Explanation of the difference between the location of Js placed in the HTML file-HTML Tutorial-php.cn

Explanation of the difference between the location of Js placed in the HTML file

高洛峰

Release： 2017-03-14 18:11:29

Original

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This question has always been a confusion for beginners. First understand where js can be placed in HTML, namely head and body. Most people put it in the head. When I was learning it, I confusedly followed it and put it in the head. I don’t know why? Today, let’s talk about the difference between these two places:
Let’s look at a piece of html code first:

<html> 
<head> 
<title> New Document </title> 
<meta http-equiv="content-type" content="text/html;charset=utf-8"> 

<script type="text/javascript" src="test.js"></script> 

</head> 

<body> 
<div id="target"> 

</div> 
<button id="btn">按钮</button> 

</body> 
</html>

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var test = function(){ 
var span = document.createElement("span"); 
span.innerHTML="添加"; 
document.getElementById("target").appendChild(span); 
} 

document.getElementById("btn").onclick=test;

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If this code is placed in the head, it will not run. Why?
This is about the running order of HTML. To be more precise, it is not the running order of HTML, but the running order of js. When HTML is run from above to , it enters the test.js file. The previous one will not run, that is, the one wrapped by function will not be run. At this time, the last sentence will be executed. Go to the page and get the element whose element ID is btn. But at this time, the HTML page has not been loaded. It is definitely not possible to get the element with id btn. An error will be reported. At this time, someone said that it can be changed to the following code:

document.body.onload = function(){ 
document.getElementById("btn").onclick=test; 
};

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But it is not as good as writing it this way, writing it in front of .
Have you noticed that there are no brackets in test above [document.getElementById("btn").onclick=test;], so what will happen if it is changed to [test()].

Explanation of the difference between the location of Js placed in the HTML file

The result is as shown in the figure. The page is loaded like this, and there is no response when clicking the button. Change the js code to the following:

var test=function(){ 
var span = document.createElement("span"); 
span.innerHTML="添加"; 
document.getElementById("target").appendChild(span); 
return function(){ 
alert("aaaa"); 
}; 
} 

document.getElementById("btn").onclick=test();

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When the page is loaded, it still looks the same as above. When the button is clicked, a box pops up with the content "aaaa"; indicating that it was executed when clicked. The value returned in function. That is, when adding parentheses, the function will be executed even if the event is not triggered. The return value of the function is executed when the event is triggered. When parentheses are not included, the function is executed only when the event is triggered.

html eventtrigger, what can be written in the content?
•For example, onclick="";what can be written inside the double quotes. Generally speaking, you can write functions, for example, onclick="test();". What else can be written besides this? Can we omit this semicolon?
•Look at the js code above, there is a semicolon in every line. The purpose of semicolons is to obfuscate statements. That means you can write js code in onclick. Write a try, as follows

<html> 
<head> 
<title> New Document </title> 
<meta http-equiv="content-type" content="text/html;charset=utf-8"> 


</head> 

<body> 
<div id="target"> 

</div> 
<button id="btn" onclick="var espan = document.createElement(&#39;span&#39;);espan.innerHTML=&#39;添加&#39;;document.getElementById(&#39;target&#39;).appendChild(espan);">按钮</button> 

</body> 
</html>

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•The running results are as follows:

Explanation of the difference between the location of Js placed in the HTML file

It means it can be run. This shows that you can put more than just function names.

Event binding method?
•There are two commonly used methods of event binding. One is adding js code to the event as introduced earlier. For example: onclick="test();". The disadvantage of this binding method is that you have to modify the code that the artist has already written.
•Another way is to use the ID as I started writing the code. The artist only needs to add the ID to each element. There is no need to modify the HTML code.

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