Release History

v5.1.1

Version v5.1.1

What’s Changed

Set up ESLint for JSDoc comments by @koddsson in https://github.com/chaijs/chai/pull/1605
build(deps-dev): bump ip from 1.1.8 to 1.1.9 by @dependabot in https://github.com/chaijs/chai/pull/1608
Correct Mocha import instructions by @MattiSG in https://github.com/chaijs/chai/pull/1611
fix: support some virtual contexts in toThrow by @43081j in https://github.com/chaijs/chai/pull/1609

New Contributors

@MattiSG made their first contribution in https://github.com/chaijs/chai/pull/1611

Full Changelog: https://github.com/chaijs/chai/compare/v5.1.0…v5.1.1

v5.1.0

Version v5.1.0

What’s Changed

Remove useless guards and add parentheses to constuctors by @koddsson in https://github.com/chaijs/chai/pull/1593
Cleanup jsdoc comments by @koddsson in https://github.com/chaijs/chai/pull/1596
Convert comments in “legal comments” format to jsdoc or normal comments by @koddsson in https://github.com/chaijs/chai/pull/1598
Implement iterable assertion by @koddsson in https://github.com/chaijs/chai/pull/1592
Assert interface fix by @developer-bandi in https://github.com/chaijs/chai/pull/1601
Set support in same members by @koddsson in https://github.com/chaijs/chai/pull/1583
Fix publish script by @koddsson in https://github.com/chaijs/chai/pull/1602

New Contributors

@developer-bandi made their first contribution in https://github.com/chaijs/chai/pull/1601

Full Changelog: https://github.com/chaijs/chai/compare/v5.0.3…v5.1.0

v5.0.3

Version v5.0.3

Fix bad v5.0.2 publish.

Full Changelog: https://github.com/chaijs/chai/compare/v5.0.2…v5.0.3

v5.0.2

Version v5.0.2

What’s Changed

build(deps): bump nanoid and mocha by @dependabot in https://github.com/chaijs/chai/pull/1558
remove bump-cli by @koddsson in https://github.com/chaijs/chai/pull/1559
Update developer dependencies by @koddsson in https://github.com/chaijs/chai/pull/1560
fix: removes ?? for node compat (5.x) by @43081j in https://github.com/chaijs/chai/pull/1576
Update loupe to latest version by @koddsson in https://github.com/chaijs/chai/pull/1579
Re-enable some webkit tests by @koddsson in https://github.com/chaijs/chai/pull/1580
Remove a bunch of if statements in test/should.js by @koddsson in https://github.com/chaijs/chai/pull/1581
Remove a bunch of unused files by @koddsson in https://github.com/chaijs/chai/pull/1582
Fix 1564 by @koddsson in https://github.com/chaijs/chai/pull/1566

Full Changelog: https://github.com/chaijs/chai/compare/v5.0.1…v5.0.2

v4.4.1

Version v4.4.1

What’s Changed

fix: removes ?? for node compat by @43081j in https://github.com/chaijs/chai/pull/1574

Full Changelog: https://github.com/chaijs/chai/compare/v4.4.0…v4.4.1

v4.4.0

Version v4.4.0

What’s Changed

Allow deepEqual fonction to be configured globally (4.x.x branch) by @forty in https://github.com/chaijs/chai/pull/1553

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.10…v4.4.0

v5.0.0-rc.0

Version v5.0.0-rc.0

The first Release Candidate of chai@v5 is here!

We’ve put out a few alpha versions and tested them out in various projects with good success. This RC includes all those changes plus any fixes that we’ve discovered since then.

Please try it out in your projects and let us know if you run into any issues so we can make fixes before version 5!

Thanks for using Chai 🙏🏻

What’s Changed

feat: use chaijs/loupe for inspection by @pcorpet in https://github.com/chaijs/chai/pull/1401
docs: fix URL in README by @Izzur in https://github.com/chaijs/chai/pull/1413
Remove get-func-name dependency by @koddsson in https://github.com/chaijs/chai/pull/1416
Convert Makefile script to npm scripts by @koddsson in https://github.com/chaijs/chai/pull/1424
Clean up README badges by @koddsson in https://github.com/chaijs/chai/pull/1422
fix: package.json - deprecation warning on exports field by @stevenjoezhang in https://github.com/chaijs/chai/pull/1400
fix: deep-eql bump package to support symbols by @snewcomer in https://github.com/chaijs/chai/pull/1458
ES module conversion PoC by @43081j in https://github.com/chaijs/chai/pull/1498
chore: drop commonjs support by @43081j in https://github.com/chaijs/chai/pull/1503
Update pathval by @koddsson in https://github.com/chaijs/chai/pull/1527
Update check-error by @koddsson in https://github.com/chaijs/chai/pull/1528
update deep-eql to latest version by @koddsson in https://github.com/chaijs/chai/pull/1542
Inline type-detect as a simple function by @koddsson in https://github.com/chaijs/chai/pull/1544
Update loupe by @koddsson in https://github.com/chaijs/chai/pull/1545
Typo ‘Test an object’ not ‘Test and object’ by @mavaddat in https://github.com/chaijs/chai/pull/1460
Update assertion-error to it’s latest major version! by @koddsson in https://github.com/chaijs/chai/pull/1543
Replacing Karma with Web Test Runner by @koddsson in https://github.com/chaijs/chai/pull/1546
remove codecov by @koddsson in https://github.com/chaijs/chai/pull/1548
remove chai version constant by @koddsson in https://github.com/chaijs/chai/pull/1550
Remove istanbul by @koddsson in https://github.com/chaijs/chai/pull/1549

