til/typescript/narrow-the-type-of-an-array-to-its-values.md

Narrow The Type Of An Array To Its Values

When an array of string values is defined in a TypeScript context, the inferred type will be string[]. That's because the values of an array can be reassigned. The most precise TypeScript can be is to say that it is an array of string.

const actions = ['increase', 'decrease'];
// typeof actions => string[]

We can freeze the array with its values using as const, the const assertion. This doesn't actually freeze the array, but it does mark it as readonly for the TypeScript compiler.

That means we can lean on the compiler for a lot more specific feedback. Consider for instance this reducer function.

const actions = ['increase', 'decrease'] as const

const reducer = (action: typeof actions[number]) => {
  switch (action) {
    case 'increase':
      // do an increase
    case 'decrease':
      // do a decrease
    case 'submit': // TYPE ERROR, "submit" is not comparable to type 'increase' | 'decrease'
      // do a submit
    default:
      throw Error('Unrecognized action!')
  }
}

// TYPE ERROR, "submit" is not comparable to type 'increase' | 'decrease'
reducer('submit')