Kevin Powell - Community•5mo ago

eventListener on click

I'm dealing with a "simple issue". Basically, i want to put my code inside a function instead of passing directly in the button, but i'm having a issue where i don't know exactly how i'm supposed to do it, so. Here's the html, very simple:

<html !doctype>
  <head></head>
  <body>
<input id="text-input"></input>
<button id="check-btn">Click</button>
<p id="result"> test </p>

</body>
<script src="script.js"/>

<html !doctype>
  <head></head>
  <body>
<input id="text-input"></input>
<button id="check-btn">Click</button>
<p id="result"> test </p>

</body>
<script src="script.js"/>

the js:

const checkButton = document.getElementById("check-btn");
const input = document.getElementById("text-input");
const result = document.getElementById("result")


checkButton.addEventListener("click", ()=> {
  if(input.value == ''){
    alert ("Please input a value");
  } //it works like this
})

//but once i put like this, i just can't make it work 
const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty())

const checkButton = document.getElementById("check-btn");
const input = document.getElementById("text-input");
const result = document.getElementById("result")


checkButton.addEventListener("click", ()=> {
  if(input.value == ''){
    alert ("Please input a value");
  } //it works like this
})

//but once i put like this, i just can't make it work 
const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty())

34 Replies

lossOP•5mo ago

I do believe i'm overthinking about something more simple than it seems to solve

ἔρως•5mo ago

you're not only overthinkinging but you're misuing it you can delete your entire javascript and just add the required attribute to the fields BUT!!!!! you have to put everything inside a form, as you should have done from the beginning and the button should have the type explicitly set to submit

lossOP•5mo ago

oooh

Jochem•5mo ago

Definitely what epic said, but to answer the actual question, when you're passing in a function like to an event listener, you need to omit the calling brackets where you pass the function reference. So this

checkButton.addEventListener("click", checkIsEmpty)

checkButton.addEventListener("click", checkIsEmpty)

lossOP•5mo ago

i tried without but still wasn't working so this made me have a little mentalbreakdown for a few seconds if was it wrong or not since the logic is right and is just to show a simple alert

ἔρως•5mo ago

you have 2 different clicks

lossOP•5mo ago

it was just an example of the right version, but once i do like this doesn't work idk can be like bcs of the order in the script?

Jochem•5mo ago

By adding the (), you're telling Javascript to run the function and pass the result to the listener. The event listener should do the calling, so you just pass it the reference to the function to run

lossOP•5mo ago

like the function going after or ?

ἔρως•5mo ago

this

Jochem•5mo ago

Make a codepen or something then, I can't run code in my head

lossOP•5mo ago

ok wait a sec

ἔρως•5mo ago

what jochem said is supposed to work

Jochem•5mo ago

I'm on mobile atm though so I can't look too much further. I'll look when I can if epic hasn't solved it already 🙂

ἔρως•5mo ago

you already solved it checkButton.addEventListener("click", checkIsEmpty) <-- this is the solution

lossOP•5mo ago

oh wait, i just had to actually put my function before like before i used in the eventlistener idk why tho, it worked before the other way around

ἔρως•5mo ago

that's not strictly a function, that's an anonymous arrow function it's important to keep that distinction because there's nuance to it if you just did function checkIsEmpty ... it would work

lossOP•5mo ago

This way wasn't working:

checkButton.addEventListener("click", checkIsEmpty)
const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty)
const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

Like this works:

const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty)

const checkIsEmpty = () => {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty)

ἔρως•5mo ago

you can put the function anywhere, since javascript is weird because you're using a constant before it is defined

lossOP•5mo ago

yeah prob is a issue with the platform i'm using on the browser than that's what made me got confused i literally just had to put the function before the eventListener weird

ἔρως•5mo ago

checkButton.addEventListener("click", checkIsEmpty);

function checkIsEmpty() {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

checkButton.addEventListener("click", checkIsEmpty);

function checkIsEmpty() {
  if(input.value == ''){
    alert ("Please input a value");
  }
}

this works as well no, it's just you confusing yourself over javascript's weirdness which is normal it's not a function, very strictly speaking: it's an arrow function assigned to a constant

lossOP•5mo ago

yeah like why wasn't working when i put the checkIsEmpty after declaring the eventListener? idk

ἔρως•5mo ago

it's treated completely differently in some aspects

lossOP•5mo ago

this just makes my mind a little lost xD

ἔρως•5mo ago

i just explained why

lossOP•5mo ago

so basically js is just weird ok got it

ἔρως•5mo ago

well, yes and no

Jochem•5mo ago

Defining an anonymous arrow function declared the function in place, and it's available to any code that runs after the arrow function definition. Declaring a function anywhere in your script with the function keyword hoists that declaration to the top of the file so it's available in the entire script.

ἔρως•5mo ago

^ this

lossOP•5mo ago

oh okay i got it now thanks guys this helped a lot

ἔρως•5mo ago

you're welcome also, be VERY careful when using arrow functions, as the this variable won't change

ἔρως•5mo ago

take this code as an example: the first one will be referring to window the 2nd one will be referring to <button>

Choo♚𝕂𝕚𝕟𝕘•5mo ago

Your last line of code has an error. The second parameter of addEventListener() must be a function. What you provided is a function call and not a function. The result is that the parameter is actually the value undefined, because it calls checkIsEmpty and that function doesn't return anything. The parentheses after checkIsEmpty calls it. Those parentheses must be removed to pass the function as a callback.

lossOP•5mo ago

oh yeah, i put bcs wasn't working when only passing without the () but i just had to move the const above and it worked without the () ofc thx for the help still, is always good to learn

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eventListener on click