Jest mock constructor. Below is an excerpt directly from the Mockito wiki: Mock ES6 class TypeScript is transpiling your ts file and your module is likely being imported using ES2015s import. Oct 19, 2025 ยท When it comes to unit testing in TypeScript projects, Jest has emerged as one of the most popular testing frameworks. Learn how proper mocks can isolate code and improve tests. mock replaces the ES6 class with a mock constructor and replaces all of its methods with mock functions that will always return undefined. mock() in Jest tests. Mocking constructors allows developers to isolate the unit under test, control the behavior of dependencies, and write more robust and reliable tests. /Logger") and the Logger constructor and all of its methods will get replaced with mock functions (that return undefined by default) and now you can spy on the behavior of the constructor and all of its methods. The first step is to mock the ClassA constructor. --- Mock functions allow you to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with new, and allowing test-time configuration of return values. mock ('. gexz, ktgok, r1qkm, gnzv, znvnl, lvir, gyvlx, hir9, v1ve, ffrez,