New Contributors

@Izzur made their first contribution in https://github.com/chaijs/chai/pull/1413
@koddsson made their first contribution in https://github.com/chaijs/chai/pull/1416
@stevenjoezhang made their first contribution in https://github.com/chaijs/chai/pull/1400
@43081j made their first contribution in https://github.com/chaijs/chai/pull/1498

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.10…v5.0.0-rc.0

v5.0.0

Version v5.0.0

BREAKING CHANGES

Chai now only supports EcmaScript Modules (ESM). This means your tests will need to either have import {...} from 'chai' or import('chai'). require('chai') will cause failures in nodejs. If you’re using ESM and seeing failures, it may be due to a bundler or transpiler which is incorrectly converting import statements into require calls.
Dropped support for Internet Explorer.
Dropped support for NodeJS < 18.
Minimum supported browsers are now Firefox 100, Safari 14.1, Chrome 100, Edge 100. Support for browsers prior to these versions is “best effort” (bug reports on older browsers will be assessed individually and may be marked as wontfix).

What’s Changed

feat: use chaijs/loupe for inspection by @pcorpet in https://github.com/chaijs/chai/pull/1401
docs: fix URL in README by @Izzur in https://github.com/chaijs/chai/pull/1413
Remove get-func-name dependency by @koddsson in https://github.com/chaijs/chai/pull/1416
Convert Makefile script to npm scripts by @koddsson in https://github.com/chaijs/chai/pull/1424
Clean up README badges by @koddsson in https://github.com/chaijs/chai/pull/1422
fix: package.json - deprecation warning on exports field by @stevenjoezhang in https://github.com/chaijs/chai/pull/1400
fix: deep-eql bump package to support symbols by @snewcomer in https://github.com/chaijs/chai/pull/1458
ES module conversion PoC by @43081j in https://github.com/chaijs/chai/pull/1498
chore: drop commonjs support by @43081j in https://github.com/chaijs/chai/pull/1503
Update pathval by @koddsson in https://github.com/chaijs/chai/pull/1527
Update check-error by @koddsson in https://github.com/chaijs/chai/pull/1528
update deep-eql to latest version by @koddsson in https://github.com/chaijs/chai/pull/1542
Inline type-detect as a simple function by @koddsson in https://github.com/chaijs/chai/pull/1544
Update loupe by @koddsson in https://github.com/chaijs/chai/pull/1545
Typo ‘Test an object’ not ‘Test and object’ by @mavaddat in https://github.com/chaijs/chai/pull/1460
Update assertion-error to it’s latest major version! by @koddsson in https://github.com/chaijs/chai/pull/1543
Replacing Karma with Web Test Runner by @koddsson in https://github.com/chaijs/chai/pull/1546

New Contributors

@Izzur made their first contribution in https://github.com/chaijs/chai/pull/1413
@stevenjoezhang made their first contribution in https://github.com/chaijs/chai/pull/1400
@43081j made their first contribution in https://github.com/chaijs/chai/pull/1498

Full Changelog: https://github.com/chaijs/chai/compare/4.3.1…v5.0.0

v5.0.0-alpha.2

Version v5.0.0-alpha.2

What’s Changed

update deep-eql to latest version by @koddsson in https://github.com/chaijs/chai/pull/1542
Inline type-detect as a simple function by @koddsson in https://github.com/chaijs/chai/pull/1544
Update loupe by @koddsson in https://github.com/chaijs/chai/pull/1545
Typo ‘Test an object’ not ‘Test and object’ by @mavaddat in https://github.com/chaijs/chai/pull/1460
Update assertion-error to it’s latest major version! by @koddsson in https://github.com/chaijs/chai/pull/1543
Replacing Karma with Web Test Runner by @koddsson in https://github.com/chaijs/chai/pull/1546

Full Changelog: https://github.com/chaijs/chai/compare/v5.0.0-alpha.1…v5.0.0-alpha.2

v4.3.10

Version v4.3.10

This release simply bumps all dependencies to their latest non-breaking versions.

What’s Changed

upgrade all dependencies by @keithamus in https://github.com/chaijs/chai/pull/1540

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.9…v4.3.10

v4.3.9

Version v4.3.9

Upgrade dependencies.

This release upgrades dependencies to address CVE-2023-43646 where a large function name can cause “catastrophic backtracking” (aka ReDOS attack) which can cause the test suite to hang.

