JS load execution order in HTML

1. Loading order: Introduce the order of the <script/> of the tag, load it sequentially

The JavaScript code on the page is part of the HTML document, so the order in which JavaScript executes when the page is loaded is the order in which the markup <script/> is introduced.
The external JS, either inside the <script/> tag or introduced by SRC, is executed in the order in which the statements appear, and the execution process is part of the document loading.

2. The global variables and functions defined by each script can be called by the script executed later.

The call to a variable must be previously declared, otherwise the value of the variable obtained is undefined.

<script type= "Text/javscrpt" >//<![ cdata[    alert (TMP);       // output undefined var     TMP = 1;    Alert (TMP);   // output 1//]] >  </script>  

3. The same script, the function definition can appear after the function call, but if it is in two pieces of code, and the function call in the first code, the function does not define the error.

<script type= "Text/javscrpt" >//<![ cdata[    aa ();            // Browser error//]] >  </script>  <script type= "text/javscrpt" >//<![ cdata[    aa ();   // Output 1     function AA ()    {      alert (1);    } // ]]>  </script>  

4, the same name JS function execution order

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd "> functionAA () {alert (' First AA ')      }     </script> <title></title> functionAA (s) {alert (' Second AA '); }      functionAA (s) { executed alert (' Last AA '); }  </script> 
executes the last function with the same name , so the result is click "Botton" to execute the result: End AA
5.document.write () writes the output to the location of the script document, and after the browser resolves the contents of the document Documemt.write (), it continues to parse the contents of the document.write () output, after parsing is complete
Then continue parsing 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 ' ("I am" + tmpstr); );        document.write (' </' + ' script> ');     </script>     <script type= "Text/javascript" >  Alert (3);   </script>
Test.js File Contents
var tmpstr=2;
alert (TMPSTR);
Order of Execution 2 2 I am 2 3
6. js in the introduction of JS file must be aware of the <script></script> separation import, otherwise it will truncate the main file JS, mainly behind the </script> end tag
document.write ("<SCR"+"IPT src= ' my.js ' ></scr"+"ipt> "
Or
document.write ('<script type= ' Text/javascript "src=" test.js "></'+'script> ');
JS load execution order in HTML