A Function Expression is a way to define a function in JavaScript as part of an expression (for example, by assigning it to a variable). Unlike a Function Declaration, a Function Expression is not hoisted and can be either anonymous or named.

Syntax of Function Expression

const variableName = function(parameters) {
  // function body
};

• function — the keyword used to create the function.
• variableName — the variable that stores the function.
• parameters — arguments passed to the function (optional).
• function body — the code executed when the function is called.

Example of a Function Expression

const greet = function(name) {
  console.log(`Hello, ${name}!`);
};

greet("Elle"); 
// Hello, Elle!

No Hoisting

Function Expressions are not hoisted to the top of their scope. This means they can only be called after their definition:

greet("Elle"); 
// Error: greet is not defined

const greet = function(name) {
  console.log(`Hello, ${name}!`);
};

Anonymous Function Expression

Function Expressions can be anonymous (without a name):

const sum = function(a, b) {
  return a + b;
};
console.log(sum(2, 3)); 
// 5

A Function Expression is created only when the execution reaches it.

When to Use Function Expressions

• For creating closures, where a function keeps access to variables from an outer scope.
• When you want a function to be created conditionally during runtime.
• For passing a function as an argument (e.g., in callbacks).

Function Expressions are a flexible way to define functions in JavaScript. They allow for anonymous or named functions that can be used in callbacks, object methods, closures, and more.