The assertion of `expect.toBeInstanceOf(Object)` fails in Jest

Question

The assertion of `expect.toBeInstanceOf(Object)` fails in Jest

Just wanted to verify if we're exporting an object, but the test is failing as shown below:

import * as Foo from './foo';

describe('Foo', () => {
  test('should export an object', () => {
    expect(Foo).toBeInstanceOf(Object);
  });
});

encountering this error message:

https://i.sstatic.net/j9FoR.png

However, I found a workaround using typeof:

import * as Foo from './foo';

describe('Foo', () => {
  test('should export an object', () => {
-    expect(Foo).toBeInstanceOf(Object);
+    expect(typeof LivingAppsCoreReact).toEqual('object');
  });
});

I am curious to know why Jest doesn't recognize the Object constructor as... the Object constructor.

Confirmed that the object is indeed exporting the correct keys ✅

Environment details:

$» npm ls jest                                                            1 ↵
<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="04697d2974766b6e61677044342a37352a31">[email protected]</a>
├── <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="771d12040337454e59445946">[email protected]</a>
└─┬ <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="9aeee9b7f0ffe9eedaa8a3b4aab4a9">[email protected]</a>
  └── <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="28424d5b5c681a11061b0619">[email protected]</a> deduped

javascript typescript jestjs javascript-objects

Answer 1

Answer №1

It appears there may be a potential issue with Jest and Node regarding global variables, as the environments of Jest and Node differ from one another. For more information, please refer to the following links:

If you are using a default export, your exported element will be of type Object. When comparing this to "Jest's Object type," it may cause the test to fail due to the different contexts.

This situation is also connected to scenarios like:

To resolve this issue in your code, consider using object destructuring if you are utilizing named exports (e.g export const Foo = {...}). This will allow Jest to evaluate your objects within the same context.

In other words, transition from

import * as Foo from './foo';

To

import { Foo } from './foo';

Alternatively, you can verify object properties using .toMatchObject(object) instead of checking if the Object is an Object.

Answer 2

It appears there may be a potential issue with Jest and Node regarding global variables, as the environments of Jest and Node differ from one another. For more information, please refer to the following links:

If you are using a default export, your exported element will be of type Object. When comparing this to "Jest's Object type," it may cause the test to fail due to the different contexts.

This situation is also connected to scenarios like:

To resolve this issue in your code, consider using object destructuring if you are utilizing named exports (e.g export const Foo = {...}). This will allow Jest to evaluate your objects within the same context.

In other words, transition from

import * as Foo from './foo';

To

import { Foo } from './foo';

Alternatively, you can verify object properties using .toMatchObject(object) instead of checking if the Object is an Object.

Answer 3

Answer №2

The Debate Between Jest Globals and Node Globals

One of the ongoing discussions revolves around the differences between Jest and Node globals. It's essential to understand that there are distinctions between the Jest global variables and the Node global variables. For more detailed information, you can refer to this open issue on Github: Jest globals differ from Node globals.

Simply put, the behavior of the toBeInstanceOf function varies depending on whether it is operating on instances of classes that you have created or instances of the global Array class. When asserting against the example provided above, using the toBeInstanceOf function is recommended. Take a look at the documentation for toBeInstanceOf and the code snippet below for clarification.

This information is sourced from the documentation itself.

describe('toMatchObject applied to arrays', () => {
  test('the number of elements must match exactly', () => {
    expect([{foo: 'bar'}, {baz: 1}]).toMatchObject([{foo: 'bar'}, {baz: 1}]);
  });

  test('.toMatchObject is called for each element, allowing for extra object properties', () => {
    expect([{foo: 'bar'}, {baz: 1, extra: 'quux'}]).toMatchObject([
      {foo: 'bar'},
      {baz: 1},
    ]);
  });
});

Answer 4

The Debate Between Jest Globals and Node Globals

One of the ongoing discussions revolves around the differences between Jest and Node globals. It's essential to understand that there are distinctions between the Jest global variables and the Node global variables. For more detailed information, you can refer to this open issue on Github: Jest globals differ from Node globals.

Simply put, the behavior of the toBeInstanceOf function varies depending on whether it is operating on instances of classes that you have created or instances of the global Array class. When asserting against the example provided above, using the toBeInstanceOf function is recommended. Take a look at the documentation for toBeInstanceOf and the code snippet below for clarification.

This information is sourced from the documentation itself.

describe('toMatchObject applied to arrays', () => {
  test('the number of elements must match exactly', () => {
    expect([{foo: 'bar'}, {baz: 1}]).toMatchObject([{foo: 'bar'}, {baz: 1}]);
  });

  test('.toMatchObject is called for each element, allowing for extra object properties', () => {
    expect([{foo: 'bar'}, {baz: 1, extra: 'quux'}]).toMatchObject([
      {foo: 'bar'},
      {baz: 1},
    ]);
  });
});

The assertion of `expect.toBeInstanceOf(Object)` fails in Jest

Answer №1

Answer №2

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