Google Closure Engels Concept

 Closure Library

The Closure library is a JavaScript library, comparable to other modern products such as jQuery, Angular, Vue.js, Dojo and MooTools. The encoding style and usage of comments in the Closure library are tailor-made for Closure Compiler. Compared to other JavaScript libraries, it is Closure Compiler's main differentiator. A simple compression experiment found that when using Closure Compiler instead of YUI Compressor, the Closure Lib code can be reduced by 85%, which can have a huge impact on the compiler's code compression capacity.

   The implementation of the closure library also focuses on readability and performance. Be thrifty when creating objects, but be generous when naming and including objects. It also has a beautiful event system, class and inheritance support, and several UI components, including a rich text editor, for example. The Closure library code is regularly tested in all browsers and, as far as possible, may also be used in non-browser JavaScript environments.
Since the library is primarily a resource for Google engineers and secondly an open source project, it is certain that every line of code in the library is developed to support at least one Google product. First, introduce the style of the library.

Closure Templates

Closure templates provide intuitive syntax for creating efficient JavaScript functions (or objects in Java) that can generate HTML. This makes it easy to create larger HTML / XML strings, which can then be used to build the DOM. Unfortunately, most programming languages ​​do not have native support for templates, so creating a separate template solution is common practice for web frameworks (Python developers often use Django's template system, J2EE has JSP ASP MVC etc.). The unique thing about disabling templates is that the same template can be compiled in Java and JavaScript at the same time. Therefore, those servers on which servers are written in Java (or JavaScript!) Can use the same template on both the server and the client.
It can be used with other template systems such as lit-html, an efficient, extensible, and extensible HTML template library for JavaScript. The next generation HTML template in JavaScript. lit-html allows you to write HTML templates with JavaScript, and then efficiently render and re-render those templates and data to create and update the DOM. Create and update the DOM efficiently lit-html is not a framework, nor does it contain a component model. It focuses on one thing, and only one thing: making and updating the DOM effective. It can be used alone for simple tasks, or combined with frameworks or component models (such as web components) for a fully functional UI development platform. For example, we saw this combination on the YouTube website.

Closure Compiler

For existing JavaScript applications, the Closure compiler is probably the most direct and useful Closure tool. Although using it to compile code written in the style of the Closure library will be the most effective method, replacing the existing jQuery or Dojo dependencies with the library can be time consuming. In contrast, Closure Compiler can be used to replace existing JavaScript compressors (such as minifier or  JSMin YUI Compressor), which saves a lot of effort. There are even online services to be found.
The Closure compiler is a JavaScript optimized compiler: it takes JavaScript source code as input and generates source code with equivalent behavior as output. In other words, when the output code is used instead of the input code, the observable effect will be the same (although the execution speed of the output code may be faster than the original code). In addition, like JSLint, the compiler can detect a large number of errors by performing static checks at compile time. This helps to find errors earlier, which greatly speeds up the development of JavaScript. Using a compiler to identify problems is not a substitute for unit testing, but it can.

Closure Testing Framework

Closure Testing Framework is a unit testing framework that runs in the browser, very similar to Jasmine.js, QUnit or Mocha.js. Most Closure Library code has corresponding tests that run in the Framework. Good programming practice is to create tests for your own code and run them regularly to identify regressions. Since Closure Testing Framework runs inside the browser, other software is needed to automate the process of starting the browser, running tests and recording results. Selenium ( python WebDriver  ) may be the best solution for this purpose.

Closure CSS

Closure Stylesheets is a system that adds many Google extensions to the standard CSS language. Using these extensions, you can define and use variables, functions, conditions, and hybrid tables in the style sheet, thus making the style sheet more readable and maintainable. The included tools can compile style sheets to standard CSS, and support minification, hair loss, directionality (flip from right to left) and class renaming.

Some words about code

  It is important to understand the design goals and principles that will prompt the implementation of closed tools. Most of the design of the toolkit is driven by the functions of the compiler and the style of the library.

