.toBeNull() is the same as .toBe(null) but the error messages are a bit nicer. A class instance with fields. We can test this with: The expect.hasAssertions() call ensures that the prepareState callback actually gets called. For example, let's say you have a drinkEach(drink, Array
) function that applies f to a bunch of flavors, and you want to ensure that when you call it, the first flavor it operates on is 'lemon' and the second one is 'octopus'. toBe uses Object.is to test exact equality. For example, let's say you have a drinkFlavor function that throws whenever the flavor is 'octopus', and is coded like this: The test for this function will look this way: And it will generate the following snapshot: Check out React Tree Snapshot Testing for more information on snapshot testing. Lastly, make ⦠Matchers are called with the argument passed to expect(x) followed by the arguments passed to .yourMatcher(y, z): These helper functions and properties can be found on this inside a custom matcher: A boolean to let you know this matcher was called with the negated .not modifier allowing you to display a clear and correct matcher hint (see example code). Use .toHaveReturnedWith to ensure that a mock function returned a specific value. That is, the expected array is not a subset of the received array. â Rob Gleeson Aug 4 '15 at 13:19. Therefore, it matches a received object which contains properties that are not in the expected object. Itâs also light on configuration so thereâs a lot to like. Use .toStrictEqual to test that objects have the same types as well as structure. For example, this code tests that the promise resolves and that the resulting value is 'lemon': Note that, since you are still testing promises, the test is still asynchronous. Although Jest always appends a number at the end of a snapshot name, short descriptive hints might be more useful than numbers to differentiate multiple snapshots in a single it or test block. Also under the alias: .nthReturnedWith(nthCall, value). You should craft a precise failure message to make sure users of your custom assertions have a good developer experience. This is especially useful for checking arrays or strings size. Here's a snapshot matcher that trims a string to store for a given length, .toMatchTrimmedSnapshot(length): It's also possible to create custom matchers for inline snapshots, the snapshots will be correctly added to the custom matchers. For example, let's say that you're testing a number theory library and you're frequently asserting that numbers are divisible by other numbers. You can provide an optional argument to test that a specific error is thrown: For example, let's say that drinkFlavor is coded like this: We could test this error gets thrown in several ways: Use .toThrowErrorMatchingSnapshot to test that a function throws an error matching the most recent snapshot when it is called. Intuitive equality comparisons often fail, because arithmetic on decimal (base 10) values often have rounding errors in limited precision binary (base 2) representation. For example, in VSCode doing Ctrl+Shift+P > TypeScript: Restart TS server helps, as sometimes it fails to recognize jest, or the test file to be a module, etc. Structure of a test file. And when pass is true, message should return the error message for when expect(x).not.yourMatcher() fails. The simplest way to test a value is with exact equality. yarn add --dev jest Or npm:. To get the first yielded value from a saga,call its next().value: A value must then be returned to assign to the action constant, which is used for the argument to the puteffect: Since there are no more yields, then next time next()is called, the generator will be done: expect.anything() matches anything but null or undefined. You can also pass an array of objects, in which case the method will return true only if each object in the received array matches (in the toMatchObject sense described above) the corresponding object in the expected array. You can do that with this test suite: Use .toHaveBeenCalledWith to ensure that a mock function was called with specific arguments. .toContain can also check whether a string is a substring of another string. However, inline snapshot will always try to append to the first argument or the second when the first argument is the property matcher, so it's not possible to accept custom arguments in the custom matchers. For example, this test fails: It fails because in JavaScript, 0.2 + 0.1 is actually 0.30000000000000004. For example, let's say you have a mock drink that returns true. You can test this with: This matcher also accepts a string, which it will try to match: Use .toMatchObject to check that a JavaScript object matches a subset of the properties of an object. Use toBeCloseTo to compare floating point numbers for approximate equality. Use toBeGreaterThan to compare received > expected for number or big integer values. When Jest runs, it tracks all the failing matchers so that it can print out nice error messages for you. selector (EnzymeSelector): The selector to match. Note: We assume you start off with a simple node package.json setup. In this code, .toBe(4)is the matcher. Use .toHaveLastReturnedWith to test the specific value that a mock function last returned. What I expect is for a attributeValue to be of type string. Use .toContain when you want to check that an item is in an array. Only the message property of an Error is considered for equality. expect gives you access to a number of "matchers" that let you validate different things. So if you want to test there are no errors after drinking some La Croix, you could write: In JavaScript, there are six falsy values: false, 0, '', null, undefined, and NaN. You can also tes⦠Most ways of comparing numbers have matcher equivalents. If the promise is rejected the assertion fails. You can compare yarn and npm commands in the yarn docs, here.. Let's get started by writing a test for ⦠You can use it inside toEqual or toBeCalledWith instead of a literal value. Let's say you have a method bestLaCroixFlavor() which is supposed to return the string 'grapefruit'. To use snapshot testing inside of your custom matcher you can import jest-snapshot and use it from within your matcher. For example, .toEqual and .toBe behave differently in this test suite, so all the tests pass: Note: .toEqual won't perform a deep equality check for two errors. Use .toThrow to test that a function throws when it is called. /* We are going to implement a matcher called toBeDivisibleByExternalValue, where the divisible number is going to be pulled from an external source. */, /* For example, if we want to test that drinkFlavor('octopus') throws, because octopus flavor is too disgusting to drink, we could write: Note: You must wrap the code in a function, otherwise the error will not be caught and the assertion will fail. Use .toHaveNthReturnedWith to test the specific value that a mock function returned for the nth call. It is recommended to use the .toThrow matcher for testing against errors. The optional numDigits argument limits the number of digits to check after the decimal point. Use .toBeFalsy when you don't care what a value is and you want to ensure a value is false in a boolean context. */, // The error (and its stacktrace) must be created before any `await`. Also under the alias: .toThrowError(error?). Async tests. Jest sorts snapshots by name in the corresponding .snap file. If you have a mock function, you can use .toHaveBeenNthCalledWith to test what arguments it was nth called with. If you want to check the value of an object, use toEqual instead: toEqual recursively checks every field of an object or array. You avoid limits to configuration that might cause you to eject from, Object types are checked to be equal. */, 'map calls its argument with a non-null argument', 'randocall calls its callback with a number', 'matches even if received contains additional elements', 'does not match if received does not contain expected elements', 'Beware of a misunderstanding! The default jest-playwright environment is node, but you can use a browser-like environment through jest-playwright-jsdom expect-playwright There is a utility package expect-playwright which simplifies the expect statements in combination with Playwright to make e.g. Letâs say that the component that you want to test using Jest snapshots has props that contain dates and times that are based on the current date/time. expect.extend({ toBeWithinRange(received, floor, ceiling) { const pass = received >= floor && received <= ceiling; if (pass) { return { message: () => `expected ${received} not to be within range ${floor} - ${ceiling} `, pass: true, }; } else { return { message: () => `expected ${received} to be within range ${floor} - ${ceiling} `, pass: false, }; } }, }); test('numeric ranges', => { expect(100).toBeWithinRange(90, 110); ⦠This is often useful when testing asynchronous code, in order to make sure that assertions in a callback actually got called. Thus, when pass is false, message should return the error message for when expect(x).yourMatcher() fails. define what to expect as the output; check if the function produces the expected output; Really, that's it. For example, if you want to check that a mock function is called with a non-null argument: expect.any(constructor) matches anything that was created with the given constructor. Use .toBeDefined to check that a variable is not undefined. It calls Object.is to compare values, which is even better for testing than === strict equality operator. Use .toBeTruthy when you don't care what a value is and you want to ensure a value is true in a boolean context. For example, you might not know what exactly essayOnTheBestFlavor() returns, but you know it's a really long string, and the substring grapefruit should be in there somewhere. When Jest is called with the --expand flag, this.expand can be used to determine if Jest is expected to show full diffs and errors. If you add a snapshot serializer in individual test files instead of adding it to snapshotSerializers configuration: See configuring Jest for more information. expect.not.stringMatching(string | regexp) matches the received value if it is not a string or if it is a string that does not match the expected string or regular expression. exports[`stores only 10 characters: toMatchTrimmedSnapshot 1`] = `"extra long"`; Use .toHaveBeenCalledTimes to ensure that a mock function got called exact number of times. Check out the Snapshot Testing guide for more information. For example, to assert whether or not elements are the same instance: Use .toHaveBeenCalled to ensure that a mock function got called. The simplest way to test a value is with exact equality. When Jest runs, it tracks all the failing matchers so that it can print out nice error messages for you. If you know how to test something, .not lets you test its opposite. You should use the matcher that most precisely corresponds to what you want your code to be doing. For example, if you want to check that a function fetchNewFlavorIdea() returns something, you can write: You could write expect(fetchNewFlavorIdea()).not.toBe(undefined), but it's better practice to avoid referring to undefined directly in your code. 2. Also all TypeScript files should be in a src folder which is always recommended (even without Jest⦠You can write: Also under the alias: .nthCalledWith(nthCall, arg1, arg2, ...). Use .toHaveProperty to check if property at provided reference keyPath exists for an object. expect(fn) .toHaveBeenCalledWith(expect.anything()) .toHaveBeenCalledWith(expect.any(constructor)) .toHaveBeenCalledWith(expect.arrayContaining([ values ])) .toHaveBeenCalledWith(expect.objectContaining({ props })) .toHaveBeenCalledWith(expect.stringContaining(string)) .toHaveBeenCalledWith(⦠For example, this code will validate some properties of the can object: Don't use .toBe with floating-point numbers. Use .toHaveReturnedTimes to ensure that a mock function returned successfully (i.e., did not throw an error) an exact number of times. 