FUNCTION EXPRESSIONS

One of the more powerful, and often confusing, parts of JavaScript is function expressions. There are two ways to define a function: by function declaration and by function expression. The first, function declaration, has the following form:

function functionName(arg0, arg1, arg2) {
 // function body
}

One of the key characteristics of function declarations is function declaration hoisting, whereby function declarations are read before the code executes. That means a function declaration may appear after code that calls it and still work:

sayHi();
function sayHi() {
 console.log("Hi!");
}

This example doesn't throw an error because the function declaration is read first before the code begins to execute.

The second way to create a function is by using a function expression. Function expressions have several forms. The most common is as follows:

let functionName = function(arg0, arg1, arg2) {
 // function body
};

This pattern of function expression...