TypeScript const and const with as const

Question

To the best of my understanding TypeScript views a const string variable as an immutable typed variable with only that value and no other possible value. I always thought that adding as const to that was redundant.

Why am I getting the following in the 2nd part of the example?

Argument of type 'string' is not assignable to parameter of type...

Example:

declare function each<T extends [any] | any[]>(cases: ReadonlyArray<T>): (name: string, fn: (...args: T) => any, timeout?: number) => void;

const foo1 = 'FOO' as const;
const bar1 = 'BAR' as const;

declare function action1(value: typeof foo1 | typeof bar1): void;

each([
  [foo1],
])('test name', (value) => {
  // okay
  action1(value);
});

const foo2 = 'FOO';
const bar2 = 'BAR';

declare function action2(value: typeof foo2 | typeof bar2): void;

each([
  [foo2],
])('test name', (value) => {
  // Argument of type 'string' is not assignable to parameter of type '"FOO" | "BAR"'.(2345)
  action2(value);
});

Playground sample above is here.

Answer 1

Don't confuse const as a keyword for declaring immutable variables (same in JS by the way) with as const, which called const assertions.

Const assertions behaves slightly different depending on the type (see attached link to the documentation), but in case of literal types it basically means that it cannot be extended (or widened) to string. It narrows it to that specific literal type, which signals to compiler not to accept string and this is the exact error you are getting.