Never Data Type in TypeScript

The never type in TypeScript represents values that never occur. It is used to indicate that a function or expression will never successfully complete. This type is particularly useful for functions that always throw an error or run indefinitely, effectively signaling that the function will never return a value.

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Understanding the `never` Type

The never type is a subtype of every type, including void. However, the never type itself is not assignable to any other type. It is commonly used for functions that do not return normally or when implementing exhaustive checks in TypeScript.

function throwError(message: string): never {
  throw new Error(message);
}

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Common Use Cases for `never`

Here are some scenarios where the never type is useful:

• Error-Throwing Functions: Functions that throw an exception and thus never complete successfully.
• Infinite Loops: Functions that contain an infinite loop and therefore never return.
• Exhaustive Checks: When handling union types to ensure all possible cases are covered in a switch statement or other conditional checks.

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Examples of Using `never`

Throwing Errors

The never type is often used in functions that throw errors. Such functions do not return a value, and TypeScript understands this with the never type:

function throwError(message: string): never {
  throw new Error(message);
}
throwError("Something went wrong!"); // Throws an error and does not return

Infinite Loops

A function that runs indefinitely can also be typed with never, as it will never complete and return:

function infiniteLoop(): never {
  while (true) {
    // This loop never ends
  }
}
infiniteLoop(); // Never returns

Exhaustive Checks in Switch Statements

When using union types in a switch statement, you can use never to ensure all cases are covered. If a new case is added and not handled, TypeScript will generate an error:

type Shape = "circle" | "square";
function getArea(shape: Shape): number {
switch (shape) {
case "circle":
return Math.PI * 1 * 1; // Example area calculation for a circle
case "square":
return 1 * 1; // Example area calculation for a square
default:
// This should never happen if all Shape types are handled
const _exhaustiveCheck: never = shape;
return _exhaustiveCheck; // TypeScript error if new Shape type is added
}
}

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Conclusion

The never type in TypeScript is a valuable tool for handling functions and scenarios where values are not expected to be returned or where specific cases are not anticipated. By using never effectively, you can write more robust and predictable TypeScript code, ensuring that functions that throw errors or run indefinitely are correctly handled and checked by the TypeScript compiler.