# TIL about typescript typing `as const`

I knew that typescript supports a type being a specific string, rather than the `string` type, so (for example) a function parameter can only receive the string “foo” and not a different string. Today I learnt that this can be done by using the `as const` syntax.

```typescript
class Foo {
  // prop1 is typed as string
  static prop1 = 'foo';

  // prop2 is typed as 'foo'
  static prop2 = 'foo' as const;

  static func1 = (input: typeof Foo.prop1) => console.log(`Input was ${input}`);
  static func2 = (input: typeof Foo.prop2) => console.log(`Input was ${input}`);
}

Foo.func1('bob'); // works fine
Foo.func2('bob'); // Argument of type '"bob"' is not assignable to parameter of type '"foo"'.
```

However, calling `typeof` will still return `'string'` rather than `'foo'`. That’s because using `as const` only affects how typescript checks types, not the javascript output which is produced from the typescript source code.