JavaScript Mastery - Functions

JavaScript functions are the building blocks of modern web development. They allow you to write reusable, modular code that makes your applications more efficient and maintainable. This comprehensive guide covers everything you need to know about JavaScript functions, from basic declarations to advanced concepts like closures and higher-order functions.

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Function Statement/Definition

A function statement (or function declaration) is the most traditional way to define a function in JavaScript. It creates a named function that can be called before it's defined due to hoisting.

Definition

Function statements are declarations that define a named function using the function keyword. They are fully hoisted, meaning they can be called before their declaration in the code.

Syntax

function functionName(parameters) {
    // function body
    return value; // optional
}

Example

function greet(name) {
    return `Hello, ${name}!`;
}

console.log(greet("Vitthal")); // Hello, Vitthal!

// Can be called before declaration due to hoisting
sayHello(); // Works!

function sayHello() {
    console.log("Hello World!");
}

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Function Expression

Function expressions involve creating a function and assigning it to a variable. Unlike function declarations, they are not hoisted and cannot be called before they are defined.

Definition

A function expression is created by assigning an anonymous or named function to a variable. The function is created at runtime when the code execution reaches that line.

Syntax

const functionName = function(parameters) {
    // function body
    return value; // optional
};

Example

const multiply = function(a, b) {
    return a * b;
};

console.log(multiply(5, 3)); // 15

// Named function expression
const factorial = function fact(n) {
    return n <= 1 ? 1 : n * fact(n - 1);
};

console.log(factorial(5)); // 120

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Arrow Functions

Arrow functions provide a more concise syntax for writing functions and have lexical this binding, making them particularly useful in certain contexts like callbacks and methods.

Definition

Arrow functions are a compact alternative to traditional function expressions, introduced in ES6. They use the => syntax and automatically bind the this value from the enclosing context.

Syntax

const functionName = (parameters) => {
    // function body
    return value; // optional
};

Example

const square = (num) => {
    return num * num;
};

const divide = (a, b) => {
    if (b === 0) {
        throw new Error("Division by zero!");
    }
    return a / b;
};

console.log(square(4)); // 16
console.log(divide(10, 2)); // 5

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Arrow Function with One Parameter

When an arrow function has exactly one parameter, you can omit the parentheses around the parameter, making the syntax even more concise.

Definition

Arrow functions with a single parameter allow you to omit the parentheses around the parameter, providing the most concise function syntax in JavaScript.

Syntax

const functionName = parameter => {
    // function body
    return value; // optional
};

Example

const cube = num => {
    return num * num * num;
};

const isEven = number => {
    return number % 2 === 0;
};

const firstChar = str => {
    return str[0];
};

console.log(cube(3)); // 27
console.log(isEven(4)); // true
console.log(firstChar("JavaScript")); // J

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Arrow Function with Implicit Return

When an arrow function body contains only a single expression, you can omit the curly braces and the return keyword for an even more concise syntax.

Definition

Arrow functions with implicit return automatically return the result of a single expression without needing explicit return statement or curly braces.

Syntax

const functionName = parameters => expression;

Example

const add = (a, b) => a + b;
const square = num => num * num;
const isPositive = num => num > 0;
const getLength = str => str.length;

console.log(add(5, 3)); // 8
console.log(square(4)); // 16
console.log(isPositive(-5)); // false
console.log(getLength("Hello")); // 5

// With array methods
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

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Anonymous Functions

Anonymous functions are functions without a name. They are commonly used as callback functions, in function expressions, or when you need a function for a single use.

Definition

Anonymous functions are functions that are declared without a name identifier. They are often used as arguments to other functions or assigned to variables.

Syntax

// As callback
array.method(function(parameters) {
    // function body
});

// In event handlers
element.addEventListener('click', function() {
    // function body
});

Example

// Anonymous function as callback
const numbers = [1, 2, 3, 4, 5];
const squared = numbers.map(function(num) {
    return num * num;
});

// Anonymous function in setTimeout
setTimeout(function() {
    console.log("This runs after 2 seconds");
}, 2000);

// Anonymous function as IIFE
(function() {
    console.log("This runs immediately");
})();

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Rest Parameters

Rest parameters allow a function to accept an indefinite number of arguments as an array, providing flexibility when you don't know how many arguments will be passed.

