---
date: 2020-10-05
id: c0bc64a7-f2dc-4e2e-afb9-5dbc63eb8287
title: Readonly
---

# Introduction

[TypeScript's](20200929161126-typescript) type system allows you to mark
individual properties on [interfaces](20200929162220-interfaces),
[types](20200929163825-typescript_type_alias) and
[class](20201009104411-typescript_classes) properties as `readonly`.

# Examples

## interface

``` typescript
function foo(config: {
    readonly bar: number,
    readonly bas: number}) {// ..
}

let config = { bar: 123, bas: 123 };
foo(config);
// You can be sure that `config` isn't changed 🌹
```

## type

``` typescript
type Foo = {
    readonly bar: number;
    readonly bas: number;
}
​
// Initialization is okay
let foo: Foo = { bar: 123, bas: 456 };
​
// Mutation is not
foo.bar = 456; // Error: Left-hand side of assignment expression cannot be a constant or a read-only property
```

## class property

``` typescript
class Foo {
    readonly bar = 1; // OK
    readonly baz: string;
    constructor() {
        this.baz = "hello"; // OK
    }
}
```

``` typescript
type Foo = {
  bar: number;
  bas: number;
}

type FooReadonly = Readonly<Foo>;

let foo: Foo = {bar: 123, bas: 456};
let fooReadonly: FooReadonly = {bar: 123, bas: 456};

foo.bar = 456; // Okay
fooReadonly.bar = 456; // ERROR: bar is readonly
```