js引入另外一个js

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The old versions of JavaScript had no import, include, or require, so many different approaches to this problem have been developed.

But recent versions of JavaScript have standards like ES6 modules to import modules, although this is not supported yet by most browsers. Many people using modules with browser applications use build and/or transpilation tools to make it practical to use new syntax with features like modules.

ES6 Modules

Note that currently, browser support for ES6 Modules is not particularly great, but it is on it's way. According to this StackOverflow answer, they are supported (but behind flags) in Chrome 60, Firefox 54 and MS Edge 15, with only Safari 10.1 providing support without flags.

Thus, you will currently still need to use build and/or transpilation tools to valid JavaScript that will run in without any requirement for the user to use those browser versions or enable any flags.

Once ES6 Modules are commonplace, here is how you would go about using them:

// module.jsexport function hello() {  return "Hello";}// main.jsimport {hello} from 'module'; // or './module'let val = hello(); // val is "Hello";

Node.js require

Node.js is currently using a module.exports/require system. You can use babel to transpile if you want the import syntax.

// mymodule.jsmodule.exports = {   hello: function() {      return "Hello";   }}// server.jsconst myModule = require('./mymodule');let val = myModule.hello(); // val is "Hello"

There are other ways for JavaScript to include external JavaScript contents in browsers that do not require preprocessing.

AJAX Loading

You could load an additional script with an AJAX call and then use eval to run it. This is the most straightforward way, but it is limited to your domain because of the JavaScript sandbox security model. Using eval also opens the door to bugs, hacks and security issues.

jQuery Loading

The jQuery library provides loading functionality in one line:

$.getScript("my_lovely_script.js", function() {   alert("Script loaded but not necessarily executed.");});

Dynamic Script Loading

You could add a script tag with the script URL into the HTML. To avoid the overhead of jQuery, this is an ideal solution.

The script can even reside on a different server. Furthermore, the browser evaluates the code. The <script> tag can be injected into either the web page <head>, or inserted just before the closing </body> tag.

Here is an example of how this could work:

function dynamicallyLoadScript(url) {    var script = document.createElement("script"); // Make a script DOM node    script.src = url; // Set it's src to the provided URL    document.head.appendChild(script); // Add it to the end of the head section of the page (could change 'head' to 'body' to add it to the end of the body section instead)}

This function will add a new <script> tag to end of the head section of the page, where the srcattribute is set to the URL which is given to the function as the first parameter.

Both of these solutions are discussed and illustrated in JavaScript Madness: Dynamic Script Loading.

Detecting when the script has been executed

Now, there is a big issue you must know about. Doing that implies that you remotely load the code. Modern web browsers will load the file and keep executing your current script because they load everything asynchronously to improve performance. (This applies to both the jQuery method and the manual dynamic script loading method.)

It means that if you use these tricks directly, you won't be able to use your newly loaded code the next line after you asked it to be loaded, because it will be still loading.

For example: my_lovely_script.js contains MySuperObject:

var js = document.createElement("script");js.type = "text/javascript";js.src = jsFilePath;document.body.appendChild(js);var s = new MySuperObject();Error : MySuperObject is undefined

Then you reload the page hitting F5. And it works! Confusing...

So what to do about it ?

Well, you can use the hack the author suggests in the link I gave you. In summary, for people in a hurry, he uses an event to run a callback function when the script is loaded. So you can put all the code using the remote library in the callback function. For example:

function loadScript(url, callback){    // Adding the script tag to the head as suggested before    var head = document.getElementsByTagName('head')[0];    var script = document.createElement('script');    script.type = 'text/javascript';    script.src = url;    // Then bind the event to the callback function.    // There are several events for cross browser compatibility.    script.onreadystatechange = callback;    script.onload = callback;    // Fire the loading    head.appendChild(script);}

Then you write the code you want to use AFTER the script is loaded in a lambda function:

var myPrettyCode = function() {   // Here, do whatever you want};

Then you run all that:

loadScript("my_lovely_script.js", myPrettyCode);

Note that the script may execute after the DOM has loaded, or before, depending on the browser and whether you included the line script.async = false;. There's a great article on Javascript loading in general which discusses this.

Source Code Merge/Preprocessing

As mentioned at the top of this answer, many developers now use build/transpilation tool(s) like WebPack, Babel, or Gulp in their projects, allowing them to use new syntax and support modules better, combine files, minify, etc.

If anyone is looking for something more advanced, try out RequireJS. You'll get added benefits such as dependency management, better concurrency, and avoid duplication (that is, retrieving a script more than once).

You can write your JavaScript files in "modules" and then reference them as dependencies in other scripts. Or you can use RequireJS as a simple "go get this script" solution.

Example:

Define dependencies as modules:

some-dependency.js

define(['lib/dependency1', 'lib/dependency2'], function (d1, d2) {     //Your actual script goes here.        //The dependent scripts will be fetched if necessary.     return libraryObject;  //For example, jQuery object});

implementation.js is your "main" JavaScript file that depends on some-dependency.js

require(['some-dependency'], function(dependency) {    //Your script goes here    //some-dependency.js is fetched.       //Then your script is executed});

Excerpt from the GitHub README:

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