Develop Reference

How to link the imported dependencies of module created by vm.SourceTextModule to it?

Let's say we are creating a module called app by constructing a new vm.SourceTextModule object:
const context = {
exports: {},
console, // custom console object
};
const sandbox = vm.createContext(context);
const app = new vm.SourceTextModule(
`import path from 'path';
console.log(path.resolve('./src'));`,
{
context: sandbox,
}
);
According to the Node.js documentation to obtain the default export from path module we should "link" the imported dependencies of app module to it.
To achieve this we should pass linker callback to app.link method:
async function linker(specifier, referencingModule) {
// the desired logic...
}
await app.link(linker);
How to implement linker function properly so that we could import path module in newly created app module and use it:
await app.evaluate(); // => /home/user/Documents/project/src
P.S. We are using TypeScript, so I checked if we have installed types for path package.
package.json:
"#types/node": "^17.0.31",

I found https://github.com/nodejs/node/issues/35848 where someone posted a code snippet.
From there I've adapted the following linker callback:
const imports = new Map();
async function linker(specifier, referencingModule) {
if (imports.has(specifier))
return imports.get(specifier);
const mod = await import(specifier);
const exportNames = Object.keys(mod);
const imported = new vm.SyntheticModule(
exportNames,
() => {
// somehow called with this === undefined?
exportNames.forEach(key => imported.setExport(key, mod[key]));
},
{ identifier: specifier, context: referencingModule.context }
);
imports.set(specifier, imported);
return imported;
}
The code snippet from the GitHub issue didn't work for me on Node 18.7.0 as is, because the evaluator callback passed to the constructor of SyntheticModule is somehow called with this set to undefined. This may be a Node bug.
I also cached the imported SyntheticModules in a Map because if they have internal state, creating a new SyntheticModule every time will reset that state.

Jest mock window object before importing a dependency

I need to have a value set in the window object before a dependency is imported. Say I have this code
// foo.test.js
import { dependency } from './foo'
describe('...', () => {
it('...', () => {
// use dependency
})
})
But for dependency to be imported I need to have a value defined in window.myValues
// foo.js
export const dependency = {
key: window.myValue.nestedValue
}
That code will give me an error when importing the file because window.myValue.nestedValue is trying to access the property nestedValue of undefined.
How can I get that done?
Edit
Following christianeide's answer below I get the following error
● Test suite failed to run
TypeError: Cannot convert undefined or null to object
2 | delete global.window.myValue
3 | global.window = Object.create(window)
> 4 | global.window.myValue = {
| ^
5 | nestedValue: 'someValue'
6 | }
7 | }
at module.exports (jest.setup.js:4:17)
at node_modules/#jest/core/build/runGlobalHook.js:82:17
at ScriptTransformer.requireAndTranspileModule (node_modules/#jest/transform/build/ScriptTransformer.js:684:24)
at node_modules/#jest/core/build/runGlobalHook.js:72:27
at pEachSeries (node_modules/p-each-series/index.js:8:9)
at async _default (node_modules/#jest/core/build/runGlobalHook.js:58:5)
at async runJest (node_modules/#jest/core/build/runJest.js:345:5)

es6 imports are "hoisted", meaning wherever you write them in the code, they'll get processed before the importing module is executed, so the imported module is always executed before the importing module. In your case, this means foo.js executes before foo.test.js, so even if you properly mock your property of window in your tests, foo.js will not see your mock.
You can get around this by using require in your tests to import foo.js after the property of window has been mocked.
// foo.test.js
window.myValue = { nestedValue: MOCK_NESTED_VALUE };
const { dependency } = require('./foo');
describe('...', () => {
it('...', () => {
// use dependency
})
})
As other answers have pointed out, if myValue is one of existing "sytem" properties of window such as window.location, you might have to delete it first. When deleting, don't forget to back it up so that you can restore it when cleaning up after your test.

