What is the never type in TypeScript? | TypeScript Interview Questions

What You'll Learn

What never means
Use cases
never vs void

What is never?

never represents values that never occur. Used for functions that never return or exhaustive checks.

Functions That Never Return

code.tsTypeScript

// Throws an error
function fail(message: string): never {
  throw new Error(message);
}

// Infinite loop
function infinite(): never {
  while (true) {
    // ...
  }
}

Exhaustive Checks

code.tsTypeScript

type Status = "pending" | "approved" | "rejected";

function handleStatus(status: Status): string {
  switch (status) {
    case "pending":
      return "Waiting...";
    case "approved":
      return "Approved!";
    case "rejected":
      return "Rejected!";
    default:
      // If all cases handled, status is never
      const _exhaustive: never = status;
      return _exhaustive;
  }
}

// If you add a new status, TypeScript will error!

never vs void

code.tsTypeScript

// void: function returns but with no value
function logMessage(msg: string): void {
  console.log(msg);
}

// never: function never returns
function throwError(msg: string): never {
  throw new Error(msg);
}