What is TypeScript?

Learn that TypeScript is a typed superset of JavaScript.

What is TypeScript?

TypeScript is a typed superset of JavaScript. That is, every valid JavaScript program is also a valid TypeScript program. TypeScript source files use the .ts extension.

let isim: string = "Ayşe";
let yas: number = 30;

Type annotations are checked only at compile time; the output is still plain JavaScript.

Type annotation syntax

In TypeScript you specify a variable's type after its name with a colon. If you try to assign a wrong type, the compiler warns you.

let puan: number = 95;
puan = "yüksek"; // Error: 'string' is not assignable to 'number'

Type annotations are optional; in most cases TypeScript infers the type from the assigned value.

Exercise Your first type annotation

Declare a variable named yas. Its type is number, its value is 25. Then print the yas variable with console.log.

Show example solution

let yas: number = 25;
console.log(yas);

Test yourself

What does it mean that TypeScript is a 'superset' of JavaScript?
- Every valid JavaScript program is also valid TypeScript.
- Every valid TypeScript program is also valid JavaScript.
- TypeScript completely replaces JavaScript.
- There is no relationship between TypeScript and JavaScript.
Show answer

Every valid JavaScript program is also valid TypeScript. — A superset includes all of JavaScript's syntax and adds types on top. So every JS code is valid in TS; however TS code with type annotations does not run directly as JS.

TypeScript file extension

// Kaynak dosya: selam.____
let mesaj: string = "Merhaba";

Show answer

// Kaynak dosya: selam.ts
let mesaj: string = "Merhaba";

TypeScript core concepts

: string

Type annotation
.ts

Source file extension
üst küme

JS code is valid in TS
JavaScript

TypeScript's foundation

Next lesson: Why TypeScript?

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