Type Annotations
Explicitly tell TypeScript what type a value should be using a colon.
Syntax
let isActive: boolean = true;A type annotation is the : type you add after a variable, parameter, or return value. It tells TypeScript exactly what is allowed.
Where you can annotate
- Variables:
let x: number - Function parameters:
function f(a: string) - Return types:
function g(): boolean
If you assign a value of the wrong type, TypeScript reports an error during compilation.
Example
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When to use it
- An API function annotates its parameter as string so callers know exactly what type to pass.
- A shared utility module annotates every function return type so consumers see the contract in their IDE.
- A database layer annotates query result variables so downstream code handles the right shape.
More examples
Variable and parameter annotations
Adds explicit colon-type annotations to variable declarations and to function parameters and return type.
let username: string = 'alice';
let age: number = 30;
function add(a: number, b: number): number {
return a + b;
}Annotating object properties
Uses an inline object type annotation to describe exactly which properties and types a variable must have.
let user: { name: string; age: number; active: boolean } = {
name: 'Bob',
age: 25,
active: true,
};Array annotation in function
Annotates a parameter as an array of numbers and the return as a number, covering array type annotation syntax.
function sumAll(numbers: number[]): number {
return numbers.reduce((acc, n) => acc + n, 0);
}
const total: number = sumAll([1, 2, 3, 4, 5]);
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