JavaScript Function Expressions

A function expression is a function that is assigned to a variable. Unlike function declarations, function expressions are not hoisted and must be defined before use.

1️⃣ Syntax

const functionName = function(parameters) {
    // Code to execute
    return result;
};

2️⃣ Example: Function Expression

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

console.log(greet("John")); // Output: Hello, John!

📌 Key Difference from Function Declarations:

Function declarations are hoisted, meaning they can be used before they are defined.
Function expressions are not hoisted, meaning they must be defined before use.

3️⃣ Anonymous Function Expression

A function without a name, assigned to a variable.

const sum = function(a, b) {
    return a + b;
};

console.log(sum(5, 3)); // Output: 8

📌 The function has no name but is stored in the sum variable.

4️⃣ Named Function Expression

A function with a name, assigned to a variable.

const factorial = function fact(n) {
    if (n === 0) return 1;
    return n * fact(n - 1);
};

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

📌 Here, fact(n) is a function name only available inside the function.

5️⃣ Function Expressions as Arguments

Function expressions can be passed as arguments to other functions.

function operate(a, b, callback) {
    return callback(a, b);
}

const multiply = function(x, y) {
    return x * y;
};

console.log(operate(4, 5, multiply)); // Output: 20

6️⃣ Arrow Functions (ES6) as Function Expressions

A shorter syntax for function expressions.

const square = (n) => n * n;

console.log(square(6)); // Output: 36

7️⃣ Immediately Invoked Function Expression (IIFE)

A function that runs immediately after being defined.

(function() {
    console.log("I run immediately!");
})();

📌 Useful for:
✅ Avoiding pollution of global scope
✅ Isolating variables in their own scope

🎯 Summary

✅ Function Expression → const sum = function(a, b) { return a + b; };
✅ Anonymous Function Expression → No function name
✅ Named Function Expression → Can reference itself
✅ Used as Arguments → Pass functions into other functions
✅ Arrow Functions → const square = n => n * n;
✅ IIFE → (function() { console.log("Runs immediately!"); })();

🚀 Now you understand function expressions! Let me know if you need help. 😊

Souy Soeng

JavaScript Function Expressions