electron/docs/development/testing.md

# Testing

We aim to keep the code coverage of Electron high. We ask that all pull
request not only pass all existing tests, but ideally also add new tests
to cover changed code and new scenarios. Ensuring that we capture as
many code paths and use cases of Electron as possible ensures that we
all ship apps with fewer bugs.

This repository comes with linting rules for both JavaScript and C++ –
as well as unit and integration tests. To learn more about Electron's
coding style, please see the [coding-style](coding-style.md) document.

## Linting
To ensure that your JavaScript is in compliance with the Electron coding
style, run `npm run lint-js`, which will run `standard` against both
Electron itself as well as the unit tests. If you are using an editor
with a plugin/addon system, you might want to use one of the many
[StandardJS addons][standard-addons] to be informed of coding style
violations before you ever commit them.

To run `standard` with parameters, run `npm run lint-js --` followed by
arguments you want passed to `standard`.

To ensure that your C++ is in compliance with the Electron coding style,
run `npm run lint-cpp`, which runs a `cpplint` script. We recommend that
you use `clang-format` and prepared [a short tutorial](clang-format.md).

There is not a lot of Python in this repository, but it too is governed
by coding style rules. `npm run lint-py` will check all Python, using
`pylint` to do so.

## Unit Tests

To run all unit tests, run `npm run test`. The unit tests are an Electron
app (surprise!) that can be found in the `spec` folder. Note that it has
its own `package.json` and that its dependencies are therefore not defined
in the top-level `package.json`.

To run only specific tests matching a pattern, run `npm run test --
-g=PATTERN`, replacing the `PATTERN` with a regex that matches the tests
you would like to run. As an example: If you want to run only IPC tests, you
would run `npm run test -- -g ipc`.

[standard-addons]: https://standardjs.com/#are-there-text-editor-plugins

### Testing on Windows 10 devices
[Some Windows 10 devices](https://docs.microsoft.com/en-us/typography/fonts/windows_10_font_list) do not ship with the Meiryo font installed, which may cause a font fallback test to fail. To install Meiryo:
1. Push the Windows key and search for _Manage optional features_.
1. Click _Add a feature_.
1. Select _Japanese Supplemental Fonts_ and click _Install_.


Some tests which rely on precise pixel measurements may not work correctly on
devices with Hi-DPI screen settings due to floating point precision errors.
To run these tests correctly, make sure the device is set to 100% scaling.

To configure display scaling:
1. Push the Windows key and search for _Display settings_.
1. Under _Scale and layout_, make sure that the device is set to 100%.