Why Exhaustive checking with never in Typescript? - Purpose & Use Cases

The Big Idea

What if your program could warn you about missing cases before it even runs?

The Scenario

Imagine you have a program that handles different types of user actions, like clicking buttons or submitting forms. You write code to handle each action manually, but as your app grows, you might forget to handle a new action type you added later.

The Problem

Manually checking every possible action is slow and easy to mess up. You might miss a case, causing bugs that are hard to find. The program might crash or behave unexpectedly because it didn't handle a new action type.

The Solution

Using exhaustive checking with never in TypeScript helps you catch missing cases at compile time. It forces you to handle every possible type, so your code is safer and more reliable without extra manual work.

Before vs After

✗ Before

switch(action.type) {
  case 'click': handleClick(); break;
  case 'submit': handleSubmit(); break;
  // forgot 'hover' case
}

✓ After

switch(action.type) {
  case 'click': handleClick(); break;
  case 'submit': handleSubmit(); break;
  case 'hover': handleHover(); break;
  default: const _exhaustiveCheck: never = action; // error if new type missed
}

What It Enables

This concept makes your code future-proof by ensuring you never miss handling a new case, catching errors before your program runs.

Real Life Example

When building a menu with different item types, exhaustive checking guarantees you handle every item type, so users never see broken or missing options.

Key Takeaways

Manual checks can miss new cases and cause bugs.

never helps catch missing cases at compile time.

It makes your code safer and easier to maintain.