Kevin Powell - Community•2mo ago

arrow function .this value

Hey, so i'm aware that in an arrow function, .this equates to the window and not the current object. If this is the case, in an arrow function, wouldn't defining a .this value essentially be the same as defining a global variable outside of the function since they both equate to the window? If so, why would you need to utilize .this is in a arrow function? thanks in advance.

10 Replies

ἔρως•2mo ago

that depends on how you use it so, without seeing some code, we have to stop here there's plenty of variations where the this points to many many many different things

snxxwyyOP•2mo ago

i don't have an example but i'll try to make one. If i do this in the arrow function, isn't it redundant since i can just have a global variable as both the this and global variable refer to the window?

const arrowFunc = () => {
  this.name = "John";
}

const arrowFunc = () => {
  this.name = "John";
}

compared to:

let name = "";

const arrowFunc = () => {
  name = "John";
}

let name = "";

const arrowFunc = () => {
  name = "John";
}

ἔρως•2mo ago

being an arrow function doesn't make a difference in this case

snxxwyyOP•2mo ago

because it can be a normal one?

ἔρως•2mo ago

yes, and will work the same now, if it was in an object, then it would be different

snxxwyyOP•2mo ago

oh? why would it be different? because of the key value pairs?

ἔρως•2mo ago

because now it is associated to an object normal functions' this changes, while arrow functions don't do that

Choo♚𝕂𝕚𝕟𝕘•2mo ago

The value of "this" is not always the global object. It is the value of "this" of the outer scope. If there is no other "this" binding, then it becomes a reference to the global object. Here is an example in which "this" inside an arrow function does not refer to the global object:

const obj = {
  num: 42,
  nonArrow(){
    const arrow =()=>{
      console.log(this.num);//42
    };
    arrow();
  }
}
obj.nonArrow();

const obj = {
  num: 42,
  nonArrow(){
    const arrow =()=>{
      console.log(this.num);//42
    };
    arrow();
  }
}
obj.nonArrow();

What happens here is nonArrow() establishes a "this" binding with its containing object, and that object has a num property. The arrow function uses the "this" of its outer scope which is the scope of nonArrow, so that "this" refers to the object that has num. Also, the global object is not always the window object. That is only true in a browser. In the back end, it can refer to Node's globalThis or it can refer to the module object, depending on if the JS code is in a module or not.

dys 🐙•2mo ago

The call & apply methods of functions allow you to set this:

const context = { a: 'testing' }
const arrow = () => console.debug({ arrow: this.a })
const vanilla = function() { console.debug({ [arguments.callee.name]: this.a }) }
arrow.call(context)
vanilla.call(context)

const context = { a: 'testing' }
const arrow = () => console.debug({ arrow: this.a })
const vanilla = function() { console.debug({ [arguments.callee.name]: this.a }) }
arrow.call(context)
vanilla.call(context)

{ arrow: undefined }
{ vanilla: 'testing' }

{ arrow: undefined }
{ vanilla: 'testing' }

snxxwyyOP•2mo ago

Ah I see, thanks everyone

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arrow function .this value