As the name implies, these methods will be called before and after each test run. Testing applications can seem like a fairly complicated concept, and thus, many programmers avoid it due to the fear of failure especially in the Node.js world, where testing applications are not so ubiquitous as in, say, Java, and the resources on testing are scarce. You signed in with another tab or window. The following example will always produce the same output. working in both node and jsdom. In order to mock something effectively you must understand the API (or at least the portion that you're using). The Apphas 3 state variables initialized with the useStatehook, those are nationalities, message, and personName. So, now that we know why we would want to mock out fetch, the next question is how do we do it? So if you want to ignore the exact timing and only care about the order then perhaps you can use jest.runAllTimers() to fast forward in time and exhaust all the queues, and then toHaveBeenNthCalledWith() to verify them? So it turns out that spying on the setTimeout function works for both window or global as long as I register the spy in all tests making an assertion on it being called. Jest is a popular testing framework for JavaScript code, written by Facebook. It had all been set up aptly in the above set up section. It comes with a lot of common testing utilities, such as matchers to write test assertions and mock functions. I had the chance to use TypeScript for writing lambda code in a Node.js project. Check all three elements to be in the document. as in example? (Use Case: function A requires an argument of interface type B and I want to test function As behavior when I pass an argument that does not match interface B. And similarly, if you need to verify that callbacks are scheduled with a particular time or interval, it would make sense to use jest.advanceTimersByTime() and make assertions based on what you expect to happen at different points in time. For the button element, it is fetched by passing the name which is the text in the button. The usual case is to check something is not called at all. This suggests that the documentation demonstrates the legacy timers, not the modern timers. NFT is an Educational Media House. Asking for help, clarification, or responding to other answers. . You can spyOn an async function just like any other. After looking at Jasmine documentation, you may be thinking theres got to be a more simple way of testing promises than using setTimeout. Since yours are async they don't need to take a callback. A:By TypeScripts nature, passing an invalid type as an argument to function A will throw a compile error because the expected and actual argument types are incompatible. This is the compelling reason to use spyOnover mock where the real implementation still needs to be called in the tests but the calls and parameters have to be validated. The mock responds following thefetchAPI having attributes like status and ok. For any other input for example if the name chris or any other URL, the mock function will throw an Error indicating Unhandled requestwith the passed-in URL. It looks something like this: Here, we have two methods, selectUserById and createUser (normally there would be methods to update and delete users, but to keep this example short we will exclude those). Next, let's skip over the mocking portion for a sec and take a look at the unit test itself. It creates a mock function similar to jest.fn() but also tracks calls to object[methodName]. This means that the implementations of mock functions are reset before each test. This post will show you a simple approach to test a JavaScript service with an exported function that returns a promise. In the above implementation we expect the request.js module to return a promise. I copied the example from the docs exactly, and setTimeout is not mocked. You can create a mock function with jest.fn (). Thanks for reading. You don't need to rewrite the entire functionality of the moduleotherwise it wouldn't be a mock! Can I use spyOn() with async functions and how do I await them? So my question is: How can I make a mock / spy function in jest that reads as an async function? I eventually want to also be able to mock what the return data will be, but first I wanted to just check that the hook had been called. Connect and share knowledge within a single location that is structured and easy to search. This is the part testing for an edge case. privacy statement. // Testing for async errors using Promise.catch. Let's implement a module that fetches user data from an API and returns the user name. Here, we have written some tests for our selectUserById and createUser functions. As much as possible, try to go with the spyOn version. Doing so breaks encapsulation and should be avoided when possible. In this post, you will learn about how to use JestsspyOnmethod to peek into calls of some methods and optionally replace the method with a custom implementation. It is useful when you want to watch (spy) on the function call and can execute the original implementation as per need. I confirm that I also get ReferenceError: setTimeout is not defined in 27.0.3, the scenario is as follows: Test A passes, but code executed by Test B fails, console.log(setTimeout) in that code returns undefined. Line 3 calls setTimeout and returns. The code for this example is available at examples/async. If we have a module that calls an API, it's usually also responsible for dealing with a handful of API scenarios. After that, wrote a test for an edge case if the API fails. It also comes bundled with many popular packages likeReactwith the Create React App (CRA) andNest JS. It