jest-diff

What is jest-diff?

The jest-diff npm package is a utility primarily used to show differences between two values, typically used in testing environments to assert the equality of objects, arrays, or other data structures. It is part of the Jest testing framework but can be used independently for its diffing capabilities.

What are jest-diff's main functionalities?

Diffing two values

This feature allows you to compare two JavaScript objects and outputs a string that visually represents the differences between them. It is useful for debugging and in test assertions to see what exactly differs when a test fails.

const diff = require('jest-diff');
const a = { a: { b: { c: 5 } } };
const b = { a: { b: { c: 6 } } };
console.log(diff(a, b));

Customizing diff output

This feature allows customization of the diff output by passing options. You can specify annotations for the compared values, which helps in making the output more understandable and tailored to specific needs.

const diff = require('jest-diff');
const options = { aAnnotation: 'First', bAnnotation: 'Second' };
const a = [1, 2, 3];
const b = [2, 1, 3];
console.log(diff(a, b, options));

Other packages similar to jest-diff

deep-diff

Deep-diff is a library that provides a detailed analysis of differences between any two JavaScript objects. It is more granular than jest-diff as it provides more detailed information about the type of change (edit, delete, add), which can be useful for more complex diffing needs.

diff

The 'diff' package is another popular choice for computing differences between texts. It is not limited to JavaScript objects and can handle strings effectively, making it suitable for applications like text editors or any scenario where text comparison is needed. Unlike jest-diff, it does not provide a structured view for object differences.

README

jest-diff

Display differences clearly so people can review changes confidently.

The diff named export serializes JavaScript values, compares them line-by-line, and returns a string which includes comparison lines.

Two named exports compare strings character-by-character:

• diffStringsUnified returns a string.
• diffStringsRaw returns an array of Diff objects.

Three named exports compare arrays of strings line-by-line:

• diffLinesUnified and diffLinesUnified2 return a string.
• diffLinesRaw returns an array of Diff objects.

Installation

To add this package as a dependency of a project, run either of the following commands:

• npm install jest-diff
• yarn add jest-diff

Usage of diff()

Given JavaScript values, diff(a, b, options?) does the following:

1. serialize the values as strings using the pretty-format package
2. compare the strings line-by-line using the diff-sequences package
3. format the changed or common lines using the chalk package

To use this function, write either of the following:

• const {diff} = require('jest-diff'); in CommonJS modules
• import {diff} from 'jest-diff'; in ECMAScript modules

Example of diff()

const a = ['delete', 'common', 'changed from'];
const b = ['common', 'changed to', 'insert'];

const difference = diff(a, b);

The returned string consists of:

• annotation lines: describe the two change indicators with labels, and a blank line
• comparison lines: similar to “unified” view on GitHub, but Expected lines are green, Received lines are red, and common lines are dim (by default, see Options)

- Expected
+ Received

  Array [
-   "delete",
    "common",
-   "changed from",
+   "changed to",
+   "insert",
  ]

Edge cases of diff()

Here are edge cases for the return value:

• ' Comparing two different types of values. …' if the arguments have different types according to the jest-get-type package (instances of different classes have the same 'object' type)
• 'Compared values have no visual difference.' if the arguments have either referential identity according to Object.is method or same serialization according to the pretty-format package
• null if either argument is a so-called asymmetric matcher in Jasmine or Jest

Usage of diffStringsUnified

Given strings, diffStringsUnified(a, b, options?) does the following:

1. compare the strings character-by-character using the diff-sequences package
2. clean up small (often coincidental) common substrings, also known as chaff
3. format the changed or common lines using the chalk package

Although the function is mainly for multiline strings, it compares any strings.

Write either of the following:

• const {diffStringsUnified} = require('jest-diff'); in CommonJS modules
• import {diffStringsUnified} from 'jest-diff'; in ECMAScript modules

Example of diffStringsUnified

const a = 'common\nchanged from';
const b = 'common\nchanged to';

const difference = diffStringsUnified(a, b);

The returned string consists of:

• annotation lines: describe the two change indicators with labels, and a blank line
• comparison lines: similar to “unified” view on GitHub, and changed substrings have inverse foreground and background colors (that is, from has white-on-green and to has white-on-red, which the following example does not show)

- Expected
+ Received

  common
- changed from
+ changed to

Performance of diffStringsUnified

To get the benefit of changed substrings within the comparison lines, a character-by-character comparison has a higher computational cost (in time and space) than a line-by-line comparison.

