Arrow Function in Object Literal [duplicate]

Question

This question already has an answer here:

• Are 'Arrow Functions' and 'Functions' equivalent / exchangeable? 1 answer
• Methods in ES6 objects: using arrow functions 4 answers

I'm trying to figure out why an arrow function in an object literal is called with window as this. Can someone give me some insight?

var arrowObject = {
  name: 'arrowObject',
  printName: () => {
    console.log(this);
  }
};

// Prints: Window {external: Object, chrome: Object ...}
arrowObject.printName();

And an object that works as expected:

var functionObject = {
  name: 'functionObject',
  printName: function() {
    console.log(this);
  }
};

// Prints: Object {name: "functionObject"}
functionObject.printName();

According to Babel REPL, they're transpiled to

var arrowObject = {
  name: 'arrowObject',
  printName: function printName() {
    console.log(undefined);
  }
};

And

var functionObject = {
  name: 'functionObject',
  printName: function printName() {
    console.log(this);
  }
};

Why isn't arrowObject.printName(); called with arrowObject as this?

Console logs are from Fiddle (where use strict; isn't used).

Answer 1

Note that the Babel translation is assuming strict mode, but your result with window indicates you're running your code in loose mode. If you tell Babel to assume loose mode, its transpilation is different:

var _this = this;                    // **

var arrowObject = {
  name: 'arrowObject',
  printName: function printName() {
    console.log(_this);              // **
  }
};

Note the _this global and console.log(_this);, instead of the console.log(undefined); from your strict-mode transpilation.

I'm trying to figure out why an arrow function in an object literal is called with window as this.

Because arrow functions inherit this from the context in which they're created. Apparently, where you're doing this:

var arrowObject = {
  name: 'arrowObject',
  printName: () => {
    console.log(this);
  }
};

...this is window. (Which suggests you're not using strict mode; I'd recommend using it where there's no clear reason not to.) If it were something else, such as the undefined of strict mode global code, this within the arrow function would be that other value instead.

It may be a bit clearer what the context is where the arrow function is created if we break your initializer into its logical equivalent:

var arrowObject = {};
arrowObject.name = 'arrowObject';
arrowObject.printName = () => {
  console.log(this);
};