What are FunctionExpression and FunctionDeclaration? What is their main difference?

FunctionDeclaration is a function declaration using the function keyword and a function name, for example:

function greet() {
  console.log('Hello');
}

FunctionExpression is a function created as an expression and can be anonymous or named, for example:

const greet = function() {
  console.log('Hello');
};

The main difference is hoisting:

• FunctionDeclaration is hoisted entirely, so it can be called before its declaration in the code.
• FunctionExpression is hoisted as a variable, but the function itself is only initialized at the point of assignment, so it cannot be called before the assignment.

Example:

sayHi(); // Works
function sayHi() {
  console.log('Hi');
}

sayHello(); // Error: sayHello is not a function
const sayHello = function() {
  console.log('Hello');
};