TypeScript 4.9 introduces the "satisfies" operator, which allows us to verify that the type of an expression matches a specific type. This operator can be used to narrow down a union type and provide more precise type checking. In the example given, the "satisfies" operator is used to restrict the keys and values of a record type.
Downleveling is the process of converting modern TypeScript code into an older version of JavaScript. This allows developers to target older JavaScript environments that may not support the latest features.
The void operator in TypeScript and JavaScript evaluates an expression and returns `undefined`. It can be used to prevent unintended leakage of return values and handle Promises for side effects. It is useful when you want to suppress the return value of a function or initiate a Promise without handling its resolution.
Function overloading in TypeScript allows you to define multiple functions with the same name but different parameters. This can be useful when you want to provide different behavior based on the arguments passed to the function. By using function overloading, you can improve the return types of your functions for different input scenarios.
TypeScript 2.1 introduced mapped types, which allow you to create new types based on the properties of an existing type. For example, you can create a mapped type that has the same keys as an existing type, but with optional values.