Type Inference in TypeScript

TypeScript infers types based on the assigned value, reducing the need for explicit type annotations.

🧠 Main idea:

Type Inference means that TypeScript automatically figures out the type of a variable based on the assigned value, so you don’t always have to manually annotate types.

Simple Example:

typescript1let name = "Alice";

TypeScript infers that name is a string, even though you didn’t explicitly write : string.

If you later try to assign a number:

typescript1name = 123; // ❌ Error: Type 'number' is not assignable to type 'string'.

TypeScript will prevent it!

Where Type Inference Happens:

Variables

typescript1let count = 42; // inferred as number

Function Return Types:

typescript1function getGreeting() {
2  return "Hello!";
3}

Inferred return type: string

Function Parameters (with Contextual Typing):

typescript1const numbers = [1, 2, 3];
2numbers.forEach(num => {
3  console.log(num.toFixed(2)); // num is inferred as number
4});

Here, num is automatically inferred as number because numbers is an array of numbers.

Destructuring:

typescript1const user = { name: "Alice", age: 30 };
2
3const { name, age } = user;
4// name: string
5// age: number

Why is it useful?

Less code — no need to annotate every variable manually.
Safer code — still gets all the type-checking benefits.