javascript 'for' loop

A for in loop iterates over every key in an object and gives it to you. Consider this code:

const names = {
                first: "Michael",
                middle: "David",
                last: "Hulet"
              }

for(const position in names){
    console.log(`Your ${position} name is ${names[position]}.`);
}

This is the console output of that script:

Your first name is Michael.
Your middle name is David.
Your last name is Hulet.

It helps you operate on all the keys/values in an object without knowing what they are head of time. Note that if you use a for in loop on an array, it will give you all the indexes in the array, but it will also still give you the length property, so you shouldn't use a for in loop to iterate over an array, unless that's the functionality you want. ECMAScript 2015 added a different loop to iterate over arrays: for of. This will give you all the values that are marked as iterable in an object. For arrays, that's just the values of an array, like this:

const numbers = [1, 2, 3];

for(const number in numbers){
    console.log(number * 2);
}

The console output of that script is:

2
4
6