There’s a shorter way to write the test that doesn’t create the mockA, mockB and mockC variables, but I like to have them around so that I can change the return values of the mocks in other tests.
Here’s an example of a test suite that replaces the dependencies of the getAll function using the jest.mock function. If you think about it that way, you might think of the other mechanisms as actually duplicating the definitions of the dependencies.īut how can we replace those dependencies for testing? Well, fairly easily with Jest.
#Jest mock constructor code
The import statements are the code that defines the dependencies, just at the module (file) level rather than the function or class level. The only useful bit of this code is the getAll function, so it’s a lot of code for a one line function.Įxport const getAll = ( param : string) => [īut… one way to think about this code is that the dependencies are injected. That allows us to pass anything that matches the interface value, not just a type of C. NET and Java worlds to take in an interface instead of a specific class, so I’ve demonstrated that. The class import C could stay as it was because it’s an upper case C and so it doesn’t clash with the lowercase field name c in the class. Coming up with new names for your imports can be suprisingly difficult. One thing to note is that some imports had to be renamed - a became externalA and the default export became externalB so that they didn’t clash with the field names on the class a and b. If no dependencies are provided, the imports are used as the default implementation. The class’s constructor takes in the required dependencies and sets them as fields in the class. TypeScript has classes too, so it’s easy to understand if you have more of your programming experience in those languages. Using a classĪ popular way to do this in the. passing the required functionality into the thing that’s using the dependencies to create the return value. This means that we’ll need some mechanism for doing dependency injection - i.e. In the examples here, we’re not doing anything except returning a single character, but in the real world, functions “a”, “b” and “c” might get data from a database, make an API call and write to a database, so we want to be able to replace their implementations with mocks during testing so that we don’t end up doing integration testing. Note that we are not testing SoundPlayer (I am telling you more shortly) //file: sound-player.We now need a way to use all 3 of these dependencies to create the return value. As a part of our test we want this faction to be called exactly one time and with a pre determined argument. We have a typescript class called SoundPlayer and it has a function called playSoundFile that expects a string argument. Scenario : Dependency Class testing with Jest in TypeScript
#Jest mock constructor how to
If you want to get your environment right, you might want to read How to setup a TypeScript project dev environments like a pro first. Otherwise you are going to get errors about import, types and etc. I trust you got your TypeScript/Jest environment right. Though, jest documentation obviously is not going to work for TypeScript, otherwise we would not have this discussion. We are going to take the same scenario as jest documentation for ES6 Class Mocks. Here I am showing you what you really need to do. There are lots of treads in stack overflow, but unfortunately most of them are not really useful. In this article I am going to show you how to mock a dependency class and its functions. Jest is an awesome and easy to use testing framework for JavaScript, but when it comes to TypeScript and mocking (specially to mock typescript classes) it can really become painful as TypeScript is not relaxed about the types as JavaScript is.
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