jest: toEqual ignores keys with undefined properties

If a property is undefined, Jest appears to ignore it when calling .toEqual().

import _ from "lodash"; describe("test", () => { it("returns an object with the correct nodes", () => { let a = {"key1": undefined, "key2": "a"}; let b = {"key2": "a"}; expect(a).toEqual(b); expect(_.isEqual(a, b)).toBeFalsy(); }); });

This simple test will pass. As you can see by adding lodash, it should be false.

My output: Using Jest CLI v0.8.2, jasmine1 Running 1 test suite… PASS processors\hockey__tests__\events.js (0.573s) 1 test passed (1 total in 1 test suite, run time 1.076s) ----------|----------|----------|----------|----------|----------------|

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Lines
----------	----------	----------	----------	----------	----------------
All files	100	100	100	100
----------	----------	----------	----------	----------	----------------

About this issue

Original URL
State: closed
Created 8 years ago
Reactions: 7
Comments: 16 (4 by maintainers)

Commits related to this issue

fix(merge): - split the implem into merge(target, ...sources) and _merge(target, source), I think it's more simple this way. - I reorganized some of the conditions - I mostly kept the description and... — committed to algolia/algoliasearch-helper-js by tkrugg 5 years ago
fix(merge): use .toStrictEqual instead of .toEqual in tests cf. https://github.com/facebook/jest/issues/711 `undefined` values being critical to in assessing merge behaviour this is necessary — committed to algolia/algoliasearch-helper-js by tkrugg 5 years ago
fix(merge): change implementation (#716) * fix(merge): use .toStrictEqual instead of .toEqual in tests cf. https://github.com/facebook/jest/issues/711 `undefined` values being critical to in ... — committed to algolia/algoliasearch-helper-js by tkrugg 5 years ago
fix(merge): change implementation (#716) * fix(merge): use .toStrictEqual instead of .toEqual in tests cf. https://github.com/facebook/jest/issues/711 `undefined` values being critical to in ... — committed to algolia/algoliasearch-helper-js by tkrugg 5 years ago

Most upvoted comments

Is anyone concerned with this? I got a bug today because of this issue.

mike1808 on Mar 19, 2018

It might help if the docs for toEqual mentioned the ways in which it differs from toStrictEqual instead of you having to discover toStrictEqual on your own.

glasser on Aug 25, 2020

This is sorta fixed, we have toStrictEqual now.

EDIT: based on this comment, I’ll call this issue fixed:

would be impossible to roll out at FB. Having a separate strict matcher in Jest is the better option.

Slightly a footgun that’s it’s not the default, but legacy.

If you want to enforce usage in your code base, you can use https://github.com/jest-community/eslint-plugin-jest/pull/134, which can autofix your code

SimenB on Sep 19, 2018

Hey guys, what about adding a feature switch for us open source users to use isEqual() without strange JSON.stringify() related surprises?

peter-leonov on Jan 27, 2017

@SimenB For now I have a couple workarounds (e.g. using toHaveProperty) but I was hoping there was some timeline on a fix or at the least a note in the docs about this issue.

JasonCust on Dec 18, 2017