What is Type Assertion in TypeScript? | TypeScript Interview Questions

What You'll Learn

What type assertion is
Two syntaxes
When to use it

Type Assertion

Type assertion tells TypeScript to treat a value as a specific type. It's like a type cast but performs no runtime checking.

Two Syntaxes

code.tsTypeScript

// as syntax (preferred)
let value: unknown = "hello";
let length: number = (value as string).length;

// angle bracket syntax (not in JSX)
let length2: number = (<string>value).length;

Common Use Cases

code.tsTypeScript

// DOM elements
const input = document.getElementById("email") as HTMLInputElement;
input.value = "test@example.com";

// API responses
interface User {
  name: string;
  email: string;
}

const data = JSON.parse(response) as User;

Caution

code.tsTypeScript

// TypeScript trusts you - be careful!
const num = "hello" as unknown as number;
console.log(num.toFixed()); // Runtime error!

const Assertion

code.tsTypeScript

// Makes values readonly and literal
const colors = ["red", "green", "blue"] as const;
// Type: readonly ["red", "green", "blue"]