REserve — Testing UI5 — Measuring code coverage

In this last article, we explain how nyc is used to instrument the sources and the runner is modified to handle code coverage. The web server switches between instrumented sources and the original ones (in case one does not want to measure the coverage of specific files). Because of the way OPA tests are designed (and the use of IFrames), the instrumented files are altered on the fly to update their scope. Once every individual coverage information is extracted, nyc is called again to merge the coverage and generate a report.


Before jumping to the implementation details, I would like to spend some time on fundamentals.

Why do we care about measuring the code coverage ?

We create tests to ensure that the code behaves as expected. We measure the code coverage to check which lines of code are being evaluated while executing the tests.

This helps to understand if :

  • the tests are relevant : if the code is not tested, it does not work.

If the code coverage shows that only 10% of your code is evaluated during your tests it means that 90% of the code is either not tested or useless.

But keep in mind that having 100% code coverage does not mean that your code is fully tested !

For instance :

function div (a, b) { return a / b; }

Function under test

The following assertion will give you 100% code coverage :

assert.strictEqual(div(4,2), 2);

Test code

But, in reality, it says almost nothing on the tested code.

What happens in the following cases ?

  • div(1,0)

So you must implement additional test cases to document and secure the beahviors.

A good guideline is to start with acceptance criterias : test whatever is expected from a behavior point of view.

That is the reason why I use to say that the coverage must reach “at least 100%” !

How does the code coverage work ?

There is no magic behind code coverage : it usually consists in three steps :

  1. Instrumentation of the code

The first step rewrites the code in such a way that :

  • The code behavior does not change (and this is obviously critical)

For instance, the previous example :

function div (a, b) { return a / b; }

Source file

Turns into (when instrumenting with nyc):

function cov_1scmxvb45(){var path="div.js";var hash="021d227dc28d530884d8c843c2806b96b01d5347";var global=new Function("return this")();var gcv="__coverage__";var coverageData={path:"div.js",statementMap:{"0":{start:{line:1,column:22},end:{line:1,column:35}}},fnMap:{"0":{name:"div",decl:{start:{line:1,column:9},end:{line:1,column:12}},loc:{start:{line:1,column:20},end:{line:1,column:37}},line:1}},branchMap:{},s:{"0":0},f:{"0":0},b:{},_coverageSchema:"1a1c01bbd47fc00a2c39e90264f33305004495a9",hash:"021d227dc28d530884d8c843c2806b96b01d5347"};var coverage=global[gcv]||(global[gcv]={});if(!coverage[path]||coverage[path].hash!==hash){coverage[path]=coverageData;}var actualCoverage=coverage[path];{// @ts-ignore
cov_1scmxvb45=function(){return actualCoverage;};}return actualCoverage;}cov_1scmxvb45();function div(a,b){cov_1scmxvb45().f[0]++;cov_1scmxvb45().s[0]++;return a/b;}
//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImRpdi5qcyJdLCJuYW1lcyI6WyJkaXYiLCJhIiwiYiJdLCJtYXBwaW5ncyI6Inl6QkFlWTt5RkFmWixRQUFTQSxDQUFBQSxHQUFULENBQWNDLENBQWQsQ0FBaUJDLENBQWpCLENBQW9CLCtDQUFFLE1BQU9ELENBQUFBLENBQUMsQ0FBR0MsQ0FBWCxDQUFlIiwic291cmNlc0NvbnRlbnQiOlsiZnVuY3Rpb24gZGl2IChhLCBiKSB7IHJldHVybiBhIC8gYjsgfSJdfQ==

Instrumented source file

The last part includes source mapping to enable debugging inside the browser.

For the second step, the tests are executed after replacing the original source files with the instrumented ones.

Finally, regarding the last step, code coverage tools consolidate the information in a global javascript variable which content can be extracted.

By default, NYC aggregates the coverage data inside window.__coverage__.


nyc is a command line wrapper for Istanbul, a JS code coverage tool.

The runner offers a function wrapping the execution of the command line and waiting for its termination. nyc being declared as a dependency, it is available inside the node_modules folder of the runner.

const { join } = require('path')
const { fork } = require('child_process')
function nyc (...args) {
const childProcess = fork(join(__dirname, '../node_modules/nyc/bin/nyc.js'), args, {
stdio: 'inherit'
let done
const promise = new Promise(resolve => { done = resolve })
childProcess.on('close', done)
return promise

nyc wrapper

The code could be improved to check for errors.

Instrumenting sources

The command nyc instrument is used.

