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Interesting Facts About Functions in TypeScript

Last Updated : 19 Mar, 2025
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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!");



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