Learning JavaScript Function Declaration and Function Expression for Beginners Functions in JavaScript are a key feature that makes your code more reusable, modular, and organized. There are two main ways to declare functions in JavaScript: Function Declarations and Function Expressions. Both allow you to declare functions, but they work differently, and it is crucial to understand the differences. Let's learn about these concepts in simple language!

What is a Function Declaration?

Function declaration is one of the most used ways of declaring a function. It's the traditional way and it looks something like this:

function greet() {
    console.log("Hello, world!");
}

Key Features of Function Declarations:

1. A function declaration starts with the function keyword, then comes the function name, parentheses (), and curly braces {} which holds the function's code.

2. Hoisting: Function declarations possess one strange feature known as hoisting. It means that the function is available for the whole scope (even before the line on which it is declared in the code).

For example:

greet(); // Outputs: "Hello, world!"

function greet() {
    console.log("Hello, world!");
}

In the above code, the greet() function is called before it is actually declared, but it works because of hoisting.

Function Declaration Example:

javascriptCopyfunction add(a, b) {
    return a + b;
}

console.log(add(2, 3));  // Output: 5

What is a Function Expression?

A function expression is another way to define a function, but it is not the same as a function declaration. In a function expression, the function is defined within an expression and is usually assigned to a variable.

This is an example of a function expression:

const greet = function() {
    console.log("Hello, world!");
};

Key Features of Function Expressions:

1. Expressions Syntax: Function expressions are declared by a function being assigned to a variable. Note that the function is not given a name (anonymous function).

2. No Hoisting: Function expressions do not get hoisted, in contrast to function declarations. What this means is you can't call the function prior to the line where it's declared.

3. For example:

    javascriptCopygreet();  // Error: greet is not a function
   
    const greet = function() {
        console.log("Hello, world!");
    };
   

   In the above example, trying to call greet() before the function expression is assigned to the variable will result in an error because it hasn’t been defined yet.

Function Expression Example:

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

console.log(multiply(2, 3));  // Output: 6

Differences Between Function Declaration and Function Expression

Now that we’ve covered both concepts, let’s look at the key differences between function declarations and function expressions.

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Understanding Function Declaration and Function Expression in JavaScript: A Beginner’s Guide

In JavaScript, functions are a key concept that makes your code more modular, reusable, and organized. There are two main ways to define functions in JavaScript: Function Declarations and Function Expressions. Although both allow you to create functions, they work in slightly different ways, and it’s essential to understand the differences. Let’s dive into these concepts in simple terms!

---

What is a Function Declaration?

A function declaration is one of the most common ways to define a function. It’s the traditional approach and looks something like this:

javascriptCopyfunction greet() {
    console.log("Hello, world!");
}

Key Features of Function Declarations:

1. Syntax: A function declaration starts with the function keyword, followed by the name of the function, parentheses (), and curly braces {} which enclose the function’s code.

2. Hoisting: One unique thing about function declarations is hoisting. This means that the function is available throughout the entire scope (even before the line where it is defined in the code).

   For example:

    javascriptCopygreet(); // Outputs: "Hello, world!"
   
    function greet() {
        console.log("Hello, world!");
    }
   

   In the above example, the greet() function is called before it is actually defined, but it still works because of hoisting.

Function Declaration Example:

javascriptCopyfunction add(a, b) {
    return a + b;
}

console.log(add(2, 3));  // Output: 5

---

What is a Function Expression?

A function expression is another way to define a function, but it differs from a function declaration. In a function expression, the function is defined within an expression, and it is usually assigned to a variable.

Here’s an example of a function expression:

javascriptCopyconst greet = function() {
    console.log("Hello, world!");
};

Key Features of Function Expressions:

1. Syntax: Function expressions are created by assigning a function to a variable. Notice the function has no name (anonymous function).

2. No Hoisting: Unlike function declarations, function expressions are not hoisted. This means you cannot call the function before the line where it is defined.

   For example:

    javascriptCopygreet();  // Error: greet is not a function
   
    const greet = function() {
        console.log("Hello, world!");
    };
   

   In the above example, trying to call greet() before the function expression is assigned to the variable will result in an error because it hasn’t been defined yet.

Function Expression Example:

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

console.log(multiply(2, 3));  // Output: 6

---

Differences Between Function Declaration and Function Expression

Now that we’ve covered both concepts, let’s look at the key differences between function declarations and function expressions.

Function Expression: A function defined as part of an expression, usually set to a variable. It is not hoisted, so it must be defined prior to its usage.

When to Use Which?

Function Declarations are excellent when you want to ensure your function is available all the time, no matter where you use it in your code. They're most suited for general-purpose utility functions or if you're not dealing with dynamic behavior.

Function Expressions are useful when you want to assign a function to a variable, typically for dynamic reasons, or when passing functions as arguments (callbacks). They're also commonly used in the event handler or array methods like.map(),.filter(), etc.