Readonly

Introduction

TypeScript’s type system allows you to mark individual properties on interfaces, types and class properties as readonly.

Examples

interface

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

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

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

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