The parameter of previewImage can be this, which refers to the <input> tag.
Operate files through this.files. For example, this.files.length is the number of files, and this.files[index] points to the corresponding file.
The parameter of previewImage can be event, which refers to the change event.

As for adding event handlers, you can
1. Write the js code directly in quotation marks, such as

<input type="file" accept="image/*" onchange="alert
(event.type);" multiple/>

2.onchange = function name, the function is declared in the script. If "previewImage" does not add (), it will not be executed.

<input type="file" accept="image/*" onchange="previewImage
(this)" multiple/>
<script> function previewImage(obj){
            alert(obj.files.length);
        };</script>

3.DOM level 0 event handler

<input type="file" accept="image/*" multiple/>
<script> document.getElementsByTagName("input")[0].onchange = function(){
            alert(event.type);
        };</script>

4.DOM level 2 event handler

<input type="file" accept="image/*"  multiple/>
<script> document.getElementsByTagName("input")[0].addEventListener("change",function(){
            alert(event.type);
        });</script>`
或者IE7 8中为`<script> document.getElementsByTagName("input")[0].attachEvent("onchange",function(){
            alert(event.type);
        });</script>

I may have written something wrong. The best way is to practice it yourself.
The book I read is "JavaScript Advanced Programming" third edition.

e表示event；
可以通过this获取相关filesData;
Recommended "JavaScript Advanced Programming"

Basic js Recommended Rhino Book--The Authoritative Guide to JavaScript (6th Edition)