Add a code coverage guide.

Author: ben-eb

docs/recipes/code-coverage.md

@@ -0,0 +1,109 @@
+# Code coverage
+
+As AVA [spawns the test files][isolated-env], you can't use [`istanbul`] for code coverage; instead, you can achieve this with [`nyc`] which is basically [`istanbul`] with sub-process support. So, firstly we'll need to install it:
+
+```
+npm install nyc --save-dev
+```
+
+For both ES2015 and ES5 environments, don't forget to add `.nyc_output` & `coverage` to your `.gitignore`.
+
+
+## ES5 coverage
+
+To cover ES5, simply prepend your test script with `nyc`. This npm script will then handle our code coverage and testing:
+
+```json
+{
+	"scripts": {
+		"test": "nyc ava"
+	}
+}
+```
+
+
+## ES2015 coverage
+
+First, we'll need a babel configuration. This will vary from developer to developer but you can use this `package.json` configuration for babel as a starting point:
+
+```json
+{
+	"babel": {
+		"presets": ["es2015"],
+		"plugins": ["transform-runtime"],
+		"ignore": "test.js",
+		"env": {
+			"development": {
+				"sourceMaps": "inline"
+			}
+		}
+	}
+}
+```
+
+Note that in development mode, we need to specify a sourcemap when we transpile our code, and in production this is unnecessary. So for your production script, use an environment other than development; for example:
+
+```json
+{
+	"scripts": {
+		"build": "BABEL_ENV=production babel --out-dir=dist index.js"
+	}
+}
+```
+
+To cover ES6, simply prepend your test script with `nyc` and the `--babel` flag. This npm script will then handle our code coverage and testing:
+
+```json
+{
+	"scripts": {
+		"test": "nyc --babel --reporter=text ava"
+	}
+}
+```
+
+
+## HTML reports
+
+To see a HTML report for either the ES6 or ES5 coverage strategies we have outlined, do:
+
+```
+nyc report --reporter=html
+```
+
+Or, convert it into an npm script for less typing:
+
+```json
+{
+	"scripts": {
+		"report": "nyc report --reporter=html"
+	}
+}
+```
+
+This will output a HTML file to the `coverage` directory.
+
+
+## Hosted coverage
+
+### Travis CI & Coveralls
+
+Firstly, you will need to activate your repository in the coveralls user interface. Once that is done, add [`coveralls`] as a development dependency:
+
+```
+npm install coveralls --save-dev
+```
+
+Then add the following to your `.travis.yml`:
+
+```
+after_success:
+	- './node_modules/.bin/nyc report --reporter=text-lcov | ./node_modules/.bin/coveralls'
+```
+
+Your coverage report will then appear on coveralls shortly after the CI service completes.
+
+[`babel`]:      https://github.com/babel/babel
+[`coveralls`]:  https://github.com/nickmerwin/node-coveralls
+[isolated-env]: https://github.com/sindresorhus/ava#isolated-environment
+[`istanbul`]:   https://github.com/gotwarlost/istanbul
+[`nyc`]:        https://github.com/bcoe/nyc

readme.md

@@ -715,6 +715,7 @@ Concurrency is not parallelism. It enables parallelism. [Learn more.](http://sta
 
 ## Recipes
 
+- [Code coverage](docs/recipes/code-coverage.md)
 - [Endpoint testing](docs/recipes/endpoint-testing.md)