Try assigning values to global.window.
Something like this:
delete global.window.myValue;
global.window = Object.create(window);
global.window.myValue = {
nestedValue: 'someValue',
};
This can be done inside jest.setup.js, but you could also probably define the values inside foo.test.js

I was able to append properties on the window object by creating a setup.js:
global.propertyA = () => {};
global.nestedPropertyB = {
propertyC: () => {}
};
And setting the file in setupFilesAfterEnv in my jest.config.js file:
module.exports = {
setupFilesAfterEnv: ['<rootDir>/tests/js/setup.js']
}

I have solved such an issue with jest.mock: as it's hoisted above import statements, it's possible to execute any code before and after importing an actual module.
In my case I needed to test some functionality and set node env to a value, different from 'test', and, as I needed this substitution only for the time of import, I set its value back to 'test' before returning the actual module. Of course, it may vary depending on use cases - it's just an example of executing code before and after importing is done. Also i tried to use a getter for a particular module field, and it also worked.
import ApiBase from "../ApiBase";
jest.mock('../ApiBase', () => {
process.env.NODE_ENV = '';
const actual = jest.requireActual('../ApiBase');
process.env.NODE_ENV = 'test';
return actual;
});

TypeError: "exports" is read-only => When exporting a module with method calling another method

I'm new to webpack and this module bundler stuff and I'm currently just experimenting around what's possible and what isn't.
Here, I'm trying the following:
//appECommerce.js
import eCommerceLogic from './lib/eCommerceLogic.js'
//eCommerceLogic.js
import name from './eCommerceJSExportTests.js';
module.exports = {
productnamesOnclick: function(){
$("#AJAXproductnames").on("click",function(){
getProductnameElements()
})
},
productNamesGetter: function(){
async function getProductnameElements(){
let productNameElements = document.getElementsByClassName('customProductCardName')
console.log("result is ", productNameElements)
let test = await name.commonAJAXCall()
console.log(test)
}
}
}
//eCommerceJSExportTests.js
module.exports = {
commonAJAXCall: function() {
//return "helloExport"
return $.get('https://jsonplaceholder.typicode.com/todos/1', {
}).then((response) => {
response = JSON.stringify(response)
console.log(response)
console.log("AJAX happened")
return response
})
}
}
So basically, I just want to know why I get this error (see title)^^
Furthermore though, I'd also like to know three specific things:
1) Is it possible to have module B import from module C and then export to module A, where the content of module B's import eventually consists of both its "own" code and the code imported from module C, since module B makes use of the imported properties and methods from module C there?
2) In code which is exported like shown above, is it actually possible to have method A containing a call to method B?
3) Can code which attaches event listeners to the DOM even be exported?

Try This:
export { toBeExported as toBeExported };

How to add global commands to Jest like describe and it?

I'm writing a little testing tool for Jest (just to learn). It is called assertTruthy(msg, fn, args), expects a message, a function and arguments and should pass if the thing that is returned when the function is invoked with the arguments is truthy and fail if its not.
I would like to add it to Jest, so that you could call it without importing it in every environment.
describe('someFn()', () => {
// like 'it', 'describe' and 'expect' it should just be available here
it('is a function', () => {
expect(typeop someFN).toEqual('Function');
});
assertTruthy('it should return true', someFn, 3, 4);
});
I know Jest has setupFiles and setupFilesAfterEnv but I can't figure out how to use them to achieve this.
How do you add commands to Jest?
PS: On a single project basis (in CRA) I managed to do this like this:
// in src/setupFiles.js
const assertTruthy = // function definition ...
global.assertTruthy = assertTruthy