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.8…v4.3.9

v4.3.8

Version v4.3.8

What’s Changed

4.x.x: Fix link to commit logs on GitHub by @bugwelle in https://github.com/chaijs/chai/pull/1487
build(deps): bump socket.io-parser from 4.0.4 to 4.0.5 by @dependabot in https://github.com/chaijs/chai/pull/1488
Small typo in test.js by @mavaddat in https://github.com/chaijs/chai/pull/1459
docs: specify return type of objDisplay by @scarf005 in https://github.com/chaijs/chai/pull/1490
Update CONTRIBUTING.md by @matheus-rodrigues00 in https://github.com/chaijs/chai/pull/1521
Fix: update exports.version to current version by @peanutenthusiast in https://github.com/chaijs/chai/pull/1534

New Contributors

@bugwelle made their first contribution in https://github.com/chaijs/chai/pull/1487
@mavaddat made their first contribution in https://github.com/chaijs/chai/pull/1459
@scarf005 made their first contribution in https://github.com/chaijs/chai/pull/1490
@matheus-rodrigues00 made their first contribution in https://github.com/chaijs/chai/pull/1521
@peanutenthusiast made their first contribution in https://github.com/chaijs/chai/pull/1534

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.7…v4.3.8

v5.0.0-alpha.1

Version v5.0.0-alpha.1

What’s Changed

chore: drop commonjs support by @43081j in https://github.com/chaijs/chai/pull/1503
Update pathval by @koddsson in https://github.com/chaijs/chai/pull/1527
Update check-error by @koddsson in https://github.com/chaijs/chai/pull/1528

Full Changelog: https://github.com/chaijs/chai/compare/v5.0.0-alpha.0…v5.0.0-alpha.1

v5.0.0-alpha.0

Version v5.0.0-alpha.0

This release is the first alpha release for Chai 5. The changes from v4.x:

This package now uses ESM modules. CommonJS environments are no longer supported. If you wish to use a CommonJS environment, please continue to use Chai 4.
Drop support for NodeJS versions less than 16. If you are using NodeJS v4-v15, please continue to use Chai 4.
Drop support for Internet Explorer 10 & 11. If you wish to test against Internet Explorer 10 or 11 please continue to use Chai 4.
Drop support for Safari versions less than 14. If you wish to test against Safari v10-v13 please continue to use Chai 4.
Drop support for EdgeHTML. If you wish to test against Edges older than 18 (as in, not Chromium Edge) please continue to use Chai 4.
Drop support for Firefox versions less than 102. If you wish to test against Firefox 60 through to 101 please continue to use Chai 4.
Drop support for Chrome versions less than 108. If you wish to test against Chrome 68 through 107 please continue to use Chai 4.

v4.3.7

Version v4.3.7

What’s Changed

fix: deep-eql bump package to support symbols comparison by @snewcomer in https://github.com/chaijs/chai/pull/1483

Full Changelog: https://github.com/chaijs/chai/compare/v4.3.6…v4.3.7

Version v4.3.6

Update loupe to 2.3.1

Version v4.3.5

build chaijs fca5bb1
build(deps-dev): bump codecov from 3.1.0 to 3.7.1 (#1446) 747eb4e
fix package.json exports 022c2fa
fix: package.json - deprecation warning on exports field (#1400) 5276af6
feat: use chaijs/loupe for inspection (#1401) (#1407) c8a4e00

https://github.com/chaijs/chai/compare/v4.3.4…v4.3.5

Version v4.3.4

This fixes broken inspect behavior with bigints (#1321) (#1383) thanks @vapier

4.3.3 / 2021-03-03

Version 4.3.3

This reintroduces Assertion as an export in the mjs file. See https://github.com/chaijs/chai/pull/1378 & https://github.com/chaijs/chai/issues/1375

4.3.2 / 2021-03-03

Version 4.3.2

This fixes a regression in IE11. See https://github.com/chaijs/chai/pull/1380 & https://github.com/chaijs/chai/issues/1379

4.3.1 / 2021-03-02

Version 4.3.1

This releases fixed an engine incompatibility with 4.3.0

The 4.x.x series of releases will be compatible with Node 4.0. Please report any errors found in Node 4 as bugs, and they will be fixed.

The 5.x.x series, when released, will drop support for Node 4.0

This fix also ensures pathval is updated to 1.1.1 to fix CVE-2020-7751

4.3.0 / 2021-02-04

Version 4.3.0

This is a minor release.

Not many changes have got in since the last release but this one contains a very important change (#1257) which will allow jest users to get better diffs. From this release onwards, jest users will be able to see which operator was used in their diffs. The operator is a property of the AssertionError thrown when assertions fail. This flag indicates what kind of comparison was made.

This is also an important change for plugin maintainers. Plugin maintainers will now have access to the operator flag, which they can have access to through an util method called getOperator`.

Thanks to all the amazing people that contributed to this release.

New Features

Allow contain.oneOf to take an array of possible values (@voliva)
Adding operator attribute to assertion error (#1257) (@rpgeeganage)
The closeTo error message will now inform the user when a delta is required (@eouw0o83hf)

Docs

Add contains flag to oneOf documentation (@voliva)

Tests

Make sure that useProxy config is checked in overwriteProperty (@vieiralucas)
Add tests for contain.oneOf (@voliva )

Chores

Update mocha to version 6.1.4
Add node v10 and v12 to ci (@vieiralucas)
Drop support for node v4, v6 and v9 (@vieiralucas)
Fix sauce config for headless chrome (@meeber)
Update dev dependencies (@meeber)
Removed phantomjs dependency (#1204)

4.2.0 / 2018-09-25

Version 4.2.0

This is a minor release. Thank you to all of our contributors and users!

