Typescript - readonly

TypeScript includes the readonly keyword that makes a property as read-only in the class, type or interface

Since read-only members cannot be changed outside the class, they either need to be initialized at declaration or initialized inside the class constructor

class Employee {
    readonly code: number;
    name: string;
    
    constructor(code: number, name: string)     {
        this.code = code;
        this.name = name;
    }
}

let peter = new Employee(10, "Peter");
peter.name = 'Peter Doan'; 
peter.code = 20; // Error - TS2540: Cannot assign to 'code' because it is a read-only property.

In the same way you can use Readonly<T> to create a readonly type

type Config = { ENV: string };
const config: Readonly<Config> = { ENV: 'development' };
config.ENV = 'test' // Error TS2540: Cannot assign to 'ENV' because it is a read-only property.

But be careful when use it inside another function with inconsistent type -> readonly property can be changed

type Config = { ENV: string };
const config: Readonly<Config> = { ENV: 'development' };

// config parameter type is Config instead of Readonly<Config>
function doSomething(config: Config) {
  config.ENV = 'test';
}

console.log(config.ENV);
// Output: 'test'

You can also use Readonly to make Array or Tuple readonly

type Person = [number, string];
const peter: Readonly<Person> = [10, 'Peter'];
peter.push('Address'); // Error TS2339: Property 'push' does not exist on type 'readonly [number, string]'.