What is type inference in TypeScript and when does it kick in?

Type inference is TypeScript's ability to automatically determine a value's type without an explicit annotation.

Where inference applies:

• Variable initialization — const x = 42 infers number
• Function return type — inferred from the return expression
• Default parameters — function greet(name = 'World') infers name: string
• Destructuring — inferred from the source type
• Generic instantiation — argument types drive the type parameter

// Variable initialization
const count = 0;       // inferred: number
const name = 'Alice';  // inferred: string
const active = true;   // inferred: boolean

// Function return
function add(a: number, b: number) {
  return a + b; // return type inferred as number
}

// Contextual typing — callback param inferred from array type
const nums = [1, 2, 3];
nums.forEach(n => console.log(n.toFixed(2))); // n inferred as number

// Generic inference
function identity<T>(value: T): T { return value; }
const result = identity('hello'); // T inferred as string