TypeScript Interfaces

interface User { # define interface
    name: string;
    age: number;
}
let user: User = {
    name: 'John',
    age: 25
};

interface User {
    name: string;
    email?: string; # optional property
}

interface User {
    readonly id: number; # cannot be changed
    name: string;
}

interface MathFunc {
    (x: number, y: number): number; # function signature
}
const add: MathFunc = (x, y) => x + y;

interface Person {
    name: string;
}
interface Employee extends Person { # extend interface
    employeeId: number;
}

interface StringMap {
    [key: string]: string; # dynamic properties
}
let map: StringMap = {
    name: 'John',
    city: 'NYC'
};