2. It is the inverse of expect.stringMatching. npm install --save-dev jest Note: Jest documentation uses yarn commands, but npm will also work. A sequence of dice rolls', 'matches even with an unexpected number 7', 'does not match without an expected number 2', 'matches if the actual array does not contain the expected elements', 'matches if the actual object does not contain expected key: value pairs', 'matches if the received value does not contain the expected substring', 'matches if the received value does not match the expected regex', 'onPress gets called with the right thing', // affects expect(value).toMatchSnapshot() assertions in the test file, 'does not drink something octopus-flavoured', 'registration applies correctly to orange La Croix', 'applying to all flavors does mango last', // Object containing house features to be tested, // Deep referencing using an array containing the keyPath, // Referencing keys with dot in the key itself, 'drinking La Croix does not lead to errors', 'drinking La Croix leads to having thirst info', 'the best drink for octopus flavor is undefined', 'the number of elements must match exactly', '.toMatchObject is called for each elements, so extra object properties are okay', // Test that the error message says "yuck" somewhere: these are equivalent, // Test that we get a DisgustingFlavorError. Here's how you would test that: In this case, toBe is the matcher function. For checking deeply nested properties in an object you may use dot notation or an array containing the keyPath for deep references. So if you want to test that thirstInfo will be truthy after drinking some La Croix, you could write: Use .toBeUndefined to check that a variable is undefined. expect.objectContaining(object) matches any received object that recursively matches the expected properties. Run a single Jest test in a file using .only; Run multiple Jest tests in a file using .only.only to run a single suite of tests in a describe.only to run multiple suites of tests in describe-s; Use .skip to ignore Jest tests or suites. If you don't care what the contents are but just that it is a string. You can use it inside toEqual or toBeCalledWith instead of a literal value. You make the dependency explicit instead of implicit. For the full list, see the expect API doc. Although the .toBe matcher checks referential identity, it reports a deep comparison of values if the assertion fails. We can do that with: expect.stringContaining(string) matches the received value if it is a string that contains the exact expected string. Jest needs additional context information to find where the custom inline snapshot matcher was used to update the snapshots properly. Ensures that a value matches the most recent snapshot. In version 23.3.0 of jest, expect (string).toMatch (string) expects a string. For example, let's say that you can register a beverage with a register function, and applyToAll(f) should apply the function f to all registered beverages. We are using toHaveProperty to check for the existence and values of various properties in the object. You typically won't do much with these expectation objects except call matchers on them. That is, the expected array is a subset of the received array. If you have floating point numbers, try .toBeCloseTo instead. As part of that goal, you want to avoid all the repetitivepatterns that arise in doing so. Once you've learned about the matchers that are available, a good next step is to check out how Jest lets you test asynchronous code. Use .toThrowErrorMatchingInlineSnapshot to test that a function throws an error matching the most recent snapshot when it is called. This matcher uses instanceof underneath. You can provide an optional hint string argument that is appended to the test name. So use .toBeNull() when you want to check that something is null. ; Returns expect.not.arrayContaining(array) matches a received array which does not contain all of the elements in the expected array. The one-page guide to Jasmine: usage, examples, links, snippets, and more. You can write: Also under the alias: .lastReturnedWith(value). jest-extended aims to add additional matchers to Jest's default ones making it easy to test everything ð ⦠This ensures that a value matches the most recent snapshot. Run a single Jest test file with the CLI; Use .only to run only certain tests. Mocking a function that returns a number (like Date.now) is a lot easier than mocking a constructor. This is often useful when testing asynchronous code, in order to make sure that assertions in a callback actually got called. Now hands on Jest! It is the inverse of expect.arrayContaining. For example, let's say that we have a function doAsync that receives two callbacks callback1 and callback2, it will asynchronously call both of them in an unknown order. The most useful ones are matcherHint, printExpected and printReceived to format the error messages nicely. Alternatively, you can use async/await in combination with .resolves: Use .rejects to unwrap the reason of a rejected promise so any other matcher can be chained. For additional Jest matchers maintained by the Jest Community check out jest-extended. For example, if getAllFlavors() returns an array of flavors and you want to be sure that lime is in there, you can write: Use .toContainEqual when you want to check that an item with a specific structure and values is contained in an array. For the default value 2, the test criterion is Math.abs(expected - received) < 0.005 (that is, 10 ** -2 / 2). expect.stringMatching(string | regexp) matches the received value if it is a string that matches the expected string or regular expression. However there are times when having more specific matchers (assertions) would be far more convenient.