Careeroza

What is TypeScript

What is TypeScript?

TypeScript is JavaScript with types. It was created by Microsoft (lead designer Anders Hejlsberg, who also designed C#) and released in 2012. Every valid JavaScript program is also a TypeScript program, but TypeScript lets you describe what kind of value each variable, parameter and object holds.

  • Superset of JavaScript: you can rename a .js file to .ts and start adding types slowly.
  • Static type checking: mistakes are found while you type, before the code runs.
  • Compiles to plain JavaScript: browsers and Node.js run the JavaScript output, not TypeScript.
  • Great editor support: autocomplete, rename, go to definition and inline errors in VS Code.

Your First Look at TypeScript

Example

The : string and : number parts are type annotations. Everything else is normal JavaScript.

const name: string = "Careeroza";
const year: number = 2026;

function greet(person: string): string {
  return "Hello, " + person + "!";
}

console.log(greet(name));
console.log("Learning TypeScript in", year);

Output

Hello, Careeroza!
Learning TypeScript in 2026

What TypeScript Catches

Example

Here we pass a number where a string is expected. JavaScript would happily run this. TypeScript stops it with error TS2345 before the code ever runs.

function greet(person: string): string {
  return "Hello, " + person.toUpperCase();
}

console.log(greet(42));

Compiler output (tsc)

Line 5: error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.

Note: Types exist only at compile time. After compiling, all annotations are removed and normal JavaScript runs.