multiImageNumber 变量已经是数字，以下代码行没有有任何意义：
for（var x = 0; x< multiImageNumber.length; x ++）{

另一种情况 - document.getElementsByClassName（）返回一个类似于数组的对象，其中包含具有指定类的所有元素。 无需迭代它并将每个元素推送到一个新数组。

如果要将类似数组的对象更改为一个数组，使用 Array＃from 。
这是一个内置函数，来自 ES6 。

  var arrayOfMultiImages = document .getElementsByClassName（ '多重图象'）; 
 var array = Array.from（arrayOfMultiImages）; 
 

一旦您已将类似数组的对象更改为数组，可以使用例如 Array＃forEach 函数可以简单快速地迭代它的元素。

  array.forEach（elem => elem.className =mySpace）
 

简单示例：

  let elems = document.getElementsByClassName（'box'）; Array.from（elems）.forEach（v => v.className ='mySpace'）;  

 < div class =box >< / div>< div class =box>< / div>< div class =box>< / div>  

I'm trying to find all tags on the page and change their class from 'multiimage' to 'mySpace' but i cannot see any changes on the page and when I log this elemets into console I can see ther className set as 'multiimage'.

My code:

window.onload = function(){
    // to store all <img> elements
    var arrayOfMultiImages = new Array();
    var multiImageNumber = document.getElementsByClassName("multiimage").length;
    // store <img> tag in array
    for(var i = 0; i < multiImageNumber; i++){
        arrayOfMultiImages.push(document.getElementsByClassName("multiimage")[i]);
    }
    // change className of <img> elements
    for(var x = 0; x < multiImageNumber.length; x++){
        arrayOfMultiImages[x].className = "mySpace";
    }
    // cannot see any changes
    console.log(arrayOfMultiImages);
};

Don't you have any idea what is wrong there?

multiImageNumber variable is already a number, following line of code doesn't have any kind of sense: for(var x = 0; x < multiImageNumber.length; x++){

Another case - document.getElementsByClassName() returns an array-like object, containing all elements with specified class. There's no need to iterate over it and push every single element into a new array.

If you want to change an array-like object into an array, use Array#from. It's a build-in function, coming from ES6.

var arrayOfMultiImages = document.getElementsByClassName('multiimage');
var array = Array.from(arrayOfMultiImages);

Once you have changed your array-like object into an array, you can use e.g. Array#forEach function to simply and quickly iterate over it's elements.

array.forEach(elem => elem.className = "mySpace")

Simple example:

let elems = document.getElementsByClassName('box');
Array.from(elems).forEach(v => v.className = 'mySpace');

.box {
  background: blue;
  height: 100px;
  width: 100px;
}

.mySpace {
  background: green;
  height: 100px;
  width: 100px;
  margin: 5px;
}

<div class="box"></div>
<div class="box"></div>
<div class="box"></div>