New Features

feat(assertions): add ‘still’ language chain (#1194; @ScottRudiger)
.lengthOf for Maps and Sets (#1110, #1131; @asbish)
feat: Add the assert.fail([message]) interface (#1116, #1117; @s-leroux)

Bug Fixes

fix: remove Chai frames from .deep.equal stack (#1124; @meeber)
add ‘catch’ keyword to proxyExcludedKeys (#1050, #1101; @generalandrew)
property assertion should only accept strings if nested (#1043, #1044; @solodynamo)
fix: Make tests pass with --use_strict (#1034, #1040; @shvaikalesh)

Performance

perf: Optimize proxify and stringDistance (#1098; @sophiebits)
fix: Avoid repeated String#slice calls in stringDistance (#1095; @bmeurer)

Style

Fix typos and remove trailing whitespaces (#1042; @simonewebdesign)
Remove unnecessary code (#1049; @abetomo)
Fix variable declaration (#1048; @abetomo)

Tests

test(assert): increase coverage (#1084, #1085; @brutalcrozt)
test: stop modifying globals in Proxy tests (#1144; @meeber)

Docs

Fix unbalanced quotes in assert.notInclude example (#1200; @haykam821)
docs: fix bad .string example (#1156, #1157; @meeber)
fixed package npm url (#1151; @wadie)
Spelling (#1145; @jsoref)
docs: corrected spelling (#1141; @Powell-v2)
docs: fix wrong .property chaining examples (https://github.com/chaijs/chai/issues/193#issuecomment-360334369, #1130; @meeber)
docs: improve throws examples (#1113; @ColinEberhardt)
Fix typos (#1107; @tbroadley)
docs: correct .ok description (#1047; @shvaikalesh)

Chores

chore: update package-lock.json (#1198; @meeber)
Update mocha to the latest version (#1127)
chore: update dependencies (#1157; @meeber)
Update browserify to the latest version (#1135)
chore: update Node versions in Travis config (#1126; @meeber)
chore: remove Opera from Sauce config (#1125; @meeber)
chore: update dependencies (#1118; @meeber)
chore: update dependencies (#1074; @meeber)
Chore: change coverage service (coverall to codecov) (#927, #1073; @brutalcrozt)
chore: add package-lock.json (#1013; @meeber)

4.1.2 / 2017-08-31

Version 4.1.2

This release fixes a bug when running in certain environments, and includes a few minor documentation fixes.

Bug Fixes

fix: update deep-eql to version 3.0.0 (#1020)
fix: replace code causing breakage under strict CSP (#1032; @Alhadis)

Docs

docs: add missing assert parameters (#1017, #1023; @Hpauric)
docs: update year in license file (#1025; @yanca018)

4.1.1 / 2017-08-05

Version 4.1.1

This release includes a few bug and documentation fixes.

Bug Fixes

fix: .instanceof to allow DOM interfaces in IE11 (#1000, #1008; @meeber)
fix: .include to work with all objects (#1009, #1012; @meeber)

Docs

fix: correct hasAnyKeys comment error (#1014; @zenHeart)
docs: re-indent hasAnyKeys code (#1016; @keithamus)

4.1.0 / 2017-07-11

Version 4.1.0

This release includes one new feature and a few bug fixes.

New Features

Add ES6 collection support to include() (#970, #994; @shvaikalesh)

Bug Fixes

Allow dates for isBelow and isAbove assertions (#980, #990; @v1adko)
fix: check target’s type in .property assertion (#992; @meeber)

Chores

Add a missing var keyword found by lgtm.com (#988; @samlanning)
refactor: expectTypes to access ssfi flag (#993; @meeber)
Create CODEOWNERS (#1004; @keithamus)

4.0.2 / 2017-06-05

Version 4.0.2

We have another bugfix release, addressing some issues for WebPack 1 users.

Bug Fixes

Revert getting version information from package.json, some bundler tools like Webpack do not come default with json loaders despite Node being able to handle this. This change moves back to hardcoding the version number in the codebase. (#985, #986)

4.0.1 / 2017-05-31

Version 4.0.1

4.0.1

Of course, any major release cannot go without a quick bugfix release shortly after - and here’s ours!

Bug Fixes

Remove package.json browser field which was mistakenly added, and caused bundler tools like Browserify or Webpack to fail as it attempted to rebundle an already bundled file. (#978, #982)

4.0.0 / 2017-05-26

Version 4.0.0

4.0.0

4.0 has been a huge undertaking by the chai community! A lot has changed to ensure Chai 4 is a stable, reliable, well documented codebase. Here are just some of the major improvements:

almost all documentation has been rewritten, with detailed instructions on how assertions work, which flags they can be combined with and the best practices for how to use them.
deep equality has been rewritten from the ground up to support ES6 types like Map and Set, and better support existing types. It is now also much, much faster than before and allows us to bring some great improvements in upcoming releases.
we have made sure the deep flag now only ever does deep equality. Beforehand, it would sometimes also be used to test nested properties (for example expect(foo).to.have.deep.property('bar.baz'). For nested assertions, please now use the .nested flag.
many assertions have become more strict, which means you get better error messages explaining where things have gone wrong. For the most part, this wont mean error messages where there weren’t error messages before, but it will mean better error messages to replace the, sometimes cryptic, default TypeError messages.
we’ve added detections and helpful error messages for common mistakes and typos. The error messages will, in some cases, point you to documentation or in other cases suggest alternatives. These messages will continue to be improved in future releases, so let us know if you have any suggestions!

