Type Inference

Syntax

const value = expression; // type is inferred automatically

Examples

let count = 10;        // inferred as: number
let name = "Alice";     // inferred as: string
let items = [1, 2, 3];  // inferred as: number[]

// count = "ten"; // Error - TypeScript remembers count is a number, even without an explicit annotation

window.addEventListener("click", (event) => {
  console.log(event.button); // "event" is inferred as MouseEvent from context - full autocomplete works
});

function add(a: number, b: number) {
  return a + b; // return type inferred as: number, no annotation needed
}

Best practices

• Let TypeScript infer types for local variables initialized with an obvious value - explicit annotations there are usually redundant noise
• Still explicitly annotate function parameters, since TypeScript cannot infer those from usage the way it can return types
• Explicitly annotate a variable's type when it starts as one value but needs to later hold a broader type, like let result: string | null = null
• Use your editor's "hover to see inferred type" feature regularly - it is the fastest way to build intuition for how TypeScript reasons about your code

At a glance

Purpose

Static types for JavaScript

File extension

.ts · .tsx

Runs in

Compiled to JavaScript

Usually used with

JavaScript and its ecosystem

Specifications & further reading

• TypeScript Handbook

Related TypeScript documentation