The main purpose of Closure Compiler is to reduce the size of JavaScript code. Since Google provides JavaScript for many pages and prides itself on speed (Google engineers wear T-shirts with the words "Fast is my favorite feature"), the JavaScript required to display the page must be as small as possible. Even if the browser caches JavaScript, when a page using JavaScript is reloaded, it must be parsed and executed again. The smaller the JavaScript, the less time it takes. The only thing that can make your users (or drive them away) is the speed and usability of the application.

Take for exapmle APM for informational pages, the difference is very big. APM pages are used 50% more than regular HTML with javascript. Because they load comfortably on mobile divices.

The way to write JavaScript code should be efficiently compiled by the compiler. This is the basis for understanding the design of closure libraries: the verbosity of the code cannot represent the size of the compiler. If you need to write more code (or comments) to produce smaller compiled code, this is better than writing less code to produce larger compiled code.
For example, it is acceptable to write a comprehensive utility library, as long as the compiler can delete the unused parts. Complementary methods should be replaced with a single parameterized method (for example, setStatus(status) is preferred to statusOn() and statusOff()). This reduces the number of method declarations and makes it easier to execute functions. Therefore, to fully understand the Closure library, you must also understand how Compiler rewrites JavaScript code.      

One might wonder whether to focus on using a compiler to produce JavaScript with better  performance. The short answer is yes, but since runtime performance is much harder than code size, it took more engineering time to improve shrinking performance. Fortunately, many reductions in code size also improve performance, because many optimizations result from evaluating expressions at compile time rather than at runtime.
The compiler is designed to immediately compile all code that can be run in the application process. There are many potential input sources, but the compiler will receive all of them at the same time. This is in contrast to other languages, where part of the source code is compiled into reusable modules. In Closure, the situation is the opposite: the source code is initially compiled together and then broken down into modules that can be gradually loaded by the web application. This is done to ensure that the variable names used in each module are globally unique.

Memory Matters caused the Gmail engineers a considerable amount of pain, it did force them to invest extra effort into managing memory on the client.

Like most modern programming languages, JavaScript manages its own memory. Fortunately, this cannot rule out the possibility of memory leaks, because the inability to release references to objects that are no longer needed can still cause the application to run out of memory. The closure library uses goog.Disposable to ensure that the reference is released as soon as possible so that the object can be garbage collected.
Closure Compiler is not the first tool to try to identify problems in JavaScript code by performing static checks. However, there are limits to how much you can infer from the source code alone. In order to supplement the information in the code itself, the compiler uses the comments provided by the developer, which appear in the form of JavaScript comments.
By commenting the code to indicate the function's parameters and return types, the compiler can determine when to pass incorrect types of parameters to the function. Similarly, annotate the code to indicate which data should be private, so that the compiler can recognize when the data is accessed illegally. By using these annotations in the code, you can use the compiler to increase confidence in the correctness of the code. 

Although the compiler provides many useful conversions for its input, the code of the Closure library is also expected to run without being processed by the compiler. This not only ensures that the input language is pure JavaScript, but also makes debugging easier because deobfuscated code is always available.
 However, the library code used with all commonly available objects (strings, arrays, functions, etc.) in the JavaScript environment does not rely on APIs that are only available to the browser. This also makes the Closure library very suitable for use with server-side JavaScript.
Built-in object prototypes, such as objects, functions, arrays and strings, should not be modified. This allows Closure to be used with other JavaScript libraries such as jQuery. However, in practice, using Closure with other libraries is usually not efficient. Each library has its own logic for event management, string processing, etc. This means that it may contain duplicate logic, which increases the amount of JavaScript code to be loaded.
Each tool in the Closure kit can be used independently. This is mainly because the decision to use a particular "closed" tool is made by a single engineering team at Google, so there is no guarantee that the team using the compiler will also use the library. Now that Closure is more mature, the main reason for adopting one tool instead of another is because the dependence on the original code already relies on similar tools. When deciding how to best incorporate Closure into an existing project, you may encounter a similar situation.
Currently, each tool in the Closure suite must be downloaded and installed separately. Since these tools are independent of each other, each tool is maintained as its own project on . Most projects include a "Featured Download" section, in which the tool and its documentation can be downloaded as some kind of zip file. Unfortunately, the Closure repository does not provide such bundled software, so the only way to get the code is to check it out from the Subversion repository associated with the project.

