'as const' Annotations

• as const can be used to mark a value as deeply readonly - i.e., it can't be mutated in any way.

• You might be used to as meaning some kind of type assertion - a lie to TypeScript. Confusingly, this use of as is completely type-safe. You're simply giving TypeScript more information about your code.

• It's different from Object.freeze in two ways. First, it only runs at compile-time, so it disappears at runtime. Second, Object.freeze only works on the top level. as const works on the entire object.

// @errors: 2540
const obj = {
  foo: {
    bar: 42,
  },
} as const;

// Error!
obj.foo.bar = 43;

const freezedObj = Object.freeze({
  foo: {
    bar: 42,
  },
});

// Works!
freezedObj.foo.bar = 43;

• It's useful on objects, but can also be used on arrays to turn them into readonly tuples:

const arr = [1, 2, 3] as const;

arr.push(4);
Property 'push' does not exist on type 'readonly [1, 2, 3]'.2339
Property 'push' does not exist on type 'readonly [1, 2, 3]'.

• Objects and arrays marked with as const get inferred as their literal types, not the wider types. This can be useful for creating type-safe enums without needing the enum keyword.

const obj = {
  foo: {
    bar: 42,
  },
};

// Inferred as number
console.log(obj.foo.bar);
                    
(property) bar: number

const obj2 = {
  foo: {
    bar: 42,
  },
} as const;

// Inferred as its literal!
console.log(obj2.foo.bar);
                     
(property) bar: 42

• as const can also be used to force values to infer as narrowly as possible - for instance, inside objects:

const buttonProps = {
  type: "button" as const,
  onClick: () => {
    console.log("clicked");
  },
} as const;

// Inferred as "button", not string
console.log(buttonProps.type);
                         
(property) type: "button"

• It can also be used to encourage a function that returns an array to infer as a tuple:

declare const useState: () => [
  string,
  (newState: string) => void
];

const useStateWrapped = () => {
  const [state, setState] = useState();

  return [state, setState] as const;
};

as is often used to lie to TypeScript. For instance, you can lie that an empty object is a DOM node, and call functions on it:

const node = {} as HTMLAudioElement;

node.play();

TypeScript won't complain, but this will throw an error at runtime.

But as const is different. It's not a lie - it's just giving TypeScript more information about your code. It's like a type annotation, but for values instead of types.