snapshot-diff

Diffing snapshot utility for Jest. Takes two values, and return their difference as a string, ready to be snapshotted with toMatchSnapshot(). Especially helpful when testing the difference between different React component states.

Installation

yarn add --dev snapshot-diff

Usage

With default jest matcher

const snapshotDiff = require('snapshot-diff');

test('snapshot difference between 2 strings', () => {
  expect(snapshotDiff(a, b)).toMatchSnapshot();
});

const React = require('react');
const Component = require('./Component');

test('snapshot difference between 2 React components state', () => {
  expect(
    snapshotDiff(<Component test="say" />, <Component test="my name" />)
  ).toMatchSnapshot();
});

With custom matcher

const { toMatchDiffSnapshot } = require('snapshot-diff');

expect.extend({ toMatchDiffSnapshot });

test('snapshot difference between 2 strings', () => {
  expect(a).toMatchDiffSnapshot(b);
});

const React = require('react');
const Component = require('./Component');

test('snapshot difference between 2 React components state', () => {
  expect(<Component test="say" />).toMatchDiffSnapshot(
    <Component test="my name" />
  );
});

... alternatively import it once, for instance in your tests setup file:

require('snapshot-diff/extend-expect');

Produced snapshot:

exports[`snapshot difference between 2 strings 1`] = `
"- First value
+ Second value


-  abcx
+  abcy
   "
`;

exports[`snapshot difference between 2 React components state 1`] = `
"- <Component test=\\"say\\" />
+ <Component test=\\"my name\\" />

@@ -27,11 +27,11 @@
   <span />
   <span />
   <span />
   <span />
   <span>
-    say
+    my name
   </span>
   <span />
   <span />
   <span />
   <span />"
`;

Custom serializers

By default, snapshot-diff uses a built in React serializer based on react-test-renderer. The serializers used can be set by calling setSerializers with an array of serializers to use. The order of serializers in this array may be important to you as serializers are tested in order until a match is found.

setSerializers can be used to add new serializers for unsupported data types, or to set a different serializer for React components. If you want to keep the default React serializer in place, don't forget to add the default serializers to your list of serializers!

Adding a new custom serializer

const snapshotDiff = require('snapshot-diff');
const myCustomSerializer = require('./my-custom-serializer');

snapshotDiff.setSerializers([
  ...snapshotDiff.defaultSerializers, // use default React serializer - add this if you want to serialise React components!
  myCustomSerializer
]);

Serializing React components with a different serializer

You can replace the default React serializer by omitting it from the serializer list. The following uses enzymes to-json serializer instead:

const snapshotDiff = require('snapshot-diff');
const enzymeToJson = require('enzyme-to-json/serializer');
const myCustomSerializer = require('./my-custom-serializer');

snapshotDiff.setSerializers([
  enzymeToJson, // using enzymes serializer instead
  myCustomSerializer
]);

Snapshot serializer

By default Jest adds extra quotes around strings so it makes diff snapshots of objects too noisy. To fix this – snapshot-diff comes with custom serializer, which you can add directly in your tests or in setupFiles script:

const snapshotDiff = require('snapshot-diff');

expect.addSnapshotSerializer(snapshotDiff.getSnapshotDiffSerializer());

test('snapshot difference between 2 objects', () => {
  expect(snapshotDiff({ foo: 'bar' }, { foo: 'baz' })).toMatchSnapshot();
});

...or add it globally to your jest config:

// jest.config.js
module.exports = {
  snapshotSerializers: [
    require.resolve('snapshot-diff/serializer.js'),
  ],
};

API

type Options = {
  expand?: boolean,
  colors?: boolean,
  contextLines?: number
};

// default export
snapshotDiff(valueA: any, valueB: any, options?: Options) => string
// custom matcher
expect(valueA: any).toMatchDiffSnapshot(valueB: any, options?: Options, testName?: string) => void

Options

• expand: boolean (default: false) – expand the diff, so the whole information is preserved
• colors: boolean (default: false) – preserve color information from Jest diff
• contextLines: number (default: 5) - number of context lines to be shown at the beginning and at the end of a snapshot
• stablePatchmarks: boolean (default: false) - prevent line number patch marks from appearing in diffs. This can be helpful when diffs are breaking only because of the patch marks. Changes @@ -1,1 +1,2 @@ to @@ --- --- @@.
• aAnnotation: string (default: 'First Value') - the annotation indicating from which serialization the - lines are
• bAnnotation: string (default: 'Second Value') - the annotation indicating from which serialization the + lines are

---

Project is MIT-licensed. Pull Requests welcome!