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;
  }
}