Using with Node.js

Install the official package from npm.

npm install google-closure-library

Or you can checkout current version from git or svn

Default Hello example:
vim hello.html
    <script src="closure-library/closure/goog/base.js"></script>
    <script src="hello.js"></script>
  <body onload="sayHi()">
vim hello.js

function sayHi() {
  var newHeader = goog.dom.createDom(goog.dom.TagName.H1, {'style': 'background-color:#EEE'},
    'Hello world!');
  goog.dom.appendChild(document.body, newHeader);
Additional JavaScript files loaded by Closure. 
Getting started with Closure Library explains how to start using Closure Library in your project. This section outlines how to use ClosureBuilder and Closure Compiler in a project to reduce the file to a minimal JavaScript file 

You may have noticed that the browser made multiple HTTP requests during the page load. For each Closure Library file required by goog.require(), a new script tag is created to load the file. Each Closure Library file is loaded as a separate HTTP request.
This is very effective for development, because you can see uncompiled source code and use existing debugging tools. But this is not conducive to providing services to users. Loading all these files separately is unnecessary and slow. For real-time applications that serve real users, you will need to provide a "compiled" version of the application. Closure Compiler will take your code and required Closure Library files, strip the comments, make some optimizations, and create a larger file as output. In this way, your server can provide only one file instead of multiple individual files. 

The tool located in closures/bin/build is a tool used to help build compiled JavaScript files. It scans your files and Closure Library files, and can calculate the order of script files in order of relevance. It can output the file name of the script or its contents in dependency order, or pass the file to Closure Compiler to generate a compiled version.
closure-library/closure/bin/build/ \
  --root=closure-library/ \
  --root=projectmap/ \
Each --root flag tells ClosureBuilder to scan the .js files in that directory. The goog.provide and goog.require statements will be scanned in each file, which indicates that the file provides a namespace or requires a namespace provided in another file. Scanning all files allows ClosureBuilder to build memory dependency trees for all namespaces. The --namespace flag tells ClosureBuilder to start from the myproject.start namespace in the tree and get all its dependencies. By default, it outputs the results as a file list in order. For each file in the list, each required namespace is provided by another file that appears earlier in the list. 
closure-library/closure/bin/build/ \
  --root=closure-library/ \
  --root=projectmap/ \
  --namespace="myproject.start" \
  --output_mode=compiled \
  --compiler_jar=compiler.jar \
  > myproject/start-compiled.js
ClosureBuilder writes the output of the compiler to standard output. Here, we transfer it to a file named start-compiled.js. Alternatively, you can use --output_file to specify the file to be written

Put start-compiled.js next to myproject.html, then modify the HTML file to use the compiled version:

<!doctype html>
<script src="start-compiled.js"></script>

Using the closure compiler and the compilation_level of ADVANCED_OPTIMIZATIONS together with the compilation level with SIMPLE_OPTIMIZATIONS or WHITESPACE_ONLY can provide better compression. Compiling with ADVANCED_OPTIMIZATIONS can be more aggressive by converting code and renaming symbols to achieve additional compression. However, this more aggressive method means that you must be extra careful when using ADVANCED_OPTIMIZATIONS to ensure that the output code works the same way as the input code.


closure-library/closure/bin/build/ \
  --root=closure-library/ \
  --root=myproject/ \
  --namespace="myproject.start" \
  --output_mode=compiled \
  --compiler_jar=compiler.jar \
  --compiler_flags="--js=closure-library/closure/goog/deps.js" \
  --compiler_flags="--compilation_level=ADVANCED_OPTIMIZATIONS" \
  > start-compiled.js


To run your code you need a web server


If you run your code via file system you will see errors related to cross origin support.

You can of course use something like
python -m SimpleHTTPServer
But I would recommend a Firebase project.

Firebase is Google's mobile platform that helps you quickly develop...
Firebase projects are backed by Google Cloud Platform, letting you scale your app and you can use them tools to publish a web application to a modern hosting.
Please see minimal installation on Firebase project website.
That is an example application from official Google get started.