@title
Ahead-of-Time Compilation
@intro
Learn how to use Ahead-of-time compilation
@description
This cookbook describes how to radically improve performance by compiling _Ahead of Time_ (AOT)
during a build process.
{@a toc}
## Table of Contents
* [Overview](#overview)
* [_Ahead-of-Time_ vs _Just-in-Time_](#aot-jit)
* [Compile with AOT](#compile)
* [Bootstrap](#bootstrap)
* [Tree Shaking](#tree-shaking)
* [Load the bundle](#load)
* [Serve the app](#serve)
* [Workflow and convenience script](#workflow)
* [Source Code](#source-code)
* [Tour of Heroes](#toh)
{@a overview}
## Overview
An Angular application consist largely of components and their HTML templates.
Before the browser can render the application,
the components and templates must be converted to executable JavaScript by the _Angular compiler_.
Watch compiler author Tobias Bosch explain the Angular Compiler at AngularConnect 2016.You can compile the app in the browser, at runtime, as the application loads, using the **_Just-in-Time_ (JIT) compiler**.
This is the standard development approach shown throughout the documentation.
It's great .. but it has shortcomings.
JIT compilation incurs a runtime performance penalty.
Views take longer to render because of the in-browser compilation step.
The application is bigger because it includes the Angular compiler
and a lot of library code that the application won't actually need.
Bigger apps take longer to transmit and are slower to load.
Compilation can uncover many component-template binding errors.
JIT compilation discovers them at runtime which is later than we'd like.
The **_Ahead-of-Time_ (AOT) compiler** can catch template errors early and improve performance
by compiling at build time as you'll learn in this chapter.
{@a aot-jit}
## _Ahead-of-time_ (AOT) vs _Just-in-time_ (JIT)
There is actually only one Angular compiler. The difference between AOT and JIT is a matter of timing and tooling.
With AOT, the compiler runs once at build time using one set of libraries;
With JIT it runs every time for every user at runtime using a different set of libraries.
### Why do AOT compilation?
*Faster rendering*
With AOT, the browser downloads a pre-compiled version of the application.
The browser loads executable code so it can render the application immediately, without waiting to compile the app first.
*Fewer asynchronous requests*
The compiler _inlines_ external html templates and css style sheets within the application JavaScript,
eliminating separate ajax requests for those source files.
*Smaller Angular framework download size*
There's no need to download the Angular compiler if the app is already compiled.
The compiler is roughly half of Angular itself, so omitting it dramatically reduces the application payload.
*Detect template errors earlier*
The AOT compiler detects and reports template binding errors during the build step
before users can see them.
*Better security*
AOT compiles HTML templates and components into JavaScript files long before they are served to the client.
With no templates to read and no risky client-side HTML or JavaScript evaluation,
there are fewer opportunities for injection attacks.
{@a compile}
## Compile with AOT
### Prepare for offline compilation
Take the Setup as a starting point.
A few minor changes to the lone `app.component` lead to these two class and html files:
{@example 'cb-aot-compiler/ts/src/app/app.component.html'}
{@example 'cb-aot-compiler/ts/src/app/app.component.ts'}
Install a few new npm dependencies with the following command:
npm install @angular/compiler-cli @angular/platform-server --save
You will run the `ngc` compiler provided in the `@angular/compiler-cli` npm package
instead of the TypeScript compiler (`tsc`).
`ngc` is a drop-in replacement for `tsc` and is configured much the same way.
`ngc` requires its own `tsconfig.json` with AOT-oriented settings.
Copy the original `src/tsconfig.json` to a file called `tsconfig-aot.json` (on the project root),
then modify it to look as follows.
{@example 'cb-aot-compiler/ts/tsconfig-aot.json'}
The `compilerOptions` section is unchanged except for one property.
**Set the `module` to `es2015`**.
This is important as explained later in the [Tree Shaking](#tree-shaking) section.
What's really new is the `ngc` section at the bottom called `angularCompilerOptions`.
Its `"genDir"` property tells the compiler
to store the compiled output files in a new `aot` folder.
The `"skipMetadataEmit" : true` property prevents the compiler from generating metadata files with the compiled application.
Metadata files are not necessary when targeting TypeScript files, so there is no reason to include them.
***Component-relative Template URLS***
The AOT compiler requires that `@Component` URLS for external templates and css files be _component-relative_.
That means that the value of `@Component.templateUrl` is a URL value _relative_ to the component class file.
For example, an `'app.component.html'` URL means that the template file is a sibling of its companion `app.component.ts` file.
While JIT app URLs are more flexible, stick with _component-relative_ URLs for compatibility with AOT compilation.
JIT-compiled applications that use the SystemJS loader and _component-relative_ URLs *must set the* `@Component.moduleId` *property to* `module.id`.
The `module` object is undefined when an AOT-compiled app runs.
The app fails with a null reference error unless you assign a global `module` value in the `index.html` like this:
{@example 'cb-aot-compiler/ts/src/index.html' region='moduleId'}
Setting a global `module` is a temporary expedient.
### Compiling the application
Initiate AOT compilation from the command line using the previously installed `ngc` compiler by executing:
node_modules/.bin/ngc -p tsconfig-aot.json
Windows users should surround the `ngc` command in double quotes:
"node_modules/.bin/ngc" -p tsconfig-aot.json
`ngc` expects the `-p` switch to point to a `tsconfig.json` file or a folder containing a `tsconfig.json` file.