Breaking Changes

We no longer support Node v0.10 and v0.12 (since their LTS has ended) (PRs: #816, #901)
Instead of allowing the user to write the path of a property, now the deep flag performs a deep equality comparison when used with the .property assertion. If you want the old behavior of using the dot or bracket notation to denote the property you want to assert against you can use the new .nested flag. (Related Issues: #745, #743, PRs: #758, #757)
```
const obj = {a: 1};

// The `.deep` flag now does deep equality comparisons
expect({foo: obj}).to.have.deep.property('foo', {a: 1});

// Use the `nested` flag if you want to assert against a nested property using the bracket or dot notation
expect({foo: obj}).to.have.nested.property('foo.a', 1);

// You can also use both flags combined
const obj2 = {a: {b: 2}};
expect({foo: obj2}).to.have.deep.nested.property('foo.a', {b: 2});
```
Please notice that the old methods which used the old behavior of the deep flag on the assert interface have been renamed. They all have had the deep word changed by the nested word. If you want to know more about this please take a look at #757.
Previously, expect(obj).not.property(name, val) would throw an Error if obj didn’t have a property named name. This change causes the assertion to pass instead. The assert.propertyNotVal and assert.deepPropertyNotVal assertions were renamed to assert.notPropertyVal and assert.notDeepPropertyVal, respectively. (Related Issues: #16, #743, #758)
You can now use the deep flag for the .include assertion in order to perform a deep equality check to see if something is included on the target. Previously, .include was using strict equality (===) for non-negated property inclusion, but deep equality for negated property inclusion and array inclusion. This change causes the .include assertion to always use strict equality unless the deep flag is set. Please take a look at this comment if you want to know more about it. (Related Issues: #743, PRs: #760, #761)
```
const obj = {a: 1};
expect([obj]).to.deep.include({a:1});
expect({foo: obj}).to.deep.include({foo: {a:1}});
```
Fix unstable behavior of the NaN assertion. Now we use the suggested ES6 implementation. The new implementation is now more correct, strict and simple. While the old one threw false positives, the new implementation only checks if something is NaN (or not if the .not flag is used) and nothing else. (Related Issues: #498, #682, #681, PRs: #508)
```
// Only `NaN` will be considered to be `NaN` and nothing else
expect(NaN).to.be.NaN;

// Anything that is not `NaN` cannot be considered as `NaN`
expect('randomString').not.to.be.NaN;
expect(true).not.to.be.NaN;
expect({}).not.to.be.NaN;
expect(4).not.to.be.NaN;
```

The Typed Array types are now truncated if they’re too long (in this case, if they exceed the truncateThreshold value on the config). (Related Issues: #441, PRs: #576)

var arr = [];
for (var i = 1; i <= 1000; i++) {
  arr.push(i);
}

// The assertion below will truncate the diff shown and the enormous typed array will be shown as:
// [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...  ] instead of printing the whole typed array
chai.expect(new Float32Array(100)).to.equal(1);

The assertions: within, above, least, below, most, increase, decrease will throw an error if the assertion’s target or arguments are not numbers. (Related Issues: #691, PRs: #692, #796)

// These will throw errors, for example:
expect(null).to.be.within(0, 1);
expect(null).to.be.above(10);
expect(null).to.be.at.least(20);
expect(null).to.be.below(20);
expect(null).to.be.at.most(20);
expect(null).to.increase.by(20);
expect(null).to.decrease.by(20);

// This will not:
expect('string').to.have.a.lengthOf.at.least(3);

Previously, expect(obj).not.ownProperty(name, val) would throw an Error if obj didn’t have an own property (non-inherited) named name. This change causes the assertion to pass instead. (Related Issues: #795, #, PRs: #744, #810)*
```
expect({ foo: 'baz' }).to.not.have.own.property('quux', 'baz');
```

The .empty assertion will now throw when it is passed non-string primitives and functions (PRs: #763, #812)

// These will throw TypeErrors:
expect(Symbol()).to.be.empty;
expect(function() {}).to.be.empty;
expect(true).to.be.empty;
expect(1).to.be.empty

Assertion subject (obj) changes when using ownProperty or own.property and thus enables chaining. (Related Issues: #281, PRs: #641)
```
expect({val: 20}).to.have.own.property('val').above(10);
```
The .change, .increase, and .decrease assertions changed from chainable method assertions to method assertions. They don’t have any chaining behavior, and there’s no generic semantic benefit to chaining them. (Related Issues: #917, PRs: #925)

// This will not work anymore because there is no benefit to chaining these assertions:
expect(function() {}).to.change.by(2)
expect(function() {}).to.increase.by(2)
expect(function() {}).to.decrease.by(2)

The utils (second argument passed to the chai.use callback function) no longer exports the getPathValue function. If you want to use that please use the pathval module, which is what chai uses internally now. (Related Issues: #457, #737, PRs: #830)

(For plugin authors) Throw when calling _superon overwriteMethodif the method being overwritten is undefined. Currently if the method you are trying to overwrite is not defined and your new method calls _super it will throw an Error.(Related Issues: #467, PRs: #528) Before this change, calling _super would simply return this.