Add global functions to Jest
In order to add global functions to jest, you need to define setupFiles in your configuration and attach that function to the global object in the setup file:
module.exports = {
// ...
setupFiles: ['<rootDir>/setupFile.js'],
// ...
};
so if you want to do something very similar to it, I would suggest you do something like this:
// /setupFile.js
// in order to change an existing function (not youre case):
global.it = function(description, fn) { /* ... */ };
// this is how you define a new function globally
global.assertTruthy = (message, func, ...args) => {
return global.it(message, () => {
expect(func(...args)).toBeTruthy();
});
// optional: implementing the same interface as `jest.It`
Supporting the same interface as jest.It
Here's an example from the Airbnb library airbnb/jest-wrap where they wrapped the describe function. If you want to implement the jest.It interface, you'll also need to implement assertTruthy.todo, assertTruthy.skip, assertTruthy.only, & assertTruthy.each (check out the it interface). todo and skip are pretty easy since you want to do exactly the same as the original ones.
For each & only, we need to change the it function inside our implementation. A simple way to support only is by using a closure and passing the correct it function from the closure's inpu. each can be a little more complex to implement.
// /setupFile.js
// normaly, use the jest `it` function
global.assertTruthy = assertTruthyCreator(it);
// bypass for todo and skip
global.assertTruthy.todo = global.it.todo;
global.assertTruthy.skip = global.it.skip;
// only calls the same function but uses `only` internaly
global.assertTruthy.only = assertTruthyCreator(it.only);
// special case which needs special implementation
// see usage below
global.assertTruthy.each = assertTruthyCreator(it.each, true);
function assertTruthyCreator(itFunc, withTable) {
if (withTable) {
return (message, func, ...args) => {
return itFunc(args)(message, (...caseArgs) => {
expect(func(...caseArgs)).toBeTruthy();
});
};
}
return (message, func, ...args) => {
return itFunc(message, () => {
expect(func(...args)).toBeTruthy();
});
};
}
// usage:
assertTruthy.each(
'add numbers',
(a, b) => a + b,
[2, 4],
[4, 5],
[7, 9]);
How to use in test files
If you're using typescript for writing jest test, the first thing you'll need to do is declare your new function somewhere:
interface IGenericFunction {
(...args: any[]): any;
}
declare const assertTruthy: (message: string, func: IGenericFunction, ...args: any[]) => any;
With javascript, you can skip that step.
After that, just use it like you use describe and it:
const funcToTest = (a: number, b: number) => a + b;
describe("Test Suite", () => {
assertTruthy('this ran with assertTruthy', funcToTest, 5, 3);
test("another test", () => {
// ...
});
});
and jest will treat this as any other it function
Using as a node_module dependency
If you want to create a library from this, you can basically just pass a node_modules path to the setupFiles array.
For example, with this repository, you can do as follows:
Install using npm install --save-dev #kibibit/jest-utils
Add the following to your jest configuration
Use the function as described above
module.exports = {
// ...
setupFiles: ['node_modules/#kibibit/jest-utils/lib/jest-utils.js'],
// ...
};
and it should work the same as importing it locally.

Mock dependency in Jest with TypeScript

When testing a module that has a dependency in a different file and assigning that module to be a jest.mock, TypeScript gives an error that the method mockReturnThisOnce (or any other jest.mock method) does not exist on the dependency, this is because it is previously typed.
What is the proper way to get TypeScript to inherit the types from jest.mock?
Here is a quick example.
Dependency
const myDep = (name: string) => name;
export default myDep;
test.ts
import * as dep from '../depenendency';
jest.mock('../dependency');
it('should do what I need', () => {
//this throws ts error
// Property mockReturnValueOnce does not exist on type (name: string)....
dep.default.mockReturnValueOnce('return')
}
I feel like this is a very common use case and not sure how to properly type this.

You can use type casting and your test.ts should look like this:
import * as dep from '../dependency';
jest.mock('../dependency');
const mockedDependency = <jest.Mock<typeof dep.default>>dep.default;
it('should do what I need', () => {
//this throws ts error
// Property mockReturnValueOnce does not exist on type (name: string)....
mockedDependency.mockReturnValueOnce('return');
});
TS transpiler is not aware that jest.mock('../dependency'); changes type of dep thus you have to use type casting. As imported dep is not a type definition you have to get its type with typeof dep.default.
Here are some other useful patterns I've found during my work with Jest and TS
When imported element is a class then you don't have to use typeof for example:
import { SomeClass } from './SomeClass';
jest.mock('./SomeClass');
const mockedClass = <jest.Mock<SomeClass>>SomeClass;
This solution is also useful when you have to mock some node native modules:
import { existsSync } from 'fs';
jest.mock('fs');
const mockedExistsSync = <jest.Mock<typeof existsSync>>existsSync;
In case you don't want to use jest automatic mock and prefer create manual one
import TestedClass from './TestedClass';
import TestedClassDependency from './TestedClassDependency';
const testedClassDependencyMock = jest.fn<TestedClassDependency>(() => ({
// implementation
}));
it('Should throw an error when calling playSomethingCool', () => {
const testedClass = new TestedClass(testedClassDependencyMock());
});
testedClassDependencyMock() creates mocked object instance
TestedClassDependency can be either class or type or interface