Definition

Rest parameters collect all remaining arguments into an array, allowing functions to accept any number of arguments. They are denoted by three dots (...) followed by a parameter name.

Syntax

function functionName(...restParameter) {
    // restParameter is an array containing all arguments
}

Example

function sum(...numbers) {
    let total = 0;
    for (let number of numbers) {
        total += number;
    }
    return total;
}

console.log(sum(1, 2, 3)); // 6
console.log(sum(1, 2, 3, 4, 5)); // 15
console.log(sum()); // 0

// Rest parameters with other parameters
function greetAll(greeting, ...names) {
    return names.map(name => `${greeting}, ${name}!`);
}

console.log(greetAll("Hello", "Alice", "Bob", "Charlie"));
// ["Hello, Alice!", "Hello, Bob!", "Hello, Charlie!"]

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Hoisting

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope during compilation, allowing them to be used before they're declared.

Definition

Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during compilation. Function declarations are fully hoisted, while variables are partially hoisted.

Syntax

// Function declarations are fully hoisted
console.log(hoistedFunction()); // Works!

function hoistedFunction() {
    return "I'm hoisted!";
}

// Variables are hoisted but not their values
console.log(myVar); // undefined (not error)
var myVar = "Hello";

Example

// This works due to hoisting
sayHello(); // "Hello World!"

function sayHello() {
    console.log("Hello World!");
}

// Variable hoisting
console.log(name); // undefined
var name = "Vitthal";
console.log(name); // "Vitthal"

// Function expressions are not hoisted
// console.log(notHoisted()); // TypeError: notHoisted is not a function
var notHoisted = function() {
    return "This won't work";
};

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IIFE (Immediately Invoked Function Expression)

IIFE is a function that runs as soon as it's defined. It's commonly used to create a private scope and avoid polluting the global namespace.

Definition

An IIFE is a JavaScript function that runs immediately after it's defined. It's wrapped in parentheses and followed by another pair of parentheses to invoke it, creating a private scope that doesn't interfere with the global scope.

Syntax

(function() {
    // code here runs immediately
})();

// Arrow function IIFE
(() => {
    // code here runs immediately
})();

Example

// Basic IIFE
(function() {
    console.log("This runs immediately!");
})();

// IIFE with parameters
(function(name, age) {
    console.log(`Hello ${name}, you are ${age} years old!`);
})("Vitthal", 25);

// Arrow function IIFE
(() => {
    const privateVar = "I'm private!";
    console.log(privateVar);
})();

// IIFE returning a value
const result = (function(a, b) {
    return a + b;
})(5, 3);

console.log(result); // 8

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Higher Order Functions

Higher order functions are functions that operate on other functions, either by taking them as arguments or by returning them. They enable functional programming patterns in JavaScript.

Definition

A higher order function is a function that either takes one or more functions as arguments, returns a function, or both. They enable powerful functional programming patterns and are essential for many JavaScript operations.

Syntax

// Function that takes another function as argument
function higherOrderFunc(callback) {
    return callback();
}

// Function that returns another function
function createFunction() {
    return function() {
        // returned function body
    };
}

Example

// Higher order function that takes a callback
function processArray(arr, callback) {
    const result = [];
    for (let item of arr) {
        result.push(callback(item));
    }
    return result;
}

const numbers = [1, 2, 3, 4, 5];
const doubled = processArray(numbers, x => x * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

// Higher order function that returns a function
function createMultiplier(multiplier) {
    return function(num) {
        return num * multiplier;
    };
}

const double = createMultiplier(2);
const triple = createMultiplier(3);

console.log(double(5)); // 10
console.log(triple(4)); // 12

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Callback Functions

Callback functions are functions passed as arguments to other functions and are called at a specific time or when a specific event occurs. They're fundamental to asynchronous programming in JavaScript.

Definition

A callback function is a function passed as an argument to another function and is executed after (or during) the execution of that function. Callbacks enable asynchronous programming and event-driven programming patterns.

