Typescript

• Inline Type

const obj: { [x: string]: string } = {};

• Interface

interface T {
  [x: string]: string;
}

const obj: T = { key: "string" };

• Type

type T = {
  [x: string]: string;
};

const obj: T = { key: "string" };

• Class

class Obj {
  key: string;

  constructor(key: string) {
    this.key = key;
  }
}

• Record

const obj: Record<string, string> = {};

• Partial

It means that every property in the resulting type can be either present or omitted, making it useful when you want to define an object type with optional properties.

type T = Partial<Record<string, string[]>>;

const obj: T = {};
obj.foo.push(""); // Error: 'obj.foo' is possibly 'undefined'.
if (obj.foo) {
  obj.foo.push("string");
}

• Required

type T = {
  str: string;
  num: number;
};

const invalid: Required<T> = { str: "string" }; // Error: Property 'num' is missing in type '{ str: string; }' but required in type 'Required<T>'.
const valid: Required<T> = { str: "string", num: 0 };

• Extends

interface Vehicle {
  brand: string;
}

// Generic function with a type constraint
function displayBrand<T extends Vehicle>(vehicle: T): void {
  console.log(vehicle.brand);
}

const car = { brand: "Toyota", year: 2000 };
displayBrand(car); // Output: Toyota
displayBrand(""); // Error: Argument of type 'string' is not assignable to parameter of type 'Vehicle'.

• Object Key

const likes = {
  tom: true,
  mary: true,
};

const objectKeys = <T extends Record<string, any>>(obj: T): (keyof T)[] => {
  return Object.keys(obj);
};

const users = objectKeys(likes); // const users: ("tom" | "mary")[]