Use the mocked helper
like explained here
// foo.spec.ts
import { foo } from './foo'
jest.mock('./foo')
// here the whole foo var is mocked deeply
const mockedFoo = jest.mocked(foo, true)
test('deep', () => {
// there will be no TS error here, and you'll have completion in modern IDEs
mockedFoo.a.b.c.hello('me')
// same here
expect(mockedFoo.a.b.c.hello.mock.calls).toHaveLength(1)
})
test('direct', () => {
foo.name()
// here only foo.name is mocked (or its methods if it's an object)
expect(jest.mocked(foo.name).mock.calls).toHaveLength(1)
})

There are two solutions tested for TypeScript version 3.x and 4.x, both are casting desired function
1) Use jest.MockedFunction
import * as dep from './dependency';
jest.mock('./dependency');
const mockMyFunction = dep.myFunction as jest.MockedFunction<typeof dep.myFunction>;
2) Use jest.Mock
import * as dep from './dependency';
jest.mock('./dependency');
const mockMyFunction = dep.default as jest.Mock;
There is no difference between these two solutions. The second one is shorter and I would therefore suggest using that one.
Both casting solutions allows to call any jest mock function on mockMyFunction like mockReturnValue or mockResolvedValue
https://jestjs.io/docs/en/mock-function-api.html
mockMyFunction.mockReturnValue('value');
mockMyFunction can be used normally for expect
expect(mockMyFunction).toHaveBeenCalledTimes(1);

I use the pattern from #types/jest/index.d.ts just above the type def for Mocked (line 515):
import { Api } from "../api";
jest.mock("../api");
const myApi: jest.Mocked<Api> = new Api() as any;
myApi.myApiMethod.mockImplementation(() => "test");

Cast as jest.Mock
Simply casting the function to jest.Mock should do the trick:
(dep.default as jest.Mock).mockReturnValueOnce('return')

Use as jest.Mock and nothing else
The most concise way of mocking a module exported as default in ts-jest that I can think of really boils down to casting the module as jest.Mock.
Code:
import myDep from '../dependency' // No `* as` here
jest.mock('../dependency')
it('does what I need', () => {
// Only diff with pure JavaScript is the presence of `as jest.Mock`
(myDep as jest.Mock).mockReturnValueOnce('return')
// Call function that calls the mocked module here
// Notice there's no reference to `.default` below
expect(myDep).toHaveBeenCalled()
})
Benefits:
does not require referring to the default property anywhere in the test code - you reference the actual exported function name instead,
you can use the same technique for mocking named exports,
no * as in the import statement,
no complex casting using the typeof keyword,
no extra dependencies like mocked.

The latest jest allows you to do this very easily with jest.mocked
import * as dep from '../dependency';
jest.mock('../dependency');
const mockedDependency = jest.mocked(dep);
it('should do what I need', () => {
mockedDependency.mockReturnValueOnce('return');
});

Here's what I did with jest#24.8.0 and ts-jest#24.0.2:
source:
class OAuth {
static isLogIn() {
// return true/false;
}
static getOAuthService() {
// ...
}
}
test:
import { OAuth } from '../src/to/the/OAuth'
jest.mock('../src/utils/OAuth', () => ({
OAuth: class {
public static getOAuthService() {
return {
getAuthorizationUrl() {
return '';
}
};
}
}
}));
describe('createMeeting', () => {
test('should call conferenceLoginBuild when not login', () => {
OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
return false;
});
// Other tests
});
});
This is how to mock a non-default class and it's static methods:
jest.mock('../src/to/the/OAuth', () => ({
OAuth: class {
public static getOAuthService() {
return {
getAuthorizationUrl() {
return '';
}
};
}
}
}));
Here should be some type conversion from the type of your class to jest.MockedClass or something like that. But it always ends up with errors. So I just used it directly, and it worked.
test('Some test', () => {
OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
return false;
});
});
But, if it's a function, you can mock it and do the type conversation.
jest.mock('../src/to/the/Conference', () => ({
conferenceSuccessDataBuild: jest.fn(),
conferenceLoginBuild: jest.fn()
}));
const mockedConferenceLoginBuild = conferenceLoginBuild as
jest.MockedFunction<
typeof conferenceLoginBuild
>;
const mockedConferenceSuccessDataBuild = conferenceSuccessDataBuild as
jest.MockedFunction<
typeof conferenceSuccessDataBuild
>;

