIC {
  ON_LOGOUT = 'ON_LOGOUT',
}

export interface Strategy<SpecificConfig extends StrategyConfig> {
  on(event: TOPIC.ON_LOGOUT, listener: (reason: string) => void): this;
}

export abstract class Strategy<SpecificConfig extends StrategyConfig> extends EventEmitter {
  // ...
}

TS2445

error TS2445: Property ‘makeNoise’ is protected and only accessible within class ‘Dog’ and its subclasses.

Broken Code ❌

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abstract class Animal {
  protected abstract makeNoise(): string;
}

class Dog extends Animal {
  protected makeNoise(): string {
    return 'Woof!';
  }
}

const laika = new Dog();
laika.makeNoise();

Fixed Code ✔️

The visibility of the makeNoise method is protected. We have to make it public if we want to call it directly from an instance of Dog:

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abstract class Animal {
  protected abstract makeNoise(): string;
}

class Dog extends Animal {
  public makeNoise(): string {
    return 'Woof!';
  }
}

const laika = new Dog();
laika.makeNoise();

Video Tutorial

TS2448

error TS2448: Block-scoped variable ‘add’ used before its declaration.

Broken Code ❌

Function expressions cannot be hoisted (used before they are declared):

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5

add(1, 2);

const add = (a: number, b: number): number => {
  return a + b;
};

Fixed Code ✔️

Turn your function expression into a function declaration (which can be hoisted):

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5

add(1, 2);

function add(a: number, b: number): number {
  return a + b;
}

TS2454

error TS2454: Variable ‘myFavoriteNumber’ is used before being assigned.

Broken Code ❌

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export function myNumber(): number {
  let myFavoriteNumber: number;
  return myFavoriteNumber;
}

Fixed Code ✔️

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5

export function myNumber(): number {
  let myFavoriteNumber: number;
  myFavoriteNumber = 72;
  return myFavoriteNumber;
}

or

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export function myNumber(): number {
  let myFavoriteNumber: number = 72;
  return myFavoriteNumber;
}

TS2475

error TS2475: ‘const’ enums can only be used in property or index access expressions or the right hand side of an import declaration or export assignment or type query.

Broken Code ❌

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export const enum TIME_INTERVAL {
  ONE_MINUTE = "1m",
  THREE_MINUTES = "3m",
  FIVE_MINUTES = "5m",
}

console.log(Object.values(TIME_INTERVAL));

Fixed Code ✔️

Defining a const enum prevents TypeScript from generating JavaScript code for this enum. With const enum TypeScript will just assume that the JavaScript equivalent of this code is already present.

If you want that TypeScript creates JS code for your enum definition, then you have to remove the const keyword:

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export enum TIME_INTERVAL {
  ONE_MINUTE = "1m",
  THREE_MINUTES = "3m",
  FIVE_MINUTES = "5m",
}

console.log(Object.values(TIME_INTERVAL));

TS2488

error TS2488: Type ‘{ [month: number]: string; }’ must have a ‘Symbol.iterator‘ method that returns an iterator.

Solution

You have to add a property called [Symbol.iterator] to your object. The value of this property has to return an iterator. Here you can learn how to create an iterator.

Alternative Solution

If you run into this problem because of a for-of loop, then you can mitigate the problem by using the forEach() method of arrays:

Broken Code ❌

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const months: {
  [month: number]: string;
} = {
  1: 'January',
  2: 'February',
};

for (const month of months) {
  console.log(month);
}

Fixed Code ✔️

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const months: {
  [month: number]: string;
} = {
  1: 'January',
  2: 'February',
};

Object.entries(months).forEach(month => {
  console.log(month);
});

TS2497

error TS2497: Module ‘logdown‘ resolves to a non-module entity and cannot be imported using this construct.

Broken Code ❌

Export: logdown.d.ts

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declare class Logdown {
  constructor(options?: Object);
  public debug(...args: any[]): void;
  public error(...args: any[]): void;
  public info(...args: any[]): void;
  public log(...args: any[]): void;
  public warn(...args: any[]): void;
  public static disable(...args: string[]): void;
  public static enable(...args: string[]): void;
}

declare module "logdown" {
  export = Logdown
}

Import: app.ts

1

import {Logdown} from "logdown";

Broken Code ❌ #2

Export

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declare module 'logdown' {
  class Logdown {
    constructor(options?: Object);
    public debug(...args: any[]): void;
    public error(...args: any[]): void;
    public info(...args: any[]): void;
    public log(...args: any[]): void;
    public warn(...args: any[]): void;
    public static disable(...args: string[]): void;
    public static enable(...args: string[]): void;
  }

  export = Logdown;
}

Import

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import {Logdown} from "logdown";
...
this.logger = new Logdown({prefix: 'abc', alignOuput: true});

Note: tsc option “allowSyntheticDefaultImports” must be set to true

Import

1

import Logdown = require('logdown');

TS2503

error TS2503: Cannot find namespace ‘Proteus’

Broken Code ❌

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import Proteus from "wireapp-proteus";

Fixed Code ✔️

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import {Proteus} from "wireapp-proteus";

Cannot find name ‘process’. Do you need to install type definitions for node? Try npm i --save-dev @types/node.

Solution

Run yarn add --dev @types/node in your npm project.

TS2507

error TS2507: Type ‘typeof EventEmitter‘ is not a constructor function type.

Error happened when importing the exported class in another project.

Broken Code ❌

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import EventEmitter from 'events';

export class WebSocketClient extends EventEmitter {
  constructor() {
    super();
  }
}

Fixed Code ✔️

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import {EventEmitter} from 'events';

export class WebSocketClient extends EventEmitter {
  constructor() {
    super();
  }
}

TS2511

error TS2511: Cannot create an instance of an abstract class.

Broken Code ❌

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myFunction = (strategyConstructor: typeof Strategy, config: StrategyConfig): Promise<Big> => {
  // ....
}

Fixed Code ✔️

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export interface Type<T> extends Function { new (...args: any[]): T; }

myFunction = (strategyConstructor: StrategyType<Strategy<StrategyConfig>>, config: StrategyConfig): Promise<Big> => {
  // ....
}

Read more: Passing a class constructor as parameter to a function.

TS2515

error TS2515: Non-abstract class ‘Dog’ does not implement inherited abstract member ‘makeNoise’ from class ‘Animal’.

Broken Code ❌

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abstract class Animal {
  abstract makeNoise(): string;
}

class Dog extends Animal {}

Fixed Code ✔️

If we derive a class from an abstract class, then we have to provide an implementation for all its abstract members:

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abstract class Animal {
  abstract makeNoise(): string;
}

class Dog extends Animal {
  makeNoise(): string {
    return 'Woof!';
  }
}

Video Tutorial

TS2532

error TS2532: Object is possibly ‘undefined’.

Broken Code ❌

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if (queueObject) {
  queueObject.retry -= 1;
}

Fixed Code ✔️

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if (queueObject) {
  queueObject.retry! -= 1;
}

Broken Code ❌

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const ownerIds: string[] = process.env.WIRE_OWNER_IDS.split(',')

Fixed Code ✔️

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const ownerIds: string[] = String(process.env.WIRE_OWNER_IDS).split(',')

TS2538

error TS2538: Type ‘Person’ cannot be used as an index type.

Broken Code ❌

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interface Person {
  name: string;
}

function getValue(person: Person, key: Person): string {
  return person[key];
}

Fixed Code ✔️

You cannot use an interface as an index type, but you can use all keys of the interface using the keyof type operator: