const-assertion.md

In TypeScript, a const assertion is a type of assertion that tells the compiler to infer the most literal type possible for a variable. It's done by appending as const to the end of the expression. Here are some use cases:

1. Immutable Arrays:

If you want to create a readonly tuple, you can use a const assertion. The inferred type will be a readonly tuple, not an array, and the length of the array will be part of the type.

const arr = [10, 20] as const;
// inferred type: readonly [10, 20]

2. Immutable Objects:

If you want to create an object with readonly properties, you can use a const assertion. The inferred type will have readonly properties.

const obj = { x: 10, y: 20 } as const;
// inferred type: { readonly x: 10; readonly y: 20; }

3. Enum-like Behavior:

If you want to create a group of related constants, you can use a const assertion with an object. This can be a lightweight alternative to enums.

const Directions = {
  Up: 'UP',
  Down: 'DOWN',
  Left: 'LEFT',
  Right: 'RIGHT',
} as const;

let dir: keyof typeof Directions = Directions.Up;

In all these cases, the const assertion prevents you from changing the values once they are set, and it also gives you a more precise type. It's a powerful tool for creating immutable data structures and for working with literal types.