Document package.json in build article

Include information about npm module dependencies and build hooks.

* Module dependencies are automatically installed during build.
* Build hooks help transpile app code during build preventing the need
  to commit transpiled code.

Include information about common good practices like bundling the app
code with rollup.js and using the transpiler's watch mode to transpile
on file change.

Change-Id: Ib4a2159f2a0c04c6e8cd36816e0da0bcdd5ccae8

doc/build.md

@@ -31,6 +31,119 @@ To prepare your project for the build, you have to create a subdirectory named `
 |- .tabrisignore
 ```
 
+### The package.json file
+
+See [package.json | npm documentation](https://docs.npmjs.com/files/package.json) for information about the package.json format.
+
+`package.json` contains, among other configurations, references to the app's main script and npm module dependencies:
+
+```json
+{
+  // ...
+  "main": "src/app.js",
+  "dependencies": {
+    "tabris": "^2.0.0",
+    "left-pad": "^1.1.3",
+    // ...
+  }
+  // ...
+}
+```
+
+Dependencies are automatically installed during the build process.
+
+#### Build scripts
+
+When a Tabris.js app is built, `build` scripts given in the `package.json` are executed before the JavaScript code is bundled into the app. They can be used to transpile the JavaScript app code.
+
+```json
+{
+  ...
+  "scripts": {
+    ...
+    "build": "..."
+  }
+  ...
+}
+```
+
+Supported build script hooks are:
+
+  - `"build"`: executed for all platform builds
+  - `"build:android"`: executed for Android builds
+  - `"build:ios"`: executed for iOS builds
+  - `"build:windows"`: executed for Windows builds
+
+Make sure the `"build"` script is executed before executing `tabris serve` (when running the app locally). This step can be automated by using the watch mode of your transpiler, which will compile the file on change (refer to your transpiler's documentation). A module like [npm-run-all](https://www.npmjs.com/package/npm-run-all) can help run the transpiler in watch mode and `tabris serve` at the same time.
+
+For bigger projects you may want to use a module bundler like [rollup.js](https://rollupjs.org). It removes the overhead of loading multiple modules and uses static analysis to remove unused code from the bundled artifact.
+
+#### Example: Transpiling ES6 code
+
+Install the Babel transpiler and the necessary plug-ins. The `--save-dev` option will add the dependencies to your `package.json`:
+
+```
+npm install --save-dev babel-cli babel-plugin-transform-es2015-modules-commonjs
+```
+
+Create a `.babelrc` file in the root of your project:
+
+```json
+{
+  "plugins": ["transform-es2015-modules-commonjs"]
+}
+```
+
+Include the following build script in the `scripts` sections of your `package.json`:
+
+```json
+{
+  "scripts": {
+    "build": "babel --compact false --out-dir dist/ src/"
+  }
+  ...
+}
+```
+
+Let the `main` field point to the *transpiled* `app.js` in `dist/`:
+
+```json
+{
+  "main": "dist/app.js",
+  ...
+}
+```
+
+In case iOS 8 and 9 support is desired, more Babel plugins can be added to compensate for missing ES6 features. See [ECMAScript 6 compatibility table](http://kangax.github.io/compat-table/es6/) for more information about supported ES6 features in iOS 8 and 9.
+
+#### Example: Transpiling TypeScript code
+
+Install the TypeScript compiler:
+
+```
+npm install --save-dev typescript
+```
+
+Include the following build script in the `scripts` sections of your `package.json`:
+
+```json
+{
+  "scripts": {
+    "build": "tsc -p ."
+  }
+  ...
+}
+```
+
+Let the `main` field point to the *transpiled* `app.js` in `dist/`:
+
+```json
+{
+  "main": "dist/app.js",
+  ...
+}
+```
+
 ### The config.xml file
 
 The minimal build configuration you need is a `cordova/config.xml` file that describes your app. It contains information like the id of your app, its version, icons and splash screens. The format of the `config.xml` is the same as a standard [Cordova config.xml](https://cordova.apache.org/docs/en/4.0.0/config_ref_index.md.html#The%20config.xml%20File) file. A minimal example config could look like this:
@@ -68,7 +181,7 @@ You can integrate all available [Cordova Plugins](http://plugins.cordova.io/#/)
 The optional `<content>` element defines the app's starting page in ordinary Cordova apps. In Tabris.js you can use it to define an alternative location of the `package.json` file. Example:
 
 ```xml
-<content src="mySubFolder/package.json"/>
+<content src="mySubFolder/package.json" />
 ```
 
 #### Preferences
@@ -186,7 +299,5 @@ Extract the content of the downloaded archive and create an environment variable
 tabris build [android|ios|windows]
 ```
 
-This command will first run the npm scripts `build` or `build:<platform>`, if they exist in your `package.json`.
-
 To build a Windows app signed with your own key, run
 `cordova build windows --release -- --packageCertificateKeyFile="<absolute-path-to-pfx>" --bundle`