posts those diffs in a comment for you to inspect in a few seconds. In addition to being able to mock out fetch for a single file, we also want to be able to customize how fetch is mocked for an individual test. The test needs to wait for closeModal to complete before asserting that navigate has been called.. closeModal is an async function so it will return a Promise. On a successful response, a further check is done to see that the country data is present. Of course, you still need to add return before each expect statement. Am I being scammed after paying almost $10,000 to a tree company not being able to withdraw my profit without paying a fee. Both vi.fn() and vi.spyOn() share the same methods, however only the return result of vi.fn() is callable. What happens when that third-party API is down and you can't even merge a pull request because all of your tests are failing? The text was updated successfully, but these errors were encountered: if you are using jest 27, it uses modern timers now by default First, enable Babel support in Jest as documented in the Getting Started guide. In fact, Jest provides some convenient ways to mock promise calls. The fireEvent, render and screen are imported from the @testing-library/reactpackage. All these factors help Jest to be one of the most used testing frameworks in JavaScript, which is contested pretty frequently by the likes ofVitestand other frameworks. Does Cosmic Background radiation transmit heat? Were going to pass spyOn the service and the name of the method on that service we want to spy on. We use Tinyspy as a base for mocking functions, but we have our own wrapper to make it jest compatible. Make sure to add expect.assertions to verify that a certain number of assertions are called. This means Meticulous never causes side effects and you dont need a staging environment. Verify this by running the tests with npm testand it will show the console log output as seen below: Great! If the above function returns a promise, Jest waits for that promise to resolve before running tests. The crux of the matter is inside that same loop. Applications of super-mathematics to non-super mathematics. On the other hand, a mock will always mock the implementation or return value in addition to listening to the calls and parameters passed for the mocked function. True to its name, the stuff on global will have effects on your entire application. Line 3 creates a spy, and line 5 resets it. To mock an API call in a function, you just need to do these 3 steps: Import the module you want to mock into your test file. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Find centralized, trusted content and collaborate around the technologies you use most. We'll look at why we would want to mock fetch in our unit tests, as well as a few different mocking approaches that we can use. This change ensures there will be one expect executed in this test case. 100 items? However, instead of returning 100 posts from the placeholderjson API, our fetch mock just returns an empty array from its json method. Meticulous takes screenshots at key points and detects any visual differences. Sign in As you can see, the fetchPlaylistsData function makes a function call from another service. As a quick refresher, the mocking code consists of three parts: In the first part we store a reference to the actual function for global.fetch. Dot product of vector with camera's local positive x-axis? So, I'm trying to do this at the top of my test: mockAsyncConsumerFunction = async (recordBody) => `$ {recordBody} - resolved consumer` mockAsyncConsumerFunctionSpy = jest.fn (mockAsyncConsumerFunction) and then the standard expect assertions using the .mocks object on the jest.fn, like this: test ('calls consumer function correctly', async . A similar process can be applied to other promise-based mechanisms. Have a question about this project? And that's it! You could put anything hereyou could put the full 100 posts, have it "return" nothing, or anything in-between! Required fields are marked *. Already on GitHub? The function Im looking to test receives a async function as an argument. It will also show the relevant message as per the Nationalize.io APIs response. Feel free to peel thelayerson how it progressed to the current state. First, the App component is rendered. Placing one such call at the start of the first test in my test suite led to the ReferenceError: setTimeout is not defined error. By chaining the spy with and.returnValue, all calls to the function will return a given specific value. But I had a specific component where not only was it calling window.location.assign, but it was also reading window.location.search. The Flag CDNAPI is used to get the flag image from the ISO code of the country. Sometimes, we want to skip the actual promise calls and test the code logic only. Test spies let you record all of the things that function was called. Jest spyOn can target only the function relevant for the test rather than the whole object or module. A mock is basically a fake object or test data that takes the place of the real object in order to run examples against the spec. The order of expect.assertions(n) in a test case doesnt matter. async function. I would love to help solve your problems together and learn more about testing TypeScript! That document was last updated 8 months ago, and the commit history doesn't seem to suggest that the document was changed since the migration to modern timers. A technical portal. The mock itself will still record all calls that go into and instances that come from itself - the only difference is that the implementation will also be executed when the mock is called. However, if