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Analysis on the placement position of script in html

小云云

Dec 11, 2017 am 11:37 AM

htmlscriptplace

I used to think that script can be placed anywhere in HTML. I corrected my misconception when I made a request today - the location of script is not random. This article mainly talks about the placement of script in HTML. The editor thinks it’s pretty good, so I’ll share it with you now and give it as a reference. Hope it helps everyone.

First of all, I want to implement a select tag with two options: yes and no. However, during initialization, the select tag is required to select a null value by default, so I added a method to let it change the null value when clicking. Delete:

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nbsp;html>


<script></script>

<script>
$(&#39;#checkcash&#39;).click(function () {   
        if ($(&#39;#checkcash&#39;).val() == &#39;0&#39;) {   
            $("#checkcash option[value=&#39;0&#39;]").remove();   
        }   
    });   
    $("#alert").click(function(){   
    alert("1123");   
    })   
</script>

 是否已提现 <select>
                            <option></option>
                            <option>是</option>
                            <option>否</option>
                            </select>
                            <input>

But this cannot achieve the desired effect. At first I thought it was a jquery syntax error and I kept checking and correcting it online, but it didn’t work. Then I suddenly thought about whether I should I put the script in the back and tried it, and it turned out to be ok. Then I suddenly realized that it was not like this.

Later I checked the reason because the html file is executed in a top-down manner, but the order of the imported css and javascript is different. When the css is introduced and loaded, the program is still executed downwards, and When the <script> script is executed, the thread is interrupted, and the program continues execution only after the execution of the script script is completed. Therefore, the script is generally placed after the body to avoid delayed blocking due to long execution of the script. To achieve the effects of some pages, some js scripts need to be dynamically loaded in advance, so these scripts should be placed before <body>. Secondly, the js that needs to access the dom element cannot be placed before the body, because the dom has not been generated at this time, so the js that accesses the dom element before the body will be wrong or invalid. It is because of this that I added methods to the dom before it was generated, which led to this. </script>

I really should study more. I think it’s so, but I don’t delve into it. Keep working hard in the future!

ps: In fact, there is another way, which is to use jquery's initialization page method. It is also possible to add the click event added to the label above to $(function(){}). The principle is the same It’s the same as above. This method is executed after the page is loaded, so it can be used no matter where it is placed!

Related recommendations:

Detailed explanation of immediate execution function examples in JavaScript

A brief introduction to JavaScript execution sequence

Detailed introduction to JavaScript execution order_Basic knowledge

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