Can't access the value of HTMLCollection

问题

test.html

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Untitled Document</title>
    <script>
    var eles = document.getElementsByClassName('review');
    console.log(eles);
    console.log(eles.length);
    console.log(eles[0]);
    // for(var i=0, max=eles.length)
    </script>
</head>
<body>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
</body>

I checked eles represents HTMLCollenction.

Here says HTMLCollection also exposes its members directly as properties by both name and index.

So I tried to debug by console.log(eles.length) and console.log(eles[0]). But unfortunately console shows 0 and undefined.(using Google Chrome Tool Developer)

How can I access the result of eles? I want to change style and add attribute to the tags gotten by ClassName. Plus, I can only use natural Javascript.

回答1:

The problem is you have placed the script in the header which gets executed before the html elements are loaded, so getElementsByClassName() will not return any elements.

One solution is to wait for the html elements to be loaded then execute your script, for that you can use the window objects load event

window.addEventListener('load', function () {
    var eles = document.getElementsByClassName('review');
    console.log(eles);
    console.log(eles.length);
    console.log(eles[0]);
})

Or you can place your script at the bottom of the body element instead of in head so that by the time the script is parsed and executed the elements are loaded in the dom

回答2:

Your html elements dont exist when you run the code. you code is processed, at that poiint your body tag doesnt exist. so:

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Untitled Document</title>
</head>
<body>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>

    <script>
      var eles = document.getElementsByClassName('review');
      console.log(eles);
      console.log(eles.length);
      console.log(eles[0]);
    </script>
</body>

Works fine because you've ran your code after your html elements have been added.

Best practices are that you generally have your javascript right at the end of your body tag.

(Or another technique is using a 'ready' script, like document.load or $(function(){)

回答3:

Use below code for access 0 position element:-

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Untitled Document</title>
</head>
<body>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
    <div class="review"></div>
</body>
<script>
    var eles = document.getElementsByClassName('review');
    console.log(eles);
    console.log(eles.length);
    console.log(eles[0]);
    // for(var i=0, max=eles.length)
    </script>
</html>

来源：https://stackoverflow.com/questions/32222255/cant-access-the-value-of-htmlcollection

标签

javascript

html5

dom

htmlcollection