Interesting Facts About Functions in TypeScript

Last Updated : 19 Mar, 2025

TypeScript enhances JavaScript functions with strong typing, better inference, and advanced features, making code safer, more readable, and more scalable. Understanding its function-related capabilities can greatly improve development efficiency.

1. TypeScript Infers Function Argument Types

TypeScript can automatically guess the types of function arguments based on how they are used, especially in callbacks.

JavaScript

const numbers = [1, 2, 3];
numbers.forEach((num) => console.log(num.toFixed(2))); // TypeScript infers `num` as a number

`2. Typing this in Callback Functions`

You can specify the type of this in callback functions to ensure it behaves as expected.

JavaScript

interface Button {
    onClick: (this: Button, event: Event) => void;
}

const button: Button = {
    onClick() {
        console.log(`Button clicked: ${this}`); // `this` is correctly typed as `Button`
    },
};

3. Function Overloading with Different Return Types

TypeScript allows you to define multiple function signatures with different return types based on input types.

JavaScript

function parseInput(input: string): string;
function parseInput(input: number): number;
function parseInput(input: string | number): string | number {
    if (typeof input === "string") {
        return `Parsed string: ${input}`;
    } else {
        return input * 2; // Return a number
    }
}

console.log(parseInput("Hello")); // Parsed string: Hello
console.log(parseInput(10));     // 20

4. Strongly Typed Rest Parameters

Rest parameters can be typed as tuples to enforce a specific structure.

JavaScript

function logDetails(...details: [string, number]): void {
  console.log(`Name: ${details[0]}, Age: ${details[1]}`);
}

logDetails("Alice", 25); // Name: Alice, Age: 25

5. Functions Can Return Different Types

TypeScript functions can return different types based on the input.

JavaScript

function process(value: string | number) {
  return typeof value === "string" ? value.toUpperCase() : value * 2;
}
console.log(process("hello")); // "HELLO"
console.log(process(5)); // 10

6. Function Currying in TypeScript

Currying allows you to create reusable functions by returning another function.

JavaScript

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

const addFive = add(5);
console.log(addFive(10)); // 15

7. Function Types for Callbacks

You can define function types for callbacks to improve readability.

JavaScript

type ClickHandler = (event: MouseEvent) => void;
const handleClick: ClickHandler = (event) => {
  console.log(event.clientX, event.clientY);
};
document.addEventListener("click", handleClick);

8. Generic Functions with Constraints

Generic functions can enforce constraints to ensure the input meets specific conditions.

JavaScript

function logLength<T extends { length: number }>(arg: T): void {
    console.log(arg.length);
}

logLength("Hello"); // 5
logLength([1, 2, 3]); // 3

9. Using void as a Return Type

The void return type indicates that a function does not return a value.

JavaScript

function logMessage(message: string): void {
    console.log(message);
    // No return statement is needed
}

logMessage("Hello, TypeScript!");

Interesting Facts About Object Types and Interfaces in TypeScript

tanmxcwi

Improve

Article Tags :

TypeScript

Interesting Facts About Functions in TypeScript

1. TypeScript Infers Function Argument Types

2. Typing this in Callback Functions

3. Function Overloading with Different Return Types

4. Strongly Typed Rest Parameters

5. Functions Can Return Different Types

6. Function Currying in TypeScript

7. Function Types for Callbacks

8. Generic Functions with Constraints

9. Using void as a Return Type

Similar Reads

`2. Typing this in Callback Functions`