The method of dynamically loading JS to improve the opening speed of Web page _javascript skills

In general, if you load all the required JavaScript code at once, it will cause the initial page to open slowly, but many of the loaded code does not need to use, this wasteful performance waste should be avoided. If you want to dynamically load JavaScript code, you can use the DOM model to add <script> nodes to the HTML document and set the SRC attribute of the node (that is, the external JavaScript file) to the JavaScript code that needs to be loaded dynamically.

Here is an example of completing this feature:

(1), new jsloadertest.html file

JavaScript document 
function SayHello () { 
  alert ("Hello world! successfully loaded JavaScript file");   
} 
JavaScript document
function SayHello () {
  alert ("Hello world! successfully loaded JavaScript file");  
}