// Considering the method `imaginaryMethod` does not exist, this would throw an error for example:
chai.use(function (chai, utilities) {
  chai.Assertion.overwriteMethod('imaginaryMethod', function (_super) {
    return function () {
      _super.apply(this, arguments);
    }
  });
});

// This will throw an error since you are calling `_super` which should be a method (in this case, the overwritten assertion) that does not exist
expect('anything').to.imaginaryMethod(); 

(For plugin authors) Now showDiff is turned on by default whenever the showDiff flag is anything other than false. This issue will mostly affect plugin creators or anyone that made extensions to the core, since this affects the Assertion.assert method. (Related Issues: #574, PRs: #515)

// Now whenever you call `Assertion.assert` with anything that is not false for the `showDiff` flag it will be true
// The assertion error that was thrown will have the `showDiff` flag turned on since it was not passed to the `assert` method
try {
  new chai.Assertion().assert(
      'one' === 'two'
    , 'expected #{this} to equal #{exp}'
    , 'expected #{this} to not equal #{act}'
    , 'one'
    , 'two'
  );
} catch(e) {
  assert.isTrue(e.showDiff);
}

// The assertion error that was thrown will have the `showDiff` flag turned off since here we passed `false` explicitly
try {
  new chai.Assertion().assert(
      'one' === 'two'
    , 'expected #{this} to equal #{exp}'
    , 'expected #{this} to not equal #{act}'
    , 'one'
    , 'two'
    , false
  );
} catch(e) {
  assert.isFalse(e.showDiff);
}

New Features

Throw when non-existent property is read. (Related Issues: #407, #766 PRs: #721, #770) This is a potentially breaking change. Your build will fail if you have typos in your property assertions Before 4.x.x when using property assertions they would not throw an error if you wrote it incorrectly. The example below, for example, would pass:
```
expect(true).to.be.ture; // Oops, typo, now Chai will throw an Error
```
Since this implementation depends on ES6 Proxies it will only work on platforms that support it.

This property can be enabled (default) or disabled through the config.useProxy property, for example:
```
chai.config.useProxy = false;  // disable use of Proxy
```

Add fix suggestions when accessing a nonexistent property in proxy mode. (Related Issues: #771, PRs: #782) When a nonexistent property is accessed in proxy mode, Chai will compute the levenshtein distance to all possible properties in order to suggest the best fix to the user.

expect(false).to.be.fals; // Error: Invalid Chai property: fals. Did you mean "false"?
expect('foo').to.be.undefind; // Error: Invalid Chai property: undefind. Did you mean "undefined"?

// If the Levenshtein distance between the word and any Chai property is greater than 4, no fix will be suggested
expect('foo').to.be.fdsakfdsafsagsadgagsdfasf // error thrown, no fix suggested

When non-chainable methods (including overwritten non-chainable methods) are used incorrectly an error will be thrown with a helpful error message. (PRs: #789)
```
expect(true).to.equal.true;  // Invalid Chai property: equal.true. See docs for proper usage of "equal".
```
Add a new configuration setting that describes which keys will be ignored when checking for non-existing properties on an assertion before throwing an error. Since this implementation depends on ES6 Proxies it will only work on platforms that support it. Also, if you disable config.useProxy, this setting will have no effect. (Related Issues: #765, PRs: #774)
```
chai.config.proxyExcludedKeys.push('nonExistingProp');

expect('my string').to.nonExistingProp; // This won't throw an error now
```
Add script that registers should as a side-effect. (Related Issues: #594, #693 PRs: #604)
```
// You can now write:
import 'chai/should';

// as opposed to:
import {should} from 'chai';
should();
```
You can also register should via a mocha option: mocha --require chai/should.

The change assertion accepts a function as object. (Related Issues: #544, PRs: #607)

// Now you can also check if the return value of a function changes, for example
assert.increases(
  someOperation,
  () => getSomething().length
)

You can also assert for a delta using the by assertion alongside the change, increase and decrease assertions. (Related Issues: #339, PRs: #621)

  // You can use `.by` to assert the amount you want something to change
  var obj = { val: 10 };
  var increaseByTwo = function() { obj.val += 2 };
  var decreaseByTwo = function() { obj.val -= 2 };
  var increaseByThree = function() { obj.val += 3 };

  expect(increaseByThree).to.change(obj, 'val').by(3);
  expect(increaseByTwo).to.increase(obj, 'val').by(2);
  expect(decreaseByTwo).to.decrease(obj, 'val').by(2);

  // Please notice that if you want to assert something did change but not by some amount you need to use `.not` **after** the `change` related assertion
  // Take a look at the examples below:
  expect(increaseByThree).to.change(obj, 'val').but.not.by(5)
  expect(increaseByTwo).to.increase(obj, 'val').but.not.by(1)
  expect(decreaseByTwo).to.decrease(obj, 'val').but.not.by(1)

