> For the complete documentation index, see [llms.txt](https://sudevgeny.gitbook.io/typescript/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sudevgeny.gitbook.io/typescript/future-javascript/const.md).

# const

`const` is a very welcomed addition offered by ES6 / TypeScript. It allows you to be immutable with variables. This is good from a documentation as well as a runtime perspective. To use const just replace `var` with `const`:

```typescript
const foo = 123;
```

> The syntax is much better (IMHO) than other languages that force the user to type something like `let constant foo` i.e. a variable + behavior specifier.

`const` is a good practice for both readability and maintainability and avoids using *magic literals* e.g.

```typescript
// Low readability
if (x > 10) {
}

// Better!
const maxRows = 10;
if (x > maxRows) {
}
```

## const declarations must be initialized

The following is a compiler error:

```typescript
const foo; // ERROR: const declarations must be initialized
```

## Left hand side of assignment cannot be a constant

Constants are immutable after creation, so if you try to assign them to a new value it is a compiler error:

```typescript
const foo = 123;
foo = 456; // ERROR: Left-hand side of an assignment expression cannot be a constant
```

## Block Scoped

A `const` is block scoped like we saw with [`let`](/typescript/future-javascript/let.md):

```typescript
const foo = 123;
if (true) {
    const foo = 456; // Allowed as its a new variable limited to this `if` block
}
```

## Deep immutability

`const` хороши работает с объектными литералами, поскольку речт идет о защите "ссылки на переменную":

```typescript
const foo = { bar: 123 };
foo = { bar: 456 }; // ERROR : Left hand side of an assignment expression cannot be a constant
```

Однако он по-прежнему позволяет изменять свойства объектов, как показано ниже:

```typescript
const foo = { bar: 123 };
foo.bar = 456; // Разрешено!
console.log(foo); // { bar: 456 }
```

## Отдавайте предпочтение const

Всегда используйте `const`, если Вы не планируете либо лениво инициализировать переменную, либо переопределять ее (для таких случаев используйте `let`).