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

General

A function expression is only defined when its line is reached. So they're not available until the browser reaches them.

sayHello() // sayHello is not a function
var sayHello = function() {
    return 'say hello';
}

A function declaration is available throughout a function's scope. They are loaded before the browser executes any code.

sayHello(); // 'say hello'
function sayHello() {
    return 'say hello';
}

This means you cannot conditionally declare a function using a function declaration.

sayHello(); // 'say hello';

if (condition) {
    function sayHello() {
        return 'say hello';
    }
}