Syntax

function mainFunction(callback) {
    // do something
    callback(); // call the callback function
}

function callbackFunction() {
    // callback implementation
}

mainFunction(callbackFunction);

Example

// Simple callback example
function greet(name, callback) {
    console.log(`Hello ${name}!`);
    callback();
}

function afterGreeting() {
    console.log("Nice to meet you!");
}

greet("Vitthal", afterGreeting);
// Output:
// Hello Vitthal!
// Nice to meet you!

// Callback with parameters
function calculate(a, b, operation) {
    return operation(a, b);
}

const add = (x, y) => x + y;
const multiply = (x, y) => x * y;

console.log(calculate(5, 3, add)); // 8
console.log(calculate(5, 3, multiply)); // 15

// Array methods using callbacks
const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers); // [2, 4]

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First Class Functions

In JavaScript, functions are first-class citizens, meaning they can be treated like any other value: assigned to variables, passed as arguments, returned from functions, and stored in data structures.

Definition

First-class functions mean that functions in JavaScript are treated as first-class citizens. They support all operations generally available to other entities like being passed as arguments, returned as values, assigned to variables, and stored in data structures.

Syntax

// Assigning function to variable
const myFunc = function() { /* ... */ };

// Passing function as argument
someFunction(myFunc);

// Returning function from function
function createFunc() {
    return function() { /* ... */ };
}

// Storing functions in array
const funcArray = [func1, func2, func3];

Example

// Function assigned to variable
const sayHello = function(name) {
    return `Hello, ${name}!`;
};

// Function stored in object
const mathOperations = {
    add: function(a, b) { return a + b; },
    subtract: (a, b) => a - b,
    multiply: function(a, b) { return a * b; }
};

// Function stored in array
const operations = [
    x => x + 1,
    x => x * 2,
    x => x - 1
];

// Using functions from array
let result = 5;
operations.forEach(operation => {
    result = operation(result);
});
console.log(result); // 11

// Function as property
const person = {
    name: "Vitthal",
    greet: function() {
        return `Hi, I'm ${this.name}`;
    }
};

console.log(person.greet()); // Hi, I'm Vitthal

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Pure Functions

Pure functions are functions that always return the same output for the same input and have no side effects. They're predictable, testable, and essential for functional programming.

Definition

A pure function is a function that, given the same input, will always return the same output and does not cause any observable side effects. Pure functions don't modify external state and don't depend on external mutable state.

Syntax

// Pure function - same input, same output, no side effects
function pureFunctionName(parameters) {
    // only uses parameters and local variables
    return result; // based only on parameters
}

Example

// Pure functions
const add = (a, b) => a + b;
const multiply = (x, y) => x * y;
const getFullName = (firstName, lastName) => `${firstName} ${lastName}`;

// Pure function with array (doesn't mutate original)
const addToArray = (arr, item) => [...arr, item];

// Pure function for object transformation
const updatePersonAge = (person, newAge) => ({
    ...person,
    age: newAge
});

// Examples
console.log(add(2, 3)); // Always 5
console.log(add(2, 3)); // Always 5

const numbers = [1, 2, 3];
const newNumbers = addToArray(numbers, 4);
console.log(numbers); // [1, 2, 3] - original unchanged
console.log(newNumbers); // [1, 2, 3, 4]

const person = { name: "Vitthal", age: 25 };
const updatedPerson = updatePersonAge(person, 26);
console.log(person); // { name: "Vitthal", age: 25 } - original unchanged
console.log(updatedPerson); // { name: "Vitthal", age: 26 }

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Impure Functions

Impure functions are functions that either depend on or modify external state, have side effects, or don't always return the same output for the same input. While sometimes necessary, they should be used carefully.

Definition

An impure function is a function that has side effects, depends on external mutable state, or doesn't always return the same output for the same input. They can modify global variables, perform I/O operations, or depend on external factors.