New parameters are added to the job :

  • covSettings : file name of a coverage settings file (default explained below)

To facilitate the option selection, the runner supports the specification of a configuration file.

A default one is provided :

"all": true,
"sourceMap": false

nyc default settings

By default, the test files are excluded from the coverage report. But since the test code must be as clean as the production code, one may want to also measure the coverage of the test files. Simply create a custom nyc.json which 'cancel' the test folder exclusion and assign its path in covSettings :

"all": true,
"exclude": [
"sourceMap": false

nyc settings to include test files

The instrumentation step is triggered before executing the tests :

/* ... */
.on('ready', ({ url, port }) => {
job.port = port
if (!job.logServer) {
console.log(`Server running at ${url}`)
await instrument()

Instrumenting the code before executing the tests inside the runner

Replacing coverage context

By default, the object used to aggregate coverage information is stored at the window level.

To be more precise, nyc uses the Function constructor pattern to retreive the global context of the current host : var global=new Function("return this")(). In a browser, it corresponds to the current window object.

Unfortunately, when running OPA tests using the IFrame mode, the code being executed inside the IFrame will aggregate the coverage information in the IFrame window. Meaning that whenever the tested application is shut down at the end of the journey, this valuable information is lost.

We must change the place where the coverage information is stored.

This is common issue and several solutions were discussed. This mechanism can be configured when Istanbul is integrated in solutions like karma (see the following change). However, I was not able to leverage these options through the nyc command line.

Another approach is to rely on REserve which offers the possibility to implement a custom file system in the file handler. This gives the opportunity to manipulate the file content before it is sent to the client.

const { promisify } = require('util')
const { readdir, readFile, stat } = require('fs')
const readdirAsync = promisify(readdir)
const readFileAsync = promisify(readFile)
const statAsync = promisify(stat)
const { Readable } = require('stream')
const globalContextSearch = 'var global=new Function("return this")();'
const globalContextReplace = 'var;'
const customFileSystem = {
stat: path => statAsync(path)
.then(stats => {
if (stats) {
stats.size -= globalContextSearch.length + globalContextReplace.length
return stats
readdir: readdirAsync,
createReadStream: async (path) => {
const buffer = (await readFileAsync(path))
.replace(globalContextSearch, globalContextReplace)
return Readable.from(buffer)

Custom file system that searches and replaces the global context definition with a custom one

Replacing sources with instrumented files

Now that the instrumented files are generated and the coverage information is stored at the right place, a new mapping is created to substitute the source files with the instrumented ones. To work properly, it must be inserted before the source mapping.

NOTE : 'ignore-if-not-found' is defined to tell the handler to not fail the request if the file is not found (allowing the subsequent mapping(s) to process the request).

match: /^\/(.*\.js)$/,
file: join(instrumentedSourceDir, '$1'),
'ignore-if-not-found': true,
'custom-file-system': customFileSystem

Coverage mapping

Extracting the code coverage

When the test page ends, the QUnit.done callback is triggered. This is the perfect time to collect the generated coverage information.

/* Injected QUnit hooks */
(function () {
'use strict'
function post (url, data) {
const xhr = new XMLHttpRequest()'POST', '/_/' + url)
/* ... */ QUnit.done(function (report) {
if (window.__coverage__) {
report.__coverage__ = window.__coverage__
post('QUnit/done', report)

Modified QUnit hook to also collect coverage information

This requires a modification in the endpoint to save the information in the coverage temporary directory.

The runner must wait for the test report to be saved before stopping the browser. Also, it must wait for the coverage report to be saved. Hence, two synchronization points are needed.

// Endpoint to receive QUnit.done
match: '/_/QUnit/done',
custom: endpoint((url, report) => {
const page = job.testPages[url]
const promises = []
if (report.__coverage__) {
const coverageFileName = join(job.covTempDir, `${filename(url)}.json`)
promises.push(writeFileAsync(coverageFileName, JSON.stringify(report.coverage__)))
delete report.__coverage__
} = report
const reportFileName = join(job.tstReportDir, `${filename(url)}.json`)
promises.push(writeFileAsync(reportFileName, JSON.stringify(page)))
Promise.all(promises).then(() => stop(url))

Modified endpoint to collect code coverage and test report

Generating the code coverage reports

When all the test pages are executed, the coverage report is generated using two commands :

  • nyc merge to merge the different coverage reports in a single one.

Coverage report

Passionate developer, always keen on finding new ways to do things, welcome to my thoughts.

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