JavaScript Function Definitions

JavaScript functions are declared using the function keyword, either as a declaration or expression. Declarations define named functions, while expressions assign functions to variables. Both enable code reuse and modularity.

Syntax

Function Declarations

function functionName( parameters ) {
    // Statements
};

Function Expressions

let variableName = function( parameter ) {
    // Statements
}; 

Function Constructor

let FunctionName = new Function("parameter", "return parameter");
let variableName = FunctionName(values); 

Parameter: It contains single parameter functionName which is mandatory and used to specify the name of function.

Examples of JavaScript Function Definitions

Example: This example we demonstrates a function declaration named myFunction, which multiplies two numbers.

function multiply(num1, num2) {
    return num1 * num2;
}

let num = multiply(40, 3);
console.log(num);

120

Example 2: This example describes a function expression assigned to num, multiplying two numbers.

let multiply = function(num1, num2) {
    return num1 * num2;
};

console.log(multiply(20, 30));

600

Example 3: This example describes a function expression created with the Function constructor, multiplying two numbers and displaying the result in a paragraph element.

let multiply = new Function(
    "num1",
    "num2",
    "return num1 * num2"
);

console.log(multiply(25, 4));

100

Function Hoisting

Function hoisting moves function declarations to the top of their scope, allowing them to be used before declaration. Function expressions are not hoisted.

Example: Defining function hoisting by invoking a function before its declaration, displaying a welcome message from GeeksForGeeks.

myFunction();

function myFunction() {
    console.log("Welcome to GeeksForGeeks");
}

Welcome to GeeksForGeeks

Self-Invoking Functions

Self-invoking functions execute automatically upon creation, without a name. Function expressions followed by () execute immediately, while function declarations cannot be invoked directly.

Example: Defining a self-invoking function that sets content in a paragraph element, showcasing its execution upon creation.

(function () {
    console.log("GeeksForGeeks is the best way to learn");
})();

GeeksForGeeks is the best way to learn

Functions are Objects

It can describe functions as objects and have both properties and methods.

• When define function as property of an object then it is known as method to the object.
• When design a function to create new objects then it is known as object constructor.

Example: Using arguments object to count the number of arguments passed to a function.

function myFunction(num1, num2) {
    return arguments.length;
}

console.log(myFunction(4, 3));

2

Arrow Functions

Arrow functions simplify writing function expressions by providing a concise syntax without the need for the function keyword, return keyword, or curly brackets.

Example: Defining an arrow function to multiply two numbers and displays the result using JavaScript.

const num = (num1, num2) => num1 * num2;

console.log(num(5, 5));

25