Functions in TypeScript

Basic Function Type

function add(a: number, b: number): number {
    return a + b;
}

graph TD A[Input Parameters] --> B[Type Check] B --> C[Function Body] C --> D[Return Value] D --> E[Return Type Check]

function greet(name: string, age?: number): string {
    return age ? `${name} (${age})` : name;
}

Rules:

function createUser(name: string, role: string = "user") {
    return { name, role };
}

Default values avoid undefined handling in many cases.

function sum(...numbers: number[]): number {
    return numbers.reduce((acc, n) => acc + n, 0);
}

Rest parameters collect many values into one typed array.

void

Function finishes, but does not return useful data.

never

Function never reaches normal completion.

function logMessage(message: string): void {
    console.log(message);
}

function fail(message: string): never {
    throw new Error(message);
}

Use never for:

type MathOp = (a: number, b: number) => number;

const multiply: MathOp = (a, b) => a * b;

This pattern is common in callbacks and dependency injection.