Arrow Function in Object Literal

Arrow Function in Object Literal

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);
};

Methods in ES6 objects: using arrow functions

Arrow functions are not designed to be used in every situation merely as a shorter version of old-fashioned functions. They are not intended to replace function syntax using the function keyword. The most common use case for arrow functions is as short "lambdas" which do not redefine this, often used when passing a function as a callback to some function.

Arrow functions cannot be used to write object methods because, as you have found, since arrow functions close over the this of the lexically enclosing context, the this within the arrow is the one that was current where you defined the object. Which is to say:

// Whatever `this` is here...
var chopper = {
    owner: 'Zed',
    getOwner: () => {
        return this.owner;    // ...is what `this` is here.
    }
};

In your case, wanting to write a method on an object, you should simply use traditional function syntax, or the method syntax introduced in ES6:

var chopper = {
    owner: 'Zed',
    getOwner: function() {
        return this.owner;
    }
};

// or

var chopper = {
    owner: 'Zed',
    getOwner() {
        return this.owner;
    }
};

(There are small differences between them, but they're only important if you use super in getOwner, which you aren't, or if you copy getOwner to another object.)

There was some debate on the es6 mailing list about a twist on arrow functions which have similar syntax but with their own this. However, this proposal was poorly received because that is mere syntax sugar, allowing people to save typing a few characters, and provides no new functionality over existing function syntax. See the topic unbound arrow functions.

Javascript arrow function lexical scope: object literal vs new operator

The new operator doesn't create a new scope.

Functions create a new scope.

The constructor function is a different function to the global-outside-of-any-function space.

ECMAScript 6 arrow function that returns an object

You must wrap the returning object literal into parentheses. Otherwise curly braces will be considered to denote the function’s body. The following works:

p => ({ foo: 'bar' });

You don't need to wrap any other expression into parentheses:

p => 10;
p => 'foo';
p => true;
p => [1,2,3];
p => null;
p => /^foo$/;

and so on.

Reference: MDN - Returning object literals

ES6 Arrow function: why this points differently when used in constructor and object literal?

function Obj() {
  this.show = () => {
    console.log(this);
  };
}
const o = new Obj();
o.show(); 

Here "this" works based on the rules of the "new" keyword, pointing to a new object with the structure defined inside Obj() (the new object is the context).
More info at : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new

const o2 = {
  show: () => {
    console.log(this);
  }
}
o2.show() // window || undefinded

Here "this" gets its value at runtime and because neither lambda functions nor object literals define a context, the context remaining is the global one and that is why you get that value.

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