TypeScript Functions

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What Are Functions in TypeScript?

Functions are fundamental building blocks in TypeScript that allow you to encapsulate logic, improve code reusability, and maintain cleaner codebases. TypeScript extends JavaScript functions with static typing, enabling you to define input and output types explicitly for better type safety.

Declaring Functions in TypeScript

Basic Syntax

function functionName(parameter1: Type1, parameter2: Type2): ReturnType {
    // Function body
}

Example: A Simple Function

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

console.log(addNumbers(5, 10)); // Output: 15

In this example:

  • Parameters a and b are of type number.
  • The return type of the function is explicitly defined as number.

Optional and Default Parameters

Optional Parameters

Use the ? syntax to define optional parameters.

function greet(name: string, greeting?: string): string {
    return `${greeting || "Hello"}, ${name}!`;
}

console.log(greet("Alice")); // Output: Hello, Alice!
console.log(greet("Bob", "Hi")); // Output: Hi, Bob!

Default Parameters

Assign default values to parameters to make them optional without using the ? syntax.

function greet(name: string, greeting: string = "Hello"): string {
    return `${greeting}, ${name}!`;
}

console.log(greet("Alice")); // Output: Hello, Alice!
console.log(greet("Bob", "Hi")); // Output: Hi, Bob!

Function Overloading

TypeScript supports function overloading, which allows multiple function signatures for a single implementation.

Example: Overloaded Function

function getValue(id: number): string;
function getValue(name: string): string;
function getValue(input: number | string): string {
    if (typeof input === "number") {
        return `Value for ID: ${input}`;
    } else {
        return `Value for name: ${input}`;
    }
}

console.log(getValue(123)); // Output: Value for ID: 123
console.log(getValue("Alice")); // Output: Value for name: Alice

Anonymous Functions

Anonymous functions are unnamed functions often assigned to variables or passed as arguments.

Example: Arrow Function

const multiply = (a: number, b: number): number => a * b;
console.log(multiply(3, 4)); // Output: 12

Example: Callback Function

function processArray(arr: number[], callback: (value: number) => void): void {
    arr.forEach(callback);
}

processArray([1, 2, 3], (value) => console.log(value * 2));
// Output: 2, 4, 6

Rest Parameters

Use rest parameters to handle an indefinite number of arguments.

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

console.log(sum(1, 2, 3, 4)); // Output: 10

Void and Never Return Types

void Return Type

Use void for functions that don’t return a value.

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

never Return Type

Use never for functions that never return, such as those that throw errors.

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

Generics in Functions

Generics enable functions to work with various data types while preserving type safety.

Example: Generic Function

function getArray<T>(items: T[]): T[] {
    return items.slice();
}

const stringArray = getArray<string>(["Alice", "Bob"]);
const numberArray = getArray<number>([1, 2, 3]);

console.log(stringArray); // Output: ["Alice", "Bob"]
console.log(numberArray); // Output: [1, 2, 3]

Function Types

You can explicitly define the type of a function using function types.

Example: Function Type Declaration

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

const add: MathOperation = (x, y) => x + y;
console.log(add(5, 10)); // Output: 15

Higher-Order Functions

A higher-order function takes another function as an argument or returns a function.

Example: Returning a Function

function createMultiplier(factor: number): (value: number) => number {
    return (value) => value * factor;
}

const double = createMultiplier(2);
console.log(double(5)); // Output: 10

Best Practices for Functions in TypeScript

  1. Always Define Return Types: This improves readability and catches errors early.
  2. Use Optional Parameters Carefully: Avoid making essential parameters optional.
  3. Leverage Generics: Use generics for reusable and flexible functions.
  4. Follow Naming Conventions: Use descriptive names for functions and parameters.
  5. Keep Functions Small: Each function should do one thing well.

Common Pitfalls

1. Ignoring Type Annotations

Avoid relying on TypeScript’s type inference for complex functions.

// Avoid
function process(data) {
    return data.length;
}

// Prefer
function process(data: string | any[]): number {
    return data.length;
}

2. Overcomplicating Overloads

Keep function overloads manageable to avoid confusion.

Conclusion

TypeScript functions bring a lot of flexibility and type safety to JavaScript, making your code more robust and maintainable. Whether you’re using basic functions, generics, or higher-order functions, TypeScript ensures that your functions remain type-safe and error-free.

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