If the input strings can have arbitrary length, we recommend that the calling code set a limit, beyond which splits the strings, and then calls diffLinesUnified instead. For example, Jest falls back to line-by-line comparison if either string has length greater than 20K characters.

Usage of diffLinesUnified

Given arrays of strings, diffLinesUnified(aLines, bLines, options?) does the following:

1. compare the arrays line-by-line using the diff-sequences package
2. format the changed or common lines using the chalk package

You might call this function when strings have been split into lines and you do not need to see changed substrings within lines.

Example of diffLinesUnified

const aLines = ['delete', 'common', 'changed from'];
const bLines = ['common', 'changed to', 'insert'];

const difference = diffLinesUnified(aLines, bLines);

- Expected
+ Received

- delete
  common
- changed from
+ changed to
+ insert

Edge cases of diffLinesUnified or diffStringsUnified

Here are edge cases for arguments and return values:

• both a and b are empty strings: no comparison lines
• only a is empty string: all comparison lines have bColor and bIndicator (see Options)
• only b is empty string: all comparison lines have aColor and aIndicator (see Options)
• a and b are equal non-empty strings: all comparison lines have commonColor and commonIndicator (see Options)

Usage of diffLinesUnified2

Given two pairs of arrays of strings, diffLinesUnified2(aLinesDisplay, bLinesDisplay, aLinesCompare, bLinesCompare, options?) does the following:

1. compare the pair of Compare arrays line-by-line using the diff-sequences package
2. format the corresponding lines in the pair of Display arrays using the chalk package

Jest calls this function to consider lines as common instead of changed if the only difference is indentation.

You might call this function for case insensitive or Unicode equivalence comparison of lines.

Example of diffLinesUnified2

import {format} from 'pretty-format';

const a = {
  text: 'Ignore indentation in serialized object',
  time: '2019-09-19T12:34:56.000Z',
  type: 'CREATE_ITEM',
};
const b = {
  payload: {
    text: 'Ignore indentation in serialized object',
    time: '2019-09-19T12:34:56.000Z',
  },
  type: 'CREATE_ITEM',
};

const difference = diffLinesUnified2(
  // serialize with indentation to display lines
  format(a).split('\n'),
  format(b).split('\n'),
  // serialize without indentation to compare lines
  format(a, {indent: 0}).split('\n'),
  format(b, {indent: 0}).split('\n'),
);

The text and time properties are common, because their only difference is indentation:

- Expected
+ Received

  Object {
+   payload: Object {
      text: 'Ignore indentation in serialized object',
      time: '2019-09-19T12:34:56.000Z',
+   },
    type: 'CREATE_ITEM',
  }

The preceding example illustrates why (at least for indentation) it seems more intuitive that the function returns the common line from the bLinesDisplay array instead of from the aLinesDisplay array.

Usage of diffStringsRaw

Given strings and a boolean option, diffStringsRaw(a, b, cleanup) does the following:

1. compare the strings character-by-character using the diff-sequences package
2. optionally clean up small (often coincidental) common substrings, also known as chaff

Because diffStringsRaw returns the difference as data instead of a string, you can format it as your application requires (for example, enclosed in HTML markup for browser instead of escape sequences for console).

The returned array describes substrings as instances of the Diff class, which calling code can access like array tuples:

The value at index 0 is one of the following:

value	named export	description
0	DIFF_EQUAL	in a and in b
-1	DIFF_DELETE	in a but not in b
1	DIFF_INSERT	in b but not in a

The value at index 1 is a substring of a or b or both.

Example of diffStringsRaw with cleanup

const diffs = diffStringsRaw('changed from', 'changed to', true);

i	diffs[i][0]	diffs[i][1]
0	0	'changed '
1	-1	'from'
2	1	'to'

Example of diffStringsRaw without cleanup

const diffs = diffStringsRaw('changed from', 'changed to', false);

i	diffs[i][0]	diffs[i][1]
0	0	'changed '
1	-1	'fr'
2	1	't'
3	0	'o'
4	-1	'm'

Advanced import for diffStringsRaw

Here are all the named imports that you might need for the diffStringsRaw function:

• const {DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff, diffStringsRaw} = require('jest-diff'); in CommonJS modules
• import {DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff, diffStringsRaw} from 'jest-diff'; in ECMAScript modules

To write a formatting function, you might need the named constants (and Diff in TypeScript annotations).

