JavaScript this keyword and object methods when passed as callback

Question

I am building a counter example using an object just for practice and ran into a problem. I am trying to update an object property using a created method within my object method. I can get it to work when I declare the objects name and increment the property's value, but if I try to use the this keyword to represent the object my problem returns NaN. I think I know why without fully understanding. Can anyone help me to understand?

Example that works:

let counter = {
  num: 0,
  increase(){
    counter.num++;
    document.querySelector("#value").textContent = counter.num;
  },
  decrease(){
    counter.num--;
    document.querySelector("#value").textContent = counter.num;
  },
  reset(){
    counter.num = 0;
    document.querySelector("#value").textContent = counter.num;
  }
}

let x = document.querySelectorAll("button");
x[0].addEventListener("click", counter.increase);
x[1].addEventListener("click", counter.decrease);
x[2].addEventListener("click", counter.reset);

<div class="container">
  <div class="display">
    <h2>Counter</h2>
    <br>
    <span id="value">0</span>
  </div>
  <div class="btn-container">
    <button id="increase">Increase</button>
    <button id="decrease">Decrease</button>
    <button id="reset">Reset</button>
  </div>
</div>

Example that doesn't work:

let counter = {
  num: 0,
  increase(){
    this.num++;
    document.querySelector("#value").textContent = this.num;
  },
  decrease(){
    this.num--;
    document.querySelector("#value").textContent = this.num;
  },
  reset(){
    this.num = 0;
    document.querySelector("#value").textContent = this.num;
  }
}

let x = document.querySelectorAll("button");
x[0].addEventListener("click", counter.increase);
x[1].addEventListener("click", counter.decrease);
x[2].addEventListener("click", counter.reset);

<div class="container">
  <div class="display">
    <h2>Counter</h2>
    <br>
    <span id="value">0</span>
  </div>
  <div class="btn-container">
    <button id="increase">Increase</button>
    <button id="decrease">Decrease</button>
    <button id="reset">Reset</button>
  </div>
</div>

Shouldn't the this keyword point to the object (counter) making this example equivalent to the one that works?

“this” changes based on how the function is called. So “this” will be the element instead of the counter. — evolutionxbox, Jan 08 '21 at 01:04
Does this answer your question? [How does the "this" keyword work?](https://stackoverflow.com/questions/3127429/how-does-the-this-keyword-work) — evolutionxbox, Jan 08 '21 at 01:07
@evolutionxbox is this because the this keyword is taking ownership of the button or method that it is attached to in this situation. I ask this because if I were to solely return this.num++; and then call the function without attaching it to an event. The increment will adhere. Also, is there a way in which to used the this keyword still while attaching the method to an event? — Storm, Jan 08 '21 at 01:11
The event listener is changing the value of “this” to be the element. You must manually bind the value of “this” before giving it to the event listener — evolutionxbox, Jan 08 '21 at 01:22
@evolutionxbox. Awesome. I figured that what the issue was, but really couldn't fully comprehend. Thx so much for you help. I will read the post that you recommended to get a clearer understanding — Storm, Jan 08 '21 at 01:31
you can also manually call the methods yourself when the callback runs: `addEventListener("click", () => counter.increase());` rather than getting the event listener to call them for you, this way, you're invoking `.increase()` and the other methods on the `counter` object, which will keep the `this` as `counter` — Nick Parsons, Jan 08 '21 at 01:34
@NickParsons originally that was an idea that I entertained. Is there any difference in calling the method manually than binding ?? Is it more or less efficient? Are there any side effect or using one over the other? — Storm, Jan 08 '21 at 01:39
@Storm With `.bind()` you can set the `this` to be anything that you pass to `.bind()`, it doesn't just have to be `counter`. It also allows you to partially apply arguments, but that isn't really relevant to what you're doing here. So `.bind()` is more flexible. Using the callback/method approach, the `this` is implicitly set based on the object that you invoke the method on (ie: it gets set to `counter` and you can't change that). In terms of efficiency, I don't see there being much of a difference. Worrying about efficiency for small things like this isn't usually worth the time anyway :P — Nick Parsons, Jan 08 '21 at 01:45
@NickParsons your 100% correct. Thx once again for the clarification. — Storm, Jan 08 '21 at 01:47

score 3 · Accepted Answer · answered Jan 08 '21 at 01:18

3

try this

x[0].addEventListener("click", counter.increase.bind(counter));
x[1].addEventListener("click", counter.decrease.bind(counter));
x[2].addEventListener("click", counter.reset.bind(counter));

you must bind with the object counter

answered Jan 08 '21 at 01:18

Ari Firmanto

334
1
15

JavaScript this keyword and object methods when passed as callback

Example that works:

Example that doesn't work:

1 Answers1