Detailed explanation of js loading and execution order in html
jsLoading execution sequence in html
1. Loading sequence: introduce the order of appearance of the tag <script></script>,
# on the page ##JavascriptThe code is part of the HTML document, so the order in which Javascript is executed when the page is loaded is the order in which the introduction tags <script></script> appear,
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题 3</title>
<script>
alert("1-最先执行");
</script>
</head>
<body onload="alert('3-最后执行');">
<script>
alert("2-接着执行");
</script>
</body>
</html>2. The global variables and functions defined by each script can be called by scripts executed later.
The call to a variable must have been declared previously, otherwise the variable value obtained will be undefined.<script type="text/javscrpt">//<![CDATA[ alert(tmp); //输出 undefined var tmp = 1; alert(tmp); //输出 1//]]> </script>
3. In the same script, the function definition can appear after the function call. However, if it is in two pieces of code and the function call is in the first piece of code, a function undefined error will be reported.
<script type="text/javscrpt">//<![CDATA[
aa(); //浏览器报错//]]>
</script>
<script type="text/javscrpt">//<![CDATA[
aa(); //输出 1
function aa()
{
alert(1);
}//]]>
</script>4.document
.write() will write the output to the location of the script document. After the browser parses the content of the document where documemt.write() is located, continue Parse the content output by document.write(), and then continue to parse the HTML document.
<script type="text/javascript">//<![CDATA[
document.write('<script type="text/javascript" src="test.js"><//script>');
document.write('<script type="text/javascript">');
document.write('alert(2);')
document.write('alert("我是" + tmpStr);');
document.write('<//script>'); //]]>
</script>
<script type="text/javascript">//<![CDATA[
alert(3); //]]>
</script>test.js的内容是： var tmpStr = 1; alert(tmpStr);
The order of the pop-up values in Firefox and Opera is: 1, 2, I am 1, 3
The order of the pop-up values in IE is: 2, 1, 3, at the same time The browser reported an error: tmpStr is undefined
The reason may be that when IE is document.write, it does not wait for the Javascript code in SRC to be loaded before executing the next line, so 2 pops up first,
and executes to the document .write('document.write("I am" + tmpStr)') When calling tmpStr, tmpStr is not defined and an error is reported.
To solve this problem, you can use HTML parsing to parse an
HTML tag
and then execute the next one by splitting the code: document.write('
In this way, the order of output values under IE and other browsers is consistent. : 1, 2, I am 1, 3
: Under IE, when using the Document.Write method to reference the js file, the js file will appear to have not been loaded. In the case of direct call, it is recommended to place the referenced JS file in a separate script block. To ensure that the referenced js file is fully loaded, continue to execute the subsequent Document.Write content5. JS function execution sequence with the same name
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<script type="text/javascript">
function aa() {
alert('First aa')
}
</script>
<title></title>
</head>
<body>
<form id="form1" runat="server">
<br />
<input id="Button1" type="button" value="button" onclick="aa();"/>
</form>
</body>
<script type="text/javascript">
function aa(s) {
alert('Second aa');
}
function aa(s) {
alert('Last aa');
}
</script>
</html>Click "botton" to execute the result: Last aa
After a function with the same name appears in js, after you call the js function in the web page, the last loaded function in the page will always be called.
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