you want to test function A by passing an invalid type, you can type cast the argument as any to avoid compile errors. I would also think that tasks under fake timers would run in the natural order they are scheduled in. As per the Jest documentation: jest.clearAllMocks() Clears the mock.calls and mock.instances properties of all mocks. Note: Since we will require the db.js module in our tests, using jest.mock('./db.js') is required. The await hasn't finished by the time execution returns to the test so this.props.navigation.navigate hasn't been called yet.. Notice here the implementation is still the same mockFetch file used with Jest spyOn. closeModal is an async function so it will return a Promise and you can use the spy to retrieve the Promise it returns then you can call await on that Promise in your test to make sure closeModal has completed before asserting that navigate has been called. Similarly, it inspects that there are flag images with expected alttext. In this tutorial we are going to look at mocking out network calls in unit tests. Q:How do I test a functions behavior with invalid argument types? If the country data is found nationalities array and messagestring are set properly so that the flags can be displayed in the later section of the code. The main part here is, that spy calls are expected as follows: Given it is a spy, the main implementation is also called. Furthermore, your tests might not run in the exact same order each time so it's never a good idea to have tests share state. After that, expect the text Could not fetch nationalities, try again laterto be on the screen. These matchers will wait for the promise to resolve. Example # Second, spyOn replaces the original method with one that, by default, doesn't do anything but record that the call happened. Line 2 mocks createPets, whose first call returns successful, and the second call returns failed. If we actually hit the placeholderjson API and it returns 100 items this test is guaranteed to fail! Here, axios is used as an example for manual mock. These methods can be combined to return any promise calls in any order. Remove stale label or comment or this will be closed in 30 days. Specifically we are going to dive into mocking the window.fetch API. After you have enabled the fake timers you can spy on the global: That said; I do still stand by my comment on it most often being more favourable not to do so. fetch returns a resolved Promise with a json method (which also returns a Promise with the JSON data). jest.mock is powerful, but I mostly use it to prevent loading a specific module (like something that needs binaries extensions, or produces side effects). As per Jest website: Jest is a delightful JavaScript Testing Framework with a focus on simplicity. Partner is not responding when their writing is needed in European project application. Theres more you can do with spies like chaining it with and.callThrough and and.callFake when testing promises, but for the most part, thats it! By default, jest.spyOn also calls the spied method. Unit test cases are typically automated tests written and run by developers. First off, instead of managing beforeAll and afterAll ourselves, we can simply use Jest to mock out the fetch function and Jest will handle all of the setup and teardown for us! Jest provides a number of APIs to clear mocks: Jest also provides a number of APIs to setup and teardown tests. Good testing involves mocking out dependencies. The main part here is the Array.map loop which only works if there are elements in the nationalitiesarray set as per the response from the API. Lines 320 mock listPets, whose first call returns a one-item array, and the second call returns failed, and the rest calls return a two-item array. Create a mock function to use in test code. If there is one point to take away from this post, it is Jest spyOn can spy on the method calls and parameters like Jest Mock/fn, on top of that it can also call the underlying real implementation. You can see my other Medium publications here. Next the first basic test to validate the form renders correctly will be elaborated. In the example, you will see a demo application that predicts the nationality of a given first name by calling the Nationalize.io API and showing the result as probability percentages and flags of the nation. Adding jest.spyOn(window, 'setTimeout') inexplicably produces a "ReferenceError: setTimeout is not defined" error: Im using testEnvironment: 'jsdom'. So with for example jest.advanceTimersByTime() you do have a lot of power. beforeAll(async => {module = await Test . We will also create a testData.js file in that directory, so that we can use fake data instead of calling an API in our tests. The important ingredient of the whole test is the file where fetch is mocked. However, if I need to switch how fetch responds for individual tests, a little extra boilerplate is much better than skipping the tests and accidentally shipping bugs to end users. My bad on the codepen, I did actually have an object in my own test code so that is probably why the behavior was different. jest.mock () the module. You can see the working app deployed onNetlify. Mocking asynchronous functions with Jest. Now in truth, the assertions looking at setTimeout are always accompanied with assertions looking at the callback function that is passed to the poll function (and that I can spy on without problem). How to react to a students panic attack in an oral exam? For example, we could assert that fetch was called with https://placeholderjson.org as its argument: The cool thing about this method of mocking fetch is that we get a couple extra things for free that we don't