Explain how `this` works in JavaScript


There's no simple explanation for this; it is one of the most confusing concepts in JavaScript. A hand-wavey explanation is that the value of this depends on how the function is called. Having read many explanations on this online, Arnav Aggrawal's explanation was the clearest. The following rules are applied:

  1. If the new keyword is used when calling the function, this inside the function is a brand new object.
  2. If apply, call, or bind are used to call/create a function, this inside the function is the object that is passed in as the argument.
  3. If a function is called as a method, such as obj.method() — this is the object that the function is a property of.
  4. If a function is invoked as a free function invocation, meaning it was invoked without any of the conditions present above, this is the global object. In a browser, it is the window object. If in strict mode ('use strict'), this will be undefined instead of the global object.
  5. If multiple of the above rules apply, the rule that is higher wins and will set the this value.
  6. If the function is an ES2015 arrow function, it ignores all the rules above and receives the this value of its surrounding scope at the time it is created.

For an in-depth explanation, do check out his article on Medium.

Can you give an example of one of the ways that working with this has changed in ES2015?

ES2015 allows you to use arrow functions which uses the enclosing lexical scope. This is usually convenient, but does prevent the caller from controlling context via .call or .apply—the consequences being that a library such as jQuery will not properly bind this in your event handler functions. Thus, it's important to keep this in mind when refactoring large legacy applications.