The .keys assertion can now operate on maps and sets. (Related Issues: #632, PRs: #633, #668)

  // The `.keys` assertion now works on `map`s and `set`s natively, like the examples below:
  expect(new Map([[{objKey: 'value'}, 'value'], [1, 2]])).to.contain.key({objKey: 'value'});
  expect(new Map([[{objKey: 'value'}, 'value'], [1, 2]])).to.contain.any.keys([{objKey: 'value'}, {anotherKey: 'anotherValue'}]);
  expect(new Map([['firstKey', 'firstValue'], [1, 2]])).to.contain.all.keys('firstKey', 1);
  expect(new Set([['foo', 'bar'], ['example', 1]])).to.have.any.keys('foo');

  // You can also use `.deep` when asserting agains `Map`s and `Set`s
  expect(new Map([[{objKey: 'value'}, 'value'], [1, 2]])).to.contain.any.deep.keys([{objKey: 'value'}, {anotherKey: 'anotherValue'}]);
  expect(new Map([['firstKey', 'firstValue'], [1, 2]])).to.contain.all.deep.keys('firstKey', 1);
  expect(new Set([['foo', 'bar'], ['example', 1]])).to.have.any.deep.keys('foo');

Add compatibility with strict mode. (Related Issues: #578, PRs: #665)

// This means you can now run your code with the `--use_strict` flag on Node
// If want to understand more about this please read the issue related to this change

Add does and but as new no-op assertion. (Related Issues: #700, #339 PRs: #621, #701)

// You can now write assertions forming phrases with these two new words:
expect(increaseByThree).to.change(obj, 'val').but.not.by(5);
expect(foobar).to.have.property("baz").which.does.not.have.property("thing");

Allow use to be imported using new ES6 module syntax. (Related Issues: #718, PRs: #724)

// You can now import `use` using the ES6 module syntax, like the example below:
import sinonChai from "sinon-chai";
import {expect, use} from "chai";

use(sinonChai);

You can also use require alongside the new ES6 destructuring feature:

const sinonChai = require('sinon-chai');
const {expect, use} = require("chai");

use(sinonChai);

Add ordered flag for members assertion. (Related Issues: #717, PRs: #728)

// You can now use the `ordered` flag to assert the order of elements when using the `members` assertion:
expect([1, 2, 3]).to.include.ordered.members([1, 2]); // This passes
expect([1, 2, 3]).to.include.ordered.members([2, 3]); // This will fail! Read the docs to know more.

Add .own flag to .property assertion. It does the same thing as .ownProperty and cannot be used alongisde the new .nested flag. (Related Issues: #795, PRs: #810)

expect({a: 1}).to.have.own.property('a');

// The example below will thrown an Error
expect({a: {b: 1}}).to.have.own.nested.property('a.b', 1);

Add .deep support to .property assertion. (Related Issues: #795, PRs: #810)

expect({ foo: { bar: 'baz' } }).to.have.deep.own.property('foo', { bar: 'baz' });
expect({ foo: { bar: { baz: 'quux' } } }).to.have.deep.nested.property('foo.bar', { baz: 'quux' });

The .empty assertion will now work with ES6 collections (PRs: #763, #812, #814) Please notice that this assertion will throw an error when it is passed a WeakMap or WeakSet.

expect(new Set()).to.be.empty;
expect(new Map()).to.be.empty;

// The examples below will throw a TypeError:
expect(new WeakSet()).to.be.empty;
expect(new WeakMap()).to.be.empty;

Add script that registers should as a side-effect. This change allows you to register should via a mocha option by using: mocha spec.js -r chai/register-should and also allows you to register the testing style globally. (Issues: #693, PRs: #868)
```
require('chai/register-should');  // Using Should style
```
Add script that registers assert as a side-effect. This change allows you to register assert via a mocha option by using: mocha spec.js -r chai/register-assert (Issues: #693, PRs: #868, #872)

 require('chai/register-assert');  // Using Assert style

Add script that registers expect as a side-effect. This change allows you to register expect via a mocha option by using: mocha spec.js -r chai/register-expect (Issues: #693, PRs: #868, #872)
```
require('chai/register-expect');  // Using Expect style
```
When the length assertion is chained directly off of an uninvoked method, it references function’s built-in length property instead of Chai’s length assertion. This commit adds a guard to Chai methods to detect this problem and throw a helpful error message that advises the user on how to correct it. (Issues: #684, #841, PRs: #897)
Allows the lockSsfi flag to be set when creating new Assertion. This flag controls whether or not the given ssfi flag should retain its current value, even as assertions are chained off of this object. This is usually set to true when creating a new assertion from within another assertion. It’s also temporarily set to true before an overwritten assertion gets called by the overwriting assertion. (Issues: #878, #904, PRs: #922)
```
// This will lock the stack stack function from this line on
// The SSFI is the reference to the starting point for removing irrelevant frames from the stack trace
new Assertion(obj, msg, ssfi, true).to.have.property('length')
```

The nestedInclude, deepNestedInclude, ownInclude and deepOwnInclude assertions and there negated pairs were added to the assert interface. (Issues: #905, PRs: #964)