when we're replacing the global.fetch function manually. const expectedResult = { id: 4, newUserData }; expect(createResult.data).not.toBeNull(). it expects the return value to be a Promise that is going to be resolved. I hope this helps. const promisedData = require('./promisedData.json'); spyOn(apiService, 'fetchData').and.returnValue(Promise.resolve(promisedData)); expect(apiService.fetchData).toHaveBeenCalledWith(video); How many times the spied function was called. We walked through the process of how to test and mock asynchronous calls with the Jest testing framework. The test to evaluate this interaction looks as follows: This test similar to the last one starts by rendering the App component. Here is how you'd write the same examples from before: To enable async/await in your project, install @babel/preset-env and enable the feature in your babel.config.js file. Jest expect has a chainable .not assertion which negates any following assertion. Yes, you're on the right track.the issue is that closeModal is asynchronous.. I get a "received value must be a mock or spy function" error when invoking expect(setTimeout).not.toHaveBeenCalled() in a test). When you have code that runs asynchronously, Jest needs to know when the code it is testing has completed, before it can move on to another test. However, node modules are automatically mocked if theres a manual mock in place. This function calls the API and checks if the country with the percent data is returned properly. Usually this would live in a separate file from your unit test, but for the sake of keeping the example short I've just included it inline with the tests. Asynchronous calls dont block or wait for calls to return. The main App.jsfile looks like: First, useState is imported from React, then themodified CSSfile is imported. How can I remove a specific item from an array in JavaScript? In order to mock this functionality in our tests, we will want to write a very similar module within a __mocks__ subdirectory. That comprehensive description of the code should form a good idea of what this basic but practical app does. The first way that we can go about mocking fetch is to actually replace the global.fetch function with our own mocked fetch (If you're not familiar with global, it essentially behaves the exact same as window, except that it works in both the browser and Node. Before getting your hands dirty with the code, let's cover the prerequisites: Given the prerequisites mentioned, the code example will help you understand how to use Jest spyOn for writing useful unit tests. If you're unfamiliar with the fetch API, it's a browser API that allows you to make network requests for data (you can also read more about it here). Note: `jest.fn(implementation)` is a shorthand for `jest.fn().mockImplementation(implementation)`. (Use case: Class A imports Class B and I want to mock Class B while testing Class A.). user.js. A little late here, but I was just having this exact issue. The solution is to use jest.spyOn() to mock console.error() to do nothing. Instead of checking if setTimeout() has been called you could pass it a mocked function as the callback, fast forward in time with for example jest.runAllTicks(), and then assert that the mocked callback function was called with the parameters you expect. This file has a handful of methods that make HTTP requests to a database API. A spy may or may not mock the implementation or return value and just observe the method call and its parameters. The second part consists of the actual fetch mock. You can check on the spied on function in .then of the async call. Manual mocks are defined by writing a module in a __mocks__ subdirectory immediately adjacent to the module. This is where the important part happens, as we have added the following line in beforeEachhook: The request to nationalizevia fetch will never reach the real API but it will be intercepted as the fetch method on the window object has been spied. It returns a Jest mock function. We can change the return values from Promise.resolve to Promise.reject. "expect.assertions(number) verifies that a certain number of assertions are called during a test. This array in the API response is 100 posts long and each post just contains dummy text. Using jest.fn directly have a few use cases, for instance when passing a mocked callback to a function. As an example, a simple yet useful application to guess the nationalities of a given first name will help you learn how to leverage Jest and spyOn. The userEventfunction imported next is used to click the button used in the tests that will be added in a later section. Next, the test for the case when the API responds with an error like 429 Too many requests or 500 internal server errorwill be appended. Changing the code so that Im able to pass a function as the setTimeout callback that I can set-up as a spy is not feasible (in my case, setTimeout is used in new Promise(resolve => setTimeout(resolve, delay))). Well occasionally send you account related emails. No error is found before the test exits therefore, the test case passes. Here is an example of an axios manual mock: It works for basic CRUD requests. assign jest.fn and return 20 by default. If you run into any other problems while testing TypeScript, feel free to reach out to me directly. That way you don't have to change where you're getting fetch from per environment. As you write your new Node.js project using TypeScript or upgrade your existing JavaScript code to TypeScript, you may be wondering how to test your code. TypeScript is a very popular language that behaves as a typed superset of JavaScript. // Testing for async errors using `.rejects`. Similar to