Question

我怎样才能让这个 javascript 函数一直工作？（未捕获的 TypeError：无法读取 null 的属性）

2022-07-18

73

javascript html

我尝试创建两个数字输入，每个数字输入都有自己的自定义“+”和“-”按钮。但是，使用我当前的代码，按钮功能似乎只在某些时候起作用。在其他情况下，我收到 Uncaught TypeError: Cannot read properties of null (reading 'value') at HTMLButtonElement.<anonymous> 错误。

这是 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>

这是 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
        })
    })
})

这是单击几个按钮后的输出示例:

Answer 1

event.target 可能是按钮内的跨度，因此代码可能找不到 data-id 属性。也许可以在不使用 event.target 的情况下获取 id。可以使用 item 或 event.currentTarget 。

Answer 2

点击事件发生在 <i class="fa fa-minus"></i>
您可以使用 Event.currentTarget 代替 event.target

document.addEventListener("DOMContentLoaded", () =>
  {
  document.querySelectorAll('.minus').forEach(item =>
    {
    item.onclick = event => 
      {
      let
        input_id        = event.currentTarget.dataset.id
      , quantityInput   = document.getElementById(input_id)
      , currentQuantity = parseInt(quantityInput.value)
        ;
      console.log('Minus button clicked for input ' + input_id)
      quantityInput.value = currentQuantity - 1
      }
    })
  document.querySelectorAll('.plus').forEach(item =>
    {
    item.onclick = event => 
      {
      let
        input_id        = event.currentTarget.dataset.id
      , quantityInput   = document.getElementById(input_id)
      , currentQuantity = parseInt(quantityInput.value)
        ;
      console.log('Plus button clicked for input ' + input_id)
      quantityInput.value = currentQuantity + 1
      }
    })
  })

您也可以简单地执行以下操作：

document.addEventListener("DOMContentLoaded", () =>
  {
  document
    .querySelectorAll('button.minus, button.plus')     
    .forEach( btn =>              // this is the button
    {
    btn.onclick = () =>         // no event argument needed
      {
      let
        input_Qte = document.getElementById( btn.dataset.id ) 
      , adding    = btn.classList.contains('plus') ? +1 : -1
        ;
      input_Qte.value = +input_Qte.value + adding   // use +input_Qte.value for integer conversion
      }
    })
  })