TypeScript Const and Readonly

A name that must stay put

Write const when that binding should not be assigned again. The compiler treats a latermaxScore = 50 as an error. You get the mistake at compile time, not as a wrong answer after the program runs.

Use it for values that are facts in this program: a maximum score, a tax rate, a label you print in several places. If you need to rebind the name, use let. C++ const also locks a name; TypeScript splits the idea: const for the binding, readonly for fields inside an object.

const variables

Initialize a const on the same line you declare it. There is no later chance to fill it in. After that, you may read it as often as you like.

const maxScore: number = 100;
const score: number = 88;
console.log(score + " / " + maxScore);

const does not freeze an object

const player means you will not write player = somethingElse. The object’s fields can still change. push on a const array is legal for the same reason: the binding is fixed, the contents are not.

const nums: number[] = [8, 3];
nums.push(5);
console.log(nums.join(" "));
console.log(nums.length);

Output is 8 3 5 then 3. nums = [] would not compile. To lock the contents too, you need readonly or as const.

readonly fields

On a type or a class, readonly means that property is not assigned after the object is created. Construction (or an object literal that matches the type) may set it. Later writes fail at compile time.

type Point = {
  readonly x: number;
  readonly y: number;
};

const origin: Point = { x: 0, y: 0 };
console.log(origin.x + "," + origin.y);

class Box {
  readonly id: number;
  constructor(id: number) {
    this.id = id;
  }
}

const box = new Box(7);
console.log(box.id);

Output is 0,0 then 7. origin.x = 1 does not compile.box.id = 8 does not compile. A getter that only returns a field is often enough; readonly is for when there is no setter on purpose.

as const narrows literals

as const tells the checker the array or object is a fixed tuple or a fixed set of literal types. Elements become readonly. String fields become the specific string, not string. Use it for lookup tables you will never mutate.

const days = ["Mon", "Tue", "Wed"] as const;
console.log(days[1]);
console.log(days.length);

const status = { ok: 200, notFound: 404 } as const;
console.log(status.ok);

Output is Tue, then 3, then 200. days.push("Thu") does not compile: the tuple is readonly. status.ok has type 200, not a generalnumber.

What each keyword locks

Mark what you only read

Start with const for names you never rebind. Add readonly on fields that must not change. Use as const for small fixed tables. Next: namespaces, which keep those names from colliding with someone else’s n or status.