If you write an application-specific cleanup algorithm, then you might need to call the Diff constructor:

const diffCommon = new Diff(DIFF_EQUAL, 'changed ');
const diffDelete = new Diff(DIFF_DELETE, 'from');
const diffInsert = new Diff(DIFF_INSERT, 'to');

Usage of diffLinesRaw

Given arrays of strings, diffLinesRaw(aLines, bLines) does the following:

• compare the arrays line-by-line using the diff-sequences package

Because diffLinesRaw returns the difference as data instead of a string, you can format it as your application requires.

Example of diffLinesRaw

const aLines = ['delete', 'common', 'changed from'];
const bLines = ['common', 'changed to', 'insert'];

const diffs = diffLinesRaw(aLines, bLines);

i	diffs[i][0]	diffs[i][1]
0	-1	'delete'
1	0	'common'
2	-1	'changed from'
3	1	'changed to'
4	1	'insert'

Edge case of diffLinesRaw

If you call string.split('\n') for an empty string:

• the result is [''] an array which contains an empty string
• instead of [] an empty array

Depending of your application, you might call diffLinesRaw with either array.

Example of split method

import {diffLinesRaw} from 'jest-diff';

const a = 'non-empty string';
const b = '';

const diffs = diffLinesRaw(a.split('\n'), b.split('\n'));

i	diffs[i][0]	diffs[i][1]
0	-1	'non-empty string'
1	1	''

Which you might format as follows:

- Expected  - 1
+ Received  + 1

- non-empty string
+

Example of splitLines0 function

For edge case behavior like the diffLinesUnified function, you might define a splitLines0 function, which given an empty string, returns [] an empty array:

export const splitLines0 = string =>
  string.length === 0 ? [] : string.split('\n');

import {diffLinesRaw} from 'jest-diff';

const a = '';
const b = 'line 1\nline 2\nline 3';

const diffs = diffLinesRaw(a.split('\n'), b.split('\n'));

i	diffs[i][0]	diffs[i][1]
0	1	'line 1'
1	1	'line 2'
2	1	'line 3'

Which you might format as follows:

- Expected  - 0
+ Received  + 3

+ line 1
+ line 2
+ line 3

In contrast to the diffLinesRaw function, the diffLinesUnified and diffLinesUnified2 functions automatically convert array arguments computed by string split method, so callers do not need a splitLine0 function.

Options

The default options are for the report when an assertion fails from the expect package used by Jest.

For other applications, you can provide an options object as a third argument:

• diff(a, b, options)
• diffStringsUnified(a, b, options)
• diffLinesUnified(aLines, bLines, options)
• diffLinesUnified2(aLinesDisplay, bLinesDisplay, aLinesCompare, bLinesCompare, options)

Properties of options object

name	default
aAnnotation	'Expected'
aColor	chalk.green
aIndicator	'-'
bAnnotation	'Received'
bColor	chalk.red
bIndicator	'+'
changeColor	chalk.inverse
changeLineTrailingSpaceColor	string => string
commonColor	chalk.dim
commonIndicator	' '
commonLineTrailingSpaceColor	string => string
compareKeys	undefined
contextLines	5
emptyFirstOrLastLinePlaceholder	''
expand	true
includeChangeCounts	false
omitAnnotationLines	false
patchColor	chalk.yellow

For more information about the options, see the following examples.

Example of options for labels

If the application is code modification, you might replace the labels:

const options = {
  aAnnotation: 'Original',
  bAnnotation: 'Modified',
};

- Original
+ Modified

  common
- changed from
+ changed to

The jest-diff package does not assume that the 2 labels have equal length.

Example of options for colors of changed lines

For consistency with most diff tools, you might exchange the colors:

import chalk = require('chalk');

const options = {
  aColor: chalk.red,
  bColor: chalk.green,
};

Example of option for color of changed substrings

Although the default inverse of foreground and background colors is hard to beat for changed substrings within lines, especially because it highlights spaces, if you want bold font weight on yellow background color:

import chalk = require('chalk');

const options = {
  changeColor: chalk.bold.bgYellowBright,
};

Example of option to format trailing spaces

Because diff() does not display substring differences within lines, formatting can help you see when lines differ by the presence or absence of trailing spaces found by /\s+$/ regular expression.