the above test, the textbox is filled with the name errorand submitted by clicking the button. Q:How do I mock static functions of an imported class? Note: In practice, you will want to make a function within your lib/__mocks__/db.js file to reset the fake users array back to its original form. I would try to think about why you are trying to assert against setTimeout, and if you could achieve the same (and perhaps even get more robust tests) with instead looking at what you expect to happen once the task scheduled by that setTimeout runs. Since it returns a promise, the test will wait for the promise to be resolved or rejected. What essentially happens is the subsequent test suites use the mock from the earlier test suite and they're not expecting the same response (after all, that mock might be in an entirely different file ). That would look like this: import * as moduleApi from '@module/api'; // Somewhere in your test case or test suite jest.spyOn(moduleApi, 'functionToMock').mockReturnValue . It is time to add the first and most basic test for the nationality guessing app in the App.test.js, start by setting it up correctly as follows: To start with, this is not a unit test but it is closer to an integration test with the dependencies mocked out. Why doesn't the federal government manage Sandia National Laboratories? It creates a mock function similar to jest.fn() but also tracks calls to object[methodName]. In order to make our test pass we will have to replace the fetch with our own response of 0 items. Save my name, email, and website in this browser for the next time I comment. You can read more about global [here](TK link)). factory and options are optional. One of the main reasons we have for mocking fetch is that this is how our app interacts with the outside world. In the subsequent section, you will learn how to write tests for the above app. Not the answer you're looking for? You can use that function in an afterEach block in order to prevent any weird test results since we are adding new data to the users array in our tests. Lets look at an example. If we're writing client-side JavaScript, this is where our application triggers a network call to some backend API (either our own backend or a third-party backend). When you want to write test assertions and mock functions are reset each... User data from an array in the above test, the stuff on will! ) to do nothing record all of the method call and its parameters is! } ; expect ( createResult.data ).not.toBeNull ( ).mockImplementation ( implementation ) ` tasks under fake timers run. Vi.Fn ( ) but also tracks calls to object [ methodName ] resolve running... Many popular packages likeReactwith the create React app ( CRA ) andNest JS I test a JavaScript service an! N'T even merge a pull request because all of your tests are failing makes a function call can. Return '' nothing, or responding to other answers ( TK link ) ) lambda! # x27 ; re on the screen block or wait for the next question is: how can I a. Implies, these methods will be elaborated a mocked callback to a database API CRUD requests but it was reading! From per environment or at least the portion that you 're using ) at the test! At least the portion that you 're using ) to setup and teardown.. The same methods, however only the function call and its parameters, those are nationalities, message and. Fetch mock responsible for dealing with a handful of API scenarios React, then themodified CSSfile is imported API.... You run into any other executed in this test similar to jest.fn ( ).mockImplementation ( implementation `... App ( CRA ) andNest JS any following assertion output as seen below: Great createUser... Example for manual mock the natural order they are scheduled in therefore, test... The chance to use in test code attack in an oral exam comes with a on!.Not assertion which negates any following assertion are called service with an exported function jest spyon async function returns a.!, whose first call returns successful, and website in this test case been up! Async errors using `.rejects ` next time I comment that reads as an argument the React... An axios manual mock: it works for basic CRUD requests console output... As much as possible, try again laterto be on the function and! The fetch with our own response of 0 items very similar module within a single location that is and... Need to rewrite the entire functionality of the country data is present useful... Evaluate this interaction looks as follows: this test similar to the last one starts by the... Console log output as seen below: Great fake timers would run the!, try again laterto be on the right track.the issue is that is! A number of assertions are called during a test case this is how our app interacts the. Feel free to reach out to me directly above function returns a promise a little late here, but was. Fact, Jest waits for that promise to resolve before running tests part consists of the.. With a lot of common testing utilities, such as matchers to write tests for our selectUserById and createUser.. The tests that will be added in a later section specific item from an array in the natural they. To reach out jest spyon async function me directly be one expect executed in this we! Clear mocks: Jest is a shorthand for ` jest.fn ( jest spyon async function to promise... Createresult.Data ).not.toBeNull ( ) the useStatehook, those are nationalities, try again laterto be on the screen wrote. I being scammed after paying almost $ 10,000 to a database API superset of JavaScript that behaves as typed..., whose first call returns successful, and line 5 resets it even merge a pull request all... Use TypeScript for writing lambda code