After `ngc` completes, look for a collection of _NgFactory_ files in the `aot` folder (the folder specified as `genDir` in `tsconfig-aot.json`).
These factory files are essential to the compiled application.
Each component factory creates an instance of the component at runtime by combining the original class file
and a JavaScript representation of the component's template.
Note that the original component class is still referenced internally by the generated factory.
The curious can open the `aot/app.component.ngfactory.ts` to see the original Angular template syntax
in its intermediate, compiled-to-TypeScript form.
JIT compilation generates these same _NgFactories_ in memory where they are largely invisible.
AOT compilation reveals them as separate, physical files.
~~~ {.alert.is-important}
Do not edit the _NgFactories_! Re-compilation replaces these files and all edits will be lost.
~~~
{@a bootstrap}
## Bootstrap
The AOT path changes application bootstrapping.
Instead of bootstrapping `AppModule`, you bootstrap the application with the generated module factory, `AppModuleNgFactory`.
Make a copy of `main.ts` and name it `main-jit.ts`.
This is the JIT version; set it aside as you may need it [later](#run-jit "Running with JIT").
Open `main.ts` and convert it to AOT compilation.
Switch from the `platformBrowserDynamic.bootstrap` used in JIT compilation to
`platformBrowser().bootstrapModuleFactory` and pass in the AOT-generated `AppModuleNgFactory`.
Here is AOT bootstrap in `main.ts` next to the original JIT version:
{@example 'cb-aot-compiler/ts/src/main.ts'}
{@example 'cb-aot-compiler/ts/src/main-jit.ts'}
Be sure to recompile with `ngc`!
{@a tree-shaking}
## Tree Shaking
AOT compilation sets the stage for further optimization through a process called _Tree Shaking_.
A Tree Shaker walks the dependency graph, top to bottom, and _shakes out_ unused code like
dead needles in a Christmas tree.
Tree Shaking can greatly reduce the downloaded size of the application
by removing unused portions of both source and library code.
In fact, most of the reduction in small apps comes from removing unreferenced Angular features.
For example, this demo application doesn't use anything from the `@angular/forms` library.
There is no reason to download Forms-related Angular code and tree shaking ensures that you don't.
Tree Shaking and AOT compilation are separate steps.
Tree Shaking can only target JavaScript code.
AOT compilation converts more of the application to JavaScript,
which in turn makes more of the application "Tree Shakable".
### Rollup
This cookbook illustrates a Tree Shaking utility called _Rollup_.
Rollup statically analyzes the application by following the trail of `import` and `export` statements.
It produces a final code _bundle_ that excludes code that is exported, but never imported.
Rollup can only Tree Shake `ES2015` modules which have `import` and `export` statements.
Recall that `tsconfig-aot.json` is configured to produce `ES2015` modules.
It's not important that the code itself be written with `ES2015` syntax such as `class` and `const`.
What matters is that the code uses ES `import` and `export` statements rather than `require` statements.Install the Rollup dependencies with this command:
npm install rollup rollup-plugin-node-resolve rollup-plugin-commonjs rollup-plugin-uglify --save-dev
Next, create a configuration file (`rollup-config.js`)
in the project root directory to tell Rollup how to process the application.
The cookbook configuration file looks like this.
{@example 'cb-aot-compiler/ts/rollup-config.js'}
It tells Rollup that the app entry point is `src/app/main.js` .
The `dest` attribute tells Rollup to create a bundle called `build.js` in the `dist` folder.
It overrides the default `onwarn` method in order to skip annoying messages about the AOT compiler's use of the `this` keyword.
Then there are plugins.
### Rollup Plugins
Optional plugins filter and transform the Rollup inputs and output.
*RxJS*
Rollup expects application source code to use `ES2015` modules.
Not all external dependencies are published as `ES2015` modules.
In fact, most are not. Many of them are published as _CommonJS_ modules.
The _RxJs_ observable library is an essential Angular dependency published as an ES5 JavaScript _CommonJS_ module.
Luckily there is a Rollup plugin that modifies _RxJs_
to use the ES `import` and `export` statements that Rollup requires.
Rollup then preserves in the final bundle the parts of `RxJS` referenced by the application.
{@example 'cb-aot-compiler/ts/rollup-config.js' region='commonjs'}
*Minification*
Rollup Tree Shaking reduces code size considerably. Minification makes it smaller still.
This cookbook relies on the _uglify_ Rollup plugin to minify and mangle the code.
{@example 'cb-aot-compiler/ts/rollup-config.js' region='uglify'}
In a production setting, you would also enable gzip on the web server to compress
the code into an even smaller package going over the wire.
### Run Rollup
Execute the Rollup process with this command:
node_modules/.bin/rollup -c rollup-config.js
Windows users should surround the `rollup` command in double quotes:
"node_modules/.bin/rollup" -c rollup-config.js
{@a load}
## Load the Bundle
Loading the generated application bundle does not require a module loader like SystemJS.
Remove the scripts that concern SystemJS.
Instead, load the bundle file using a single `script` tag **_after_** the `` tag:
{@example 'cb-aot-compiler/ts/src/index.html' region='bundle'}
{@a serve}
## Serve the app
You'll need a web server to host the application.
Use the same _Lite Server_ employed elsewhere in the documentation: