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.toBeTruthy()​ 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, let's say you have some application code that looks like: drinkSomeLaCroix(); if (thirstInfo()) { drinkMoreLaCroix(); } You may not care what thirstInfo returns, specifically - it might return true or a complex object, and your code would still work. So if ..

RegExp The RegExp object is used for matching text with a pattern. For an introduction to regular expressions, read the Regular Expressions chapter in the JavaScript Guide. Description Literal notation and constructor There are two ways to create a RegExp object: a literal notation and a constructor. The literal notation's parameters are enclosed between slashes and do not use quotation marks. T..

Setting up configuration for a create-react-app based project Let's say you have created your project using the create-react-app Facebook helper. In this case, you don't have direct access to jest, so you have to execute react-scripts to get your tests working. To follow this guide: As a starting point, you can take the (create-react-app/00-start) sample. Or you can create a project from scratch..

const getMedia = async (mediaId) => (await makeRequest({ url: `${MEDIA_BASE_URL}/admin/media/${mediaId}`, method: 'GET' })).data If this is the function that is making the request through the Axios, we can make it with some curry function and Axios innate data of config to make it and return it with Promise.resolve. const createMockImplementation = ({ contentId, mediaId }) => (config = {}) => { ..

Reset/Clear with beforeEach/beforeAll and clearAllMocks/resetAllMocks Assuming we have a global stub or spy that is potentially called mutliple times throughout our tests. const mockFn = jest.fn(); function fnUnderTest(args1) { mockFn(args1); } test('Testing once', () => { fnUnderTest('first-call'); expect(mockFn).toHaveBeenCalledWith('first-call'); expect(mockFn).toHaveBeenCalledTimes(1); }); t..

Default parameters Default function parameters allow named parameters to be initialized with default values if no value or undefined is passed. Syntax function fnName(param1 = defaultValue1, ..., paramN = defaultValueN) { /* ... */ } Copy to Clipboard Description In JavaScript, function parameters default to undefined. However, it's often useful to set a different default value. This is where de..

Promise.resolve() The Promise.resolve() method returns a Promise object that is resolved with a given value. If the value is a promise, that promise is returned; if the value is a thenable (i.e. has a "then" method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. This function flattens nested layers ..

With the dramatic rise of functional programming in mainstream JavaScript, curried functions have become common in many applications. It’s important to understand what they are, how they work, and how to put them to good use. What is a curried function? A curried function is a function that takes multiple arguments one at a time. Given a function with 3 parameters, the curried version will take ..

Rest parameters The rest parameter syntax allows a function to accept an indefinite number of arguments as an array, providing a way to represent variadic functions in JavaScript. Syntax function f(a, b, ...theArgs) { // ... } Copy to Clipboard Description A function definition's last parameter can be prefixed with "..." (three U+002E FULL STOP characters), which will cause all remaining (user s..

Arrow function expressions An arrow function expression is a compact alternative to a traditional function expression, but is limited and can't be used in all situations. There are differences between arrow functions and traditional functions, as well as some limitations: Arrow functions don't have their own bindings to this or super, and should not be used as methods. Arrow functions don't have..