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JavaScript Practice Object Basics in JavaScript Practicing Object Basics Practice Adding a Method to an Object Literal

Janina Alvarez
PLUS
Janina Alvarez
Courses Plus Student 6,297 Points

Why does an arrow function not allow "this" access, but function() {} does?

In my solution to this part of the challenge, I provided:

const myString = { string: "Programming with Treehouse is fun!", countWords: () => { const words = this.string.split(' '); return words.length; }, }

this was giving me an error that this.string is undefined. When I changed it to the following, it passed:

const myString = { string: "Programming with Treehouse is fun!", countWords: function() { const words = this.string.split(' '); return words.length; }, }

Why is that the case?

mystring.js
const myString = {
    string: "Programming with Treehouse is fun!",
  countWords: () => {
    const words = this.string.split(' ');
    return words.length;
  },
}

1 Answer

Steven Parker
Steven Parker
230,248 Points

The arrow function was designed to provide better support for object-oriented programming. With a conventional function, the scope is the function itself instead of the object. But within an arrow function, the this keyword refers to the object from which you define the function.

When used outside of an object (as in the example above), this refers to the global Window object.

Janina Alvarez
Janina Alvarez
Courses Plus Student 6,297 Points

Thanks for your response, Steven Parker ! Can you add some clarification on your last statement, "When used outside of an object (as in the example above), this refers to the global Window object."

I thought because the function was defined within the object literal, it would be considered inside the object, not outside?

Steven Parker
Steven Parker
230,248 Points

The MDN page says that Arrow functions … inherit "this" from the parent scope at the time they are defined.

That same reference page warns that Arrow function expressions … cannot be used as methods … because they do not have their own "this". It goes on to illustrate the concept showing a code example very similar to your own.