• If change lines have a background color, then you can see trailing spaces.
• If common lines have default dim color, then you cannot see trailing spaces. You might want yellowish background color to see them.

const options = {
  aColor: chalk.rgb(128, 0, 128).bgRgb(255, 215, 255), // magenta
  bColor: chalk.rgb(0, 95, 0).bgRgb(215, 255, 215), // green
  commonLineTrailingSpaceColor: chalk.bgYellow,
};

The value of a Color option is a function, which given a string, returns a string.

If you want to replace trailing spaces with middle dot characters:

const replaceSpacesWithMiddleDot = string => '·'.repeat(string.length);

const options = {
  changeLineTrailingSpaceColor: replaceSpacesWithMiddleDot,
  commonLineTrailingSpaceColor: replaceSpacesWithMiddleDot,
};

If you need the TypeScript type of a Color option:

import {DiffOptionsColor} from 'jest-diff';

Example of options for no colors

To store the difference in a file without escape codes for colors, provide an identity function:

const noColor = string => string;

const options = {
  aColor: noColor,
  bColor: noColor,
  changeColor: noColor,
  commonColor: noColor,
  patchColor: noColor,
};

Example of options for indicators

For consistency with the diff command, you might replace the indicators:

const options = {
  aIndicator: '<',
  bIndicator: '>',
};

The jest-diff package assumes (but does not enforce) that the 3 indicators have equal length.

Example of options to limit common lines

By default, the output includes all common lines.

To emphasize the changes, you might limit the number of common “context” lines:

const options = {
  contextLines: 1,
  expand: false,
};

A patch mark like @@ -12,7 +12,9 @@ accounts for omitted common lines.

Example of option for color of patch marks

If you want patch marks to have the same dim color as common lines:

import chalk = require('chalk');

const options = {
  expand: false,
  patchColor: chalk.dim,
};

Example of option to include change counts

To display the number of changed lines at the right of annotation lines:

const a = ['common', 'changed from'];
const b = ['common', 'changed to', 'insert'];

const options = {
  includeChangeCounts: true,
};

const difference = diff(a, b, options);

- Expected  - 1
+ Received  + 2

  Array [
    "common",
-   "changed from",
+   "changed to",
+   "insert",
  ]

Example of option to omit annotation lines

To display only the comparison lines:

const a = 'common\nchanged from';
const b = 'common\nchanged to';

const options = {
  omitAnnotationLines: true,
};

const difference = diffStringsUnified(a, b, options);

  common
- changed from
+ changed to

Example of option for empty first or last lines

If the first or last comparison line is empty, because the content is empty and the indicator is a space, you might not notice it.

The replacement option is a string whose default value is '' empty string.

Because Jest trims the report when a matcher fails, it deletes an empty last line.

Therefore, Jest uses as placeholder the downwards arrow with corner leftwards:

const options = {
  emptyFirstOrLastLinePlaceholder: '↵', // U+21B5
};

If a content line is empty, then the corresponding comparison line is automatically trimmed to remove the margin space (represented as a middle dot below) for the default indicators:

Indicator	untrimmed	trimmed
aIndicator	'-·'	'-'
bIndicator	'+·'	'+'
commonIndicator	' ·'	''

Example of option for sorting object keys

When two objects are compared their keys are printed in alphabetical order by default. If this was not the original order of the keys the diff becomes harder to read as the keys are not in their original position.

Use compareKeys to pass a function which will be used when sorting the object keys.

const a = {c: 'c', b: 'b1', a: 'a'};
const b = {c: 'c', b: 'b2', a: 'a'};

const options = {
  // The keys will be in their original order
  compareKeys: () => 0,
};

const difference = diff(a, b, options);

- Expected
+ Received

  Object {
    "c": "c",
-   "b": "b1",
+   "b": "b2",
    "a": "a",
  }

Depending on the implementation of compareKeys any sort order can be used.

const a = {c: 'c', b: 'b1', a: 'a'};
const b = {c: 'c', b: 'b2', a: 'a'};

const options = {
  // The keys will be in reverse order
  compareKeys: (a, b) => (a > b ? -1 : 1),
};

const difference = diff(a, b, options);

- Expected
+ Received

  Object {
    "a": "a",
-   "b": "b1",
+   "b": "b2",
    "c": "c",
  }