What is the difference between a function declaration and a function expression?

The difference is how the function is introduced in the code.

A function declaration uses the function keyword followed by a function name and body:

function double(n) {
  return n * 2;
}

const result = double(4);
console.log(result); // 8

A function expression creates a function as part of an expression. A common pattern assigns it to a variable:

const double = function(n) {
  return n * 2;
};

const result = double(4);
console.log(result); // 8

In both examples, calling double(4) runs the function, and return n * 2 supplies the value 8. MDN describes declarations as the function name(...) { ... } form and expressions as functions created within expressions, often assigned to variables. [S1] [S2]

An expression can contain a named or anonymous function. In this example, function(n) has no name of its own, and the variable double is the name you use to refer to the assigned function:

const double = function(n) {
  return n * 2;
};

You can also give the function inside the expression a name:

const double = function multiplyByTwo(n) {
  return n * 2;
};

Here, the expression contains the name multiplyByTwo, while the assigned variable is double. For a basic call from surrounding code, use double(4). MDN confirms that a function expression may be named or anonymous and can be assigned to a variable. [S1] [S2]

Choose based on how you want to organize the function, not because the two forms are interchangeable in every situation. A declaration makes a named function directly in the surrounding code. An expression is useful when you want to assign a function as a value, as in the const example, or use a function as part of an expression.

Do not assume they have identical initialization behavior. MDN notes that declarations and expressions differ in initialization behavior and availability. The examples here deliberately call each function after its definition or assignment; they do not demonstrate calling either one earlier. If order matters in your code, check the exact form and scope you are using rather than relying on a blanket rule. [S1] [S2]

Use this side-by-side example. It defines each function before calling it and shows each returned value:

function declarationDouble(n) {
  return n * 2;
}

const expressionDouble = function(n) {
  return n * 2;
};

console.log(declarationDouble(4)); // 8
console.log(expressionDouble(4));  // 8

Read it in three steps:

1. The declaration starts with function declarationDouble.
2. The expression starts with const expressionDouble = and assigns a function to that variable.
3. Each call passes 4; each function returns 8.

This comparison shows the syntax and result for these specific examples. It does not establish that declarations and expressions are available at the same point in every program. [S1] [S2]