Jest-style diffs on Chai equalities

Chai’s assertions have a built-in showDiff flag, but do not produce the diff themselves; it’s up to the test runner to produce such output.

The very popular Mocha does that, but Jest doesn’t: its neat diffs are produced internally by its built-in matchers; it doesn’t rely on any sort of metadata to produce diff for third-party assertions.

When you start using Jest, if you still prefer Chai for your assertions, you quickly miss Jest’s neat JSON diffs, especially if you used to run tests with Mocha, which produced similar diffs on its own.

This Chai plugin tries to fix that, piggybacking on Jest’s jest-diff and jest-matcher-utils for maximum compatibility with Jest’s built-in expectation output.

It focuses on deep equality testing (through any variation of .deep.equal or .eql), where such diffs are most relevant. Because Jest uses diffs even for its ===-based .toBe expectation, it also provides it for shallow equal testing, just for consistency’s sake.

On the other hand, it currently doesn’t provide it for .property value testing, unlike Jest’s .toHaveProperty expectation. This seemed to require too much duplication of code between Chai’s internal assertions and this plugin, as there are many edge cases. We’ll see.

Usage

const chai = require('chai')
const chaiJestDiff = require('chai-jest-diff')

chai.use(chaiJestDiff())

Options

The plugin factory accepts a boolean argument, expand, that mirrors Jest’s expand configuration setting, for full-length diffs (instead of close-context diffs). I couldn’t figure out a way to get this information dynamically from Jest’s exposed API, so you’d have to pass that in manually if you need it.

License

© 2017 Delicious Insights

This plugin is provided under the MIT license. See LICENSE.md for details.