Why Never type and unreachable code in Typescript? - Purpose & Use Cases

The Big Idea

What if your code could tell you when you're writing lines that will never run?

The Scenario

Imagine you write a program and add extra lines of code that never run because the program stops before reaching them. You keep typing and fixing these lines, but they never do anything.

The Problem

This wastes your time and can hide real problems. You might think your code works, but some parts are just dead weight. It's easy to miss bugs or misunderstand what your program really does.

The Solution

The never type in TypeScript helps you catch these unreachable parts early. It tells you when some code can never run, so you can clean it up or fix your logic. This keeps your code clear and error-free.

Before vs After

✗ Before

function example(x: string | number) {
  if (typeof x === 'string') {
    console.log('String');
  } else if (typeof x === 'number') {
    console.log('Number');
  } else {
    console.log('Unknown'); // This is unreachable
  }
}

✓ After

function example(x: string | number) {
  if (typeof x === 'string') {
    console.log('String');
  } else if (typeof x === 'number') {
    console.log('Number');
  } else {
    const _exhaustiveCheck: never = x;
    return _exhaustiveCheck;
  }
}

What It Enables

You can write safer code that clearly shows when all cases are handled and no hidden errors lurk.

Real Life Example

When building a menu with fixed options, never helps ensure you handle every choice. If a new option is added later, TypeScript warns you to update your code, preventing bugs.

Key Takeaways

Unreachable code wastes time and hides bugs.

never type catches unreachable or impossible cases.

This leads to clearer, safer, and easier-to-maintain code.