The Road to 100% test coverage

NodeConf EU 2023
100% test coverage
The road to
ruyadorno.com

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The road to 100% test coverage
Ruy Adorno
Senior Software Developer

working at Google

Node.js TSC

NodeConf EU 2023 ruyadorno.com

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TOC
Setup the work environment for the workshop

Understanding test coverage

Techniques

Questions / Consultation

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Setup
Install node 20 (or 18): https://nodejs.org/en/download

Open your terminal, navigate to a suitable folder, e.g:
cd ~/tmp/100-workshop

Run the workshop generator:
npm init @ruyadorno/100-workshop@latest

Run npm install and tests:
npm it

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What’s in it?

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Give it a try:
List all LTS releases:
node cli.js --filter=lts

Last month’s releases:
node cli.js --recent=month

Only return the resulting count:
node cli.js --count

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What is a test
A way to programatically confirm your program
does what you think it does.

If your tests are also a program, what validates that
program?

Surprise: It’s you, the developer!


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Test Coverage
Tracking coverage is sort of a meta test

It tracks what parts of the program are executed

Gives you a hint of how much of a test it really is

Definitely does not prove that either the source
code OR the test code are good


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100% Test Coverage
It means you covered all lines, functions and
statements

Still not enough, 100% code coverage does NOT
mean 100% of features tested

Gives you super powers

🦸

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Notice: This workshop has a cheat code available
To quickly complete a step, it’s possible to use the
same generator ran to create the boilerplate.

Run the generator using the step option. In this
example we’re using the generator to complete
Step 1:
npx @ruyadorno/create-100-workshop --step=1
⚠

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Step 1
Let’s add an initial test to the main index.js module!


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./test/index.js

۪
۪ $ npx @ruyadorno/create-100-workshop --step=1
./src/index.js

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Step 2
Let’s avoid the network request


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./test/index.js

۪
۪ $ npx @ruyadorno/create-100-workshop —step=2

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Let’s visualize better the coverage results:
Opens an html-based interface in your default browser:
npm t -- --coverage-report=html

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Step 3
Let’s add tests for format-count.js

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./test/format-count.js

۪

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🤩

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Step 4
Let’s refactor and add tests for filter-recent.js

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./src/format-recent.js

۪
./src/date.js

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./test/format-recent.js

۪

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Step 5
Let’s add tests for filter-npm.js

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./test/filter-npm.js

۪

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Step 6
Let’s add tests for filter-booleans.js

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./test/filter-booleans.js

۪

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Step 7
Let’s add tests for the output.js module

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./test/output.js

۪

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Step 8
Let’s test retrieve-nodejs-versions.js

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./test/retrieve-nodejs-versions.js

۪

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Step 9
Let’s add the missing test to the index.js module

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./test/retrieve-nodejs-versions.js

۪

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Awesome Job!
👏

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Less than 100%
If you can’t delete it, you should probably write tests
for it so that you can maybe know what it does

It means there are entire components of your
application without tests

What kind of program needs no test at all?

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Less than 100%
Would you rely on a module with no tests?

A 95% test-covered program is two parts: the
covered part and the uncovered part

The uncovered part is a module with no tests

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Bene
fi
ts
Testable code === maintainable code

Full coverage forces you to look in dark corners

Investigation process tends to uncover bugs

Let you focus on what you’re good at while the
computer keeps track of everything else you’ve
already checked

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Human aspect
Going from 98% to 96% looks and sounds ok

Going from 100% to 99.5% is a huge loss!

It feels great to see all these rows of 100%
covered modules

It’s addictive 😁 and will trick you into being a
better developer!

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Extra costs
Full coverage adds up-front to development cost

In return it reduces the incidence of bugs, and the
costs of changing a program

Less technical debt!

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Extra costs
Full test coverage will not prevent all bugs, what it
does is gives you the power to fix these bugs more
efficiently

What about TypeScript and eslint preventing most
errors already?

Soon after you wrote types you no long knows
what the code is actually supposed to do

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Bad tests
People will write bad tests with less than 100%
coverage too

The solution is to write better tests, and more
easily tested components, not to accept untested
code

Only testing the happy path is the equivalent of
“works on my machine”

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Levelling Up
Limit coverage values to only account for a test
written for a specific module

Avoiding accidental coverage, e.g: a base class
method from module A gets coverage when run from
module B tests

tap supports it out-of-the-box with its coverage-map
config option

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Techniques
Tests with long assertion output that have to be
updated on every change? Use snapshots for that,
save them in git, review the diff.

Have to mock the whole world to hit a buried path?
Refactor it into a discrete submodule, and only mock
that module once. You’re now writing better code.

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Techniques
Full coverage isn’t about writing code with more
tests, it’s about writing code that is more testable.

Monkey-patching and other “bad” software
development practices are usually totally fine in
tests!

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Techniques
Things that really cannot be tested for some reason?
Use ignore comments.

This is an explicit (and well-documented, it’s going to
live there with your code) way of opting-out of
coverage for a very valid reason.

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End of the workshop
Questions? Consultation time.
⏰

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Isaac Z. Schlueter
node-tap author & author of the original
“99% is not enough” talk this workshop is
based from.

Special thanks to:

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Ruy Adorno
Linkedin: /in/ruyadorno

@ruyadorno

@[email protected]

@ruyadorno.bsky.social

Thanks! Find me at:

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The Road to 100% test coverage

The Road to 100% test coverage

Ruy Adorno

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