How can I get this javascript function to work all of the time? (Uncaught TypeError: Cannot read properties of null)

Issue

I'm trying to create two numerical inputs, each of which has their own custom '+' and '-' buttons. However, with my current code, it appears that the button functions only work some of the time. On the other occasions I get a Uncaught TypeError: Cannot read properties of null (reading 'value') at HTMLButtonElement.<anonymous> error.

This is the HTML:

<div class="col">
    <div class="row">
        <div class="input-group mx-auto w-50">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm minus" data-id="one"
                    type="button"><i class="fa fa-minus"></i></button>
            </div>
            <input type="number" id="one" 
                class="form-control form-control-sm text-center" value="0">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm plus" data-id="one" type="button"><i
                        class="fa fa-plus"></i></button>
            </div>
        </div>
    </div>

    <div class="row">
        <div class="input-group mx-auto w-50">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm minus" data-id="two"
                    type="button"><i class="fa fa-minus"></i></button>
            </div>
            <input type="number" id="two"
                class="form-control form-control-sm text-center" value="0">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm plus" data-id="two" type="button"><i
                        class="fa fa-plus"></i></button>
            </div>
        </div>
    </div>
</div>

And this is the Javascript:

<script>
document.addEventListener("DOMContentLoaded", () => {
    document.querySelectorAll('.minus').forEach(item => {
        item.addEventListener('click', event => {
            var input_id = event.target.getAttribute('data-id')
            console.log('Minus button clicked for input ' + input_id)
            var quantityInput = document.getElementById(input_id);
            var currentQuantity = parseInt(quantityInput.value);
            quantityInput.value = currentQuantity - 1
        })
    })
    document.querySelectorAll('.plus').forEach(item => {
        item.addEventListener('click', event => {

            var input_id = event.target.getAttribute('data-id')
            console.log('Plus button clicked for input ' + input_id)
            var quantityInput = document.getElementById(input_id);
            var currentQuantity = parseInt(quantityInput.value);
            quantityInput.value = currentQuantity + 1
        })
    })
})

Here is a sample of the output after clicking a few of the buttons:

Solution

The event.target may be the span inside the button, and therefore the code may not find the data-id attribute. Instead perhaps get the id without using event.target. Either use item or event.currentTarget.

Answered By - dqhendricks