Why my DOM manipulation giving error in JS

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I was making a simple project of Counter.
In this I,am unable to use nextElementSibling method to access the next sibling element.

What I have tried:

ON WEB

Counter

counter
0

decrease
reset
increase

HTML DOCUMENT

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Counter</title>

    <!-- styles -->
    <link rel="stylesheet" href="styles.css" />
  </head>
  <body>
    <main>
      <div class="container">
        <h1>counter</h1>
        <span id="value">0</span>
        <div class="button-container">
          <button class="btn decrease">decrease</button>
          <button class="btn reset">reset</button>
          <button class="btn increase">increase</button>
        </div>
      </div>
    </main>
    <!-- javascript -->
    <script src="app.js"></script>
  </body>
</html>

JS DOCUMENT

let value = document.getElementById("value");
//console.log(value);
let count = 0;
const parent = document.getElementsByClassName("button-container");
const button_dec = parent.firstChildElement;
const button_reset = button_dec.nextSibling;
const button_inc = button_reset.lastChildElement;
//console.log(button_dec);
//console.log(button_inc);
//console.log(button_reset);
button_dec.addEventListener("click",function () {
    count--;
    let value_dec = value.innerHTML;
    value_dec = count;
})

button_reset.addEventListener("click",function () {
    count = 0;
    let value_dec = value.innerHTML;
    value_dec = count;
})

button_inc.addEventListener("click",function () {
    count++;
    let value_dec = value.innerHTML;
    value_dec = count;
})

ERROR
app.js:35 Uncaught TypeError: Cannot read properties of undefined (reading 'nextSibling')

Posted 29-Jan-23 15:19pm

Parth Vijay

Updated 29-Jan-23 16:43pm

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Dave Kreskowiak · Answer 1 · 2023-01-29T16:43:00

Well, it's telling you that the object you're calling a property getter or method on doesn't exist, or in javascript terms, undefined.

So, work backwards from the point of the error. Why does button_dec not refer to an object? Probably because the call to parent.firstChildElement didn't return an element, but instead couldn't find a child element. Why did that happen? Probably because your call to getElementsByClassName didn't return anything, or didn't return the element you think it did.

You'll have to insert console.log statements after each step in the code to see what it returned, like this:

JavaScript

const parent = document.getElementsByClassName("button-container");
console.log(parent);
const button_dec = parent.firstChildElement;
console.log(button_dec);
const button_reset = button_dec.nextSibling;
console.log(button_reset);
const button_inc = button_reset.lastChildElement;

So why did firstChildElement return undefined? Google for "javascript firstChildElement" and you'll find something interesting. Look very closely at the name of the function you're calling.