Mastering JavaScript Functional Programming - Second Edition

Handling the this value

A classic problem with JavaScript is the handling of this, whose value isn't always what you expect it to be. ES2015 solved this with arrow functions, which inherit the proper this value so that problems are avoided. Look at the following code for an example of the possible problems: by the time the timeout function is called, this will point to the global (window) variable instead of the new object, so you'll get undefined in the console:

function ShowItself1(identity) {  this.identity = identity;  setTimeout(function() {    console.log(this.identity);  }, 1000);}var x = new ShowItself1("Functional");// after one second, undefined is displayed

There are two classic ways of solving this with old-fashioned JavaScript and ...