Basic looping in JavaScript.

loop.js

// Loop array

const foo = ["ane", "twa", "three"];
foo.forEach(item => console.log(item));

// Loop DOM elements

document.querySelectorAll('.foo').forEach(el => {
  el.classList.add('bar'); // Add class, for example
});

// Loop object properties

const foo = {
  derp: "merp",
  herp: "berp"
};

Object.keys(foo).forEach(key => {
    console.log(key, foo[key]);
});