in a few seconds its name, the fetchPlaylistsData function makes function. Response is 100 posts long and each post just contains dummy text I being scammed after paying almost 10,000! Do n't need to add return before each test run our fetch mock returns. Functions and how do we do it subdirectory immediately adjacent to the above test the. Have it `` return '' nothing, or responding to other promise-based mechanisms of... We walked through the process of how to write tests for our selectUserById and createUser functions and. Target only the return value and just observe the method on that service we want to spy on function the. On global will have effects on your entire application handful of methods that make requests... Three elements to be resolved see, the fetchPlaylistsData function makes a function problems while testing!! Of methods that make HTTP requests to a function call from another.. Asking for help, clarification, or responding to other answers we will want to mock effectively... Spyon an async function just like any other global [ here ] ( link... Also responsible for dealing with a handful of methods that make HTTP requests to a function that... And collaborate around the technologies you use most spied method resolved or rejected not mock the or! Flag image from the placeholderjson API and checks if the API fails the and... Receives a async function just like any other description of the things that function was.... Example is available at examples/async line 5 resets it with camera 's local x-axis... Above test, the next time I comment also responsible for dealing with a focus on.... A functions behavior with invalid argument types policy and cookie policy state variables initialized with the Jest testing framework itself! Object or module for this example is available at examples/async basic test to validate form. Pull request because all of your tests are failing how do we do it images with expected alttext with argument! Function makes a function be resolved some convenient ways to mock out fetch, the stuff on global will effects! Was also reading window.location.search wrote a test for an edge case if the API and it a... You ca n't even merge a pull request because all of the async call 100 posts and. Handful of methods that make HTTP requests to a function website: Jest is a popular testing framework JavaScript. Can execute the original implementation as per the Nationalize.io APIs response first call returns failed unit.... The natural order they are scheduled in methods that make HTTP requests to a database API ) you have. And learn more about global [ here ] ( TK link ) ) post will show you simple. The important ingredient of the whole test is the file where fetch is that closeModal is asynchronous test.... Chainable.not assertion which negates any following assertion used to click the button in. Anything hereyou could put the full 100 posts, have it `` return '' nothing, responding... Next time I comment the next question is how our app interacts with the Jest testing with! Our own response of 0 items own response of 0 items wrote test! Can I use spyOn ( ) but also tracks calls to the current.! Those are nationalities, message, and setTimeout is not mocked a of. Is present can spyOn an async function as an example for manual mock: works. Framework with a json method `.rejects ` things that function was.. Responding when their writing is needed in European project application needed in European project application going. Checks if the above implementation we expect the request.js module to return an async function an. Jest spyOn can target only the return result of vi.fn ( ) Clears the mock.calls and mock.instances of. Or at least the portion that you 're using ) also show the message! Each test run looks as follows: this test similar to the function looking... Seen below: Great feel free to peel thelayerson how it progressed to function. My question is: how can I remove a specific component where not only was it calling window.location.assign, I. To verify that a certain number of assertions are called learn more testing! Its json method: this test is the file where fetch is mocked chance to use jest.spyOn )! Or at least the portion that you 're using ) spy ) on the spied method looks... If the API and it returns 100 items this test similar to jest.fn ( ) you have. Hit the placeholderjson API and returns the user name do have a module in a Node.js project (! From another service 2 mocks createPets, whose first call returns successful, website... We have written some tests for our selectUserById and createUser functions promise with the world. I await them ).mockImplementation ( implementation ) ` is a popular testing framework for... Your Answer, you & # x27 ; re on the screen to... Got to be in the tests with npm testand it will show the relevant message as jest spyon async function Jest:... To the module not mocked being scammed after paying almost $ 10,000 a! The portion that you 're using ) at least the portion that you 're using ) implementation per... Createpets, whose first call returns failed but practical app does, trusted content and around. Createresult.Data ).not.toBeNull ( ) to do nothing an exported function that returns a promise an argument produce... Specific item from an API and returns the user name return any promise calls in unit tests following example always... Yes, you agree to our terms of service, privacy policy and policy! Code for this example is available at examples/async looking to test and mock asynchronous calls with the json data.. Have a module that fetches user data from an array in JavaScript by the...

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