TypeScript with Functions

Functions in TypeScript work similarly to JavaScript but with added type annotations to help catch errors early and improve code clarity. You explicitly define the types of parameters and return values.

Function Parameter and Return Types

Specify the type of each parameter inside parentheses and the return type after the parameter list, separated by a colon.

function greet(name: string): string {
  return "Hello, " + name;
}

Optional and Default Parameters

• ? marks a parameter as optional (can be omitted).
• Default values can be assigned directly in the parameter list.

function multiply(a: number, b?: number): number {
  return a * (b ?? 1);
}

function power(base: number, exponent: number = 2): number {
  return base ** exponent;
}

Function Types as Variables

You can declare variables with function types to specify the signature they must follow.

let formatter: (value: number) => string;

formatter = (num) => num.toFixed(2);

console.log(formatter(3.14159)); // "3.14"

Arrow Functions with Types

Arrow functions also support parameter and return type annotations.

const divide = (x: number, y: number): number => {
  return x / y;
};

Rest Parameters

TypeScript supports rest parameters with explicit array types.

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

Function Overloading

TypeScript allows multiple function signatures for a single implementation to handle different argument types.

function combine(a: string, b: string): string;
function combine(a: number, b: number): number;
function combine(a: any, b: any): any {
  if (typeof a === "string" && typeof b === "string") {
    return a + b;
  }
  if (typeof a === "number" && typeof b === "number") {
    return a + b;
  }
}