// '[]' and '.' in property names can be escaped using double backslashes.

assert.nestedInclude({'.a': {'b': 'x'}}, {'\\.a.[b]': 'x'});
assert.notNestedInclude({'.a': {'b': 'x'}}, {'\\.a.b': 'y'});

assert.deepNestedInclude({a: {b: [{x: 1}]}}, {'a.b[0]': {x: 1}});
assert.notDeepNestedInclude({a: {b: [{x: 1}]}}, {'a.b[0]': {y: 1}});

assert.ownInclude({ a: 1 }, { a: 1 });
assert.notOwnInclude({ a: 1 }, { b: 2 });

assert.deepOwnInclude({a: {b: 2}}, {a: {b: 2}});
assert.notDeepOwnInclude({a: {b: 2}}, {a: {c: 3}});

Bug Fixes

Fix missing msg argument for change related assertions. (Related Issues: None, PRs: #606)
The addMethod function returns a new assertion with flags copied over instead of this. (Related Issues: #562, #684, #723, PRs: #642, #660)
Fix stacktraces for overwritten properties and methods. (Related Issues: #659, #661)
Fix bug when testing Symbol equality with should syntax. (Related Issues: #669, PRs: #672)
Fix bug when asserting some valid ES6 keys. (Related Issues: #674, PRs: #676)
Fix bug caused when custom inspect method is used and this method calls stylize. (PRs: #680)
Fix ownProperty on objects with no prototype. (Related Issues: #688, PRs: #689)
Fix swapped expected and actual results for the .members assertion. (Related Issues: #511, PRs: #702)
Fix same.members to properly handle duplicates by treating each one as a unique member. (Related Issues: #590, PRs: #739)
Fix deep.have.same.members() that was not printing a diff. (PRs: #509)
Diff will be shown for assert’s equal and notEqual methods. (Related Issues: #790, PRs: #794)
The overwriteMethod, overwriteProperty, addChainableMethod, overwriteChainableMethod functions will return new assertion with flags copied over instead of this. (Related Issues: #562, #642, #791, PRs: #799)
Proxy-related implementation frames were showing up in the stack traces for failed property assertions. Now we remove them by setting the proxy getter (instead of the property getter) as the starting point to remove all implementation frames. (PRs: #884)
Negated keys assertions will now consider size of sets. (Related Issues: #919, PRs: #924)
Whenever passed something that is not an instance of a function, the instanceof assertion will now throw an error informing the user that he should pass a constructor to this assertion, but instead he has passed . _(Related Issues: #893, PRs: #924)_
Fix all issues on expect/should interfaces that were causing implementation frames to leak through. Also improve the internal Chai error test helper to validate stack traces. (Issues: #878, #904, PRs: #922)
This fixes custom messages not always being respected. (Issues: #922, #947, #904, PRs: #916, #923)
Fix PhantomJS 1.x incompatibility (Issues: #966, PRs: #890)

4.0.0-canary.2 / 2017-04-17

Version 4.0.0-canary.2

4.0.0-canary.2 / 2017-04-17

The second release in the 4.0.0 canary release cycle. Around the end of April, barring any showstopper bugs, this will be released as 4.0.0

Breaking Changes

We’ve dropped support for Node 0.10 (See #816) and Node 0.12 (#901). The minimum supported version is now Node 4.0. If you wish to continue using older versions of Node with Chai, please continue to use the 3.5.0 release. Officially, version 4.0.0 of chai supports Nodes 4, 6, 7, as well as the moving LTS version (currently 6.10.2). We plan to support Node 4 until at-least April 2018 (inline with Node Foundation support).
.not.keys on its own is now the equivalent of .not.all.keys (#924). The docs mention this, and suggest to always pair .keys with something.

New Features

Added side-effectful “register” style scripts for each interface, (See #868). This allows one to require('chai/should') which will automatically call global.should = chai.should(). This is useful for users who wish to automatically get a global for their codebase. Read the docs for more info!
Added the browser field to the package json (#875). This will help with browser bundling tools.
Added .own.include and .nested.include (#926).
If you attempt to call .throws with something that isn’t a constructor (like TypeError) you will now get a more helpful error message about this. (before it would say Right-hand side of 'instanceof' is not an object, now it says The instanceof assertion needs a constructor but <type> was given). (#899).

Bug Fixes

Minor update deep-eql to 2.0.1 (#871) which fixes bugs around Memoization, and lets comparators override evaluation of primitives.
We’ve updated documentation and error handling around using .length (#897) - which can, in some cases, be problematic. Typically, you’ll want to use .lengthOf instead - but the documentation now makes this clear, and errors are more helpful when bad things happen.
We’ve improved stack traces to strip out chai’s internals, especially in newer environments which use Proxies (#884 and #922).
We’ve gone through and made sure every assertion honors the custom error message you pass it - some didn’t! (#947).
getFuncName has had an update since we pulled out the behaviour in 4.0.0-canary.1 (#915). Practically this doesn’t change anything in Chai.

Documentation

Added LICENSE file (#854)
Major documentation rewrite including the plugin docs (#911), .throw docs (#866), and, well, just about all other docs (#920). These docs are far more detailed now, explaining caveats, algorithms and best practices.