Syntax

// Impure function - may have side effects or depend on external state
function impureFunctionName(parameters) {
    // may modify external variables
    // may depend on external state
    // may perform side effects
    return result; // may vary for same input
}

Example

// Impure functions examples
let counter = 0;

// Impure - modifies external state
function incrementCounter() {
    counter++; // side effect
    return counter;
}

// Impure - depends on external state
function getCounterValue() {
    return counter; // depends on external variable
}

// Impure - different output for same input
function getRandomNumber(max) {
    return Math.floor(Math.random() * max); // not predictable
}

// Impure - has side effects (logging)
function calculateWithLogging(a, b) {
    console.log(`Calculating ${a} + ${b}`); // side effect
    return a + b;
}

// Impure - modifies input array
function addItemImpure(arr, item) {
    arr.push(item); // mutates original array
    return arr;
}

// Examples
console.log(incrementCounter()); // 1
console.log(incrementCounter()); // 2 - different output
console.log(getRandomNumber(10)); // unpredictable

const myArray = [1, 2, 3];
addItemImpure(myArray, 4);
console.log(myArray); // [1, 2, 3, 4] - original modified

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Global and Local Scope

Scope determines where variables and functions can be accessed in your code. Understanding scope is crucial for avoiding naming conflicts and creating maintainable code.

Definition

Scope refers to the visibility and accessibility of variables and functions in different parts of your code. Global scope means accessible everywhere, while local scope means accessible only within a specific function or block.

Syntax

// Global scope
var globalVar = "I'm global";
let globalLet = "I'm also global";

function myFunction() {
    // Local/Function scope
    var localVar = "I'm local to this function";
    let localLet = "I'm also local";

    if (true) {
        // Block scope (for let and const)
        let blockScoped = "I'm block scoped";
        var functionScoped = "I'm function scoped";
    }
}

Example

// Global scope
const globalMessage = "Hello from global scope!";
let globalCounter = 0;

function demonstrateScope() {
    // Local scope - can access global variables
    console.log(globalMessage); // "Hello from global scope!"

    // Local variables
    const localMessage = "Hello from local scope!";
    let localCounter = 10;

    // Nested function - has access to outer function's scope
    function innerFunction() {
        console.log(globalMessage); // Can access global
        console.log(localMessage);  // Can access outer function's local

        const innerMessage = "Hello from inner scope!";
        console.log(innerMessage);
    }

    innerFunction();
    // console.log(innerMessage); // Error! innerMessage not accessible here
}

// Block scope with let and const
if (true) {
    let blockScopedLet = "I'm block scoped";
    const blockScopedConst = "I'm also block scoped";
    var functionScopedVar = "I'm function scoped";
}

// console.log(blockScopedLet); // Error!
// console.log(blockScopedConst); // Error!
console.log(functionScopedVar); // "I'm function scoped" - accessible!

demonstrateScope();

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Closures

Closures are one of JavaScript's most powerful features, allowing inner functions to access variables from their outer (enclosing) scope even after the outer function has finished executing.

Definition

A closure is created when an inner function has access to variables from its outer (enclosing) scope even after the outer function has finished executing. Closures give you access to an outer function's scope from an inner function, creating a persistent local scope.

Syntax

function outerFunction(parameter) {
    const outerVariable = "I'm from outer scope";

    function innerFunction() {
        // Can access outerVariable and parameter
        console.log(outerVariable);
        console.log(parameter);
    }

    return innerFunction; // Returns the inner function
}

const closure = outerFunction("Hello");
closure(); // Can still access outer variables

Example

// Basic closure example
function createGreeting(name) {
    const greeting = "Hello";

    return function(message) {
        return `${greeting} ${name}, ${message}`;
    };
}

const greetVitthal = createGreeting("Vitthal");
console.log(greetVitthal("how are you?")); // "Hello Vitthal, how are you?"

// Closure with private variables
function createCounter() {
    let count = 0;

    return {
        increment: function() {
            count++;
            return count;
        },
        decrement: function() {
            count--;
            return count;
        },
        getCount: function() {
            return count;
        }
    };
}

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2
console.log(counter.getCount()); // 2
// console.log(counter.count); // undefined - count is private!

// Closure in loops (common gotcha)
for (var i = 0; i < 3; i++) {
    setTimeout((function(index) {
        return function() {
            console.log("Timer", index);
        };
    })(i), 1000);
}

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Tips and Tricks

Function Performance Tips

• Use arrow functions for short, simple operations and callbacks

• Prefer const For function expressions to prevent reassignment

• Use default parameters instead of checking for undefined values

• Leverage rest parameters for flexible argument handling

Best Practices

• Keep functions small and focused on a single task

• Use descriptive names that explain what the function does

• Avoid deeply nested functions when possible

• Return early to reduce nesting and improve readability

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