Jest Spy on an Exported Function

The problem

Consider the scenario where we’re importing a specific function, targetFunction, from a module called @example/library:

import { targetFunction } from '@example/library'

You might wonder how to correctly use jest.spyOn to monitor this function, especially what argument to pass first. Jest’s spyOn expects an object or module as the first parameter, and the name of the method to spy on as the second.

The trick to effectively monitoring a named export is to import the entire module’s contents as an object, then pass this object into jest.spyOn.

The Solution

import * as exampleLibrary from '@example/library'
jest.spyOn(exampleLibrary, 'targetFunction').mockReturnValue({ key: 42 })

Be aware that attempting this might sometimes result in a TypeError: Cannot redefine property: targetFunction due to Jest’s handling of property definition. If you encounter this, it means spyOn cannot be used directly to mock this particular function. You’ll need to employ an alternate strategy for mocking in such scenarios

Also published on Medium: Javascript in Plain English