声明函数内this.id返回undefined问题排查求助

Hey there! I totally get how frustrating it is when you’ve sifted through hundreds of similar questions and still can’t crack the issue—let’s work through this step by step.

First, let’s recap your setup to make sure I’m on the same page:

• You have an HTML element: <li class='field' id='field_2'>
• You’re using this jQuery code to bind a mouseenter event:

  $('.field').on('mouseenter', function(){
    console.log(this.id);
  });
  

Assuming the event isn’t firing (since you mentioned confusion about the cause/solution), here are the most common fixes to check:

1. Verify jQuery is properly loaded #

This is the #1 culprit for jQuery code failing. Double-check:

• You’ve included the jQuery library before your custom script. For example:

  <!-- Load jQuery first -->
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <!-- Then your custom code -->
  <script>
    // Your mouseenter binding here
  </script>
  

• If using a local jQuery file, confirm the file path is correct (check for 404 errors in your browser’s Network tab).

2. Wait for the DOM to load before binding events #

If your script runs before the <li class='field'> element renders, jQuery can’t find the element to bind to. Fix this in one of two ways:

• Move your script to the very end of the <body> tag (right before </body>)—this ensures all HTML elements exist before your code runs.
• Wrap your code in a $(document).ready() handler (or its shorthand):

  $(document).ready(function() {
    $('.field').on('mouseenter', function(){
      console.log(this.id);
    });
  });
  
  // Shorthand version:
  $(function() {
    $('.field').on('mouseenter', function(){
      console.log(this.id);
    });
  });
  

3. Handle dynamically added elements #

If the <li class='field'> is added after your initial script runs (via AJAX, dynamic rendering, etc.), direct event binding won’t work. Use event delegation instead—bind the event to a parent element that exists on page load (or even document):

$(document).on('mouseenter', '.field', function(){
  console.log(this.id);
});

This tells jQuery to listen for mouseenter events on any element matching .field, even if it’s added later.

4. Check for CSS/JS interference #

Sometimes visual or script-related issues block mouse events:

• Inspect the <li> element in dev tools and check if it has pointer-events: none in its CSS—this disables all mouse interactions.
• Make sure no other elements are overlapping the <li> (check z-index and positioning). If an overlay covers it, the mouseenter event will fire on the overlay instead.

Quick Debugging Step #

Open your browser’s JavaScript console and run $('.field').length. If it returns 0, jQuery can’t find the element—go back to steps 1 and 2. If it returns 1 (or the number of .field elements you have), the element exists, so check steps 3 and 4.

内容的提问来源于stack exchange，提问作者O Lopez