As of Jest 24.9.0 here is how you can mock and correctly type both your Class/Object/function and Jest properties.
jest.MockedFunction
jest.MockedClass
What we would like for a typed mock is that the mocked object type contains the union of the mocked object type and the type of Jest mocks.
import foo from 'foo';
jest.mock('foo');
const mockedFoo = foo as jest.MockedFunction<typeof foo>;
// or: const mockedFooClass = foo as jest.MockedClass<typeof FooClass>;
mockedFoo.mockResolvedValue('mockResult');
// Or:
(mockedFoo.getSomething as jest.MockedFunction<typeof mockedFoo.getSomething>).mockResolvedValue('mockResult');
As you can see, you can either manually cast what you need or you'll need something to traverse all foo's properties/methods to type/cast everything.
To do that (deep mock types) you can use jest.mocked() introduced in Jest 27.4.0
import foo from 'foo';
jest.mock('foo');
const mockedFoo = jest.mocked(foo, true);
mockedFoo.mockImplementation() // correctly typed
mockedFoo.getSomething.mockImplementation() // also correctly typed

I have found this in #types/jest:
/**
* Wrap a function with mock definitions
*
* #example
*
* import { myFunction } from "./library";
* jest.mock("./library");
*
* const mockMyFunction = myFunction as jest.MockedFunction<typeof myFunction>;
* expect(mockMyFunction.mock.calls[0][0]).toBe(42);
*/
Note: When you do const mockMyFunction = myFunction and then something like mockFunction.mockReturnValue('foo'), you're a changing myFunction as well.
Source: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L1089

The top rated solution by Artur Górski does not work with the last TS and Jest.
Use MockedClass
import SoundPlayer from '../sound-player';
jest.mock('../sound-player'); // SoundPlayer is now a mock constructor
const SoundPlayerMock = SoundPlayer as jest.MockedClass<typeof SoundPlayer>;

A recent library solves this problem with a babel plugin: https://github.com/userlike/joke
Example:
import { mock, mockSome } from 'userlike/joke';
const dep = mock(import('./dependency'));
// You can partially mock a module too, completely typesafe!
// thisIsAMock has mock related methods
// thisIsReal does not have mock related methods
const { thisIsAMock, thisIsReal } = mockSome(import('./dependency2'), () => ({
thisIsAMock: jest.fn()
}));
it('should do what I need', () => {
dep.mockReturnValueOnce('return');
}
Be aware that dep and mockReturnValueOnce are fully type safe. On top, tsserver is aware that depencency was imported and was assigned to dep so all automatic refactorings that tsserver supports will work too.
Note: I maintain the library.

This is ugly, and in fact getting away from this ugliness is why I even looked at this question, but to get strong typing from a module mock, you can do something like this:
const myDep = (require('./dependency') as import('./__mocks__/dependency')).default;
jest.mock('./dependency');
Make sure you require './dependency' rather than the mock directly, or you will get two different instantiations.

For me this was enough:
let itemQ: queueItemType
jest.mock('../dependency/queue', () => {
return {
add: async (item: queueItemType, ..._args: any) => {
// then we can use the item that would be pushed to the queue in our tests
itemQ = item
return new Promise(resolve => {
resolve('Mocked')
})
},
}
})
Then, whenever the add method is called it will execute this code above instead of pushing it to the queue, in this case.

With TypeScript 2.8 we can do like this with ReturnType:
import * as dep from "./depenendency"
jest.mock("./dependency")
const mockedDependency = <jest.Mock<ReturnType<typeof dep.default>>>dep.default
it("should do what I need", () => {
mockedDependency.mockReturnValueOnce("return")
})