How can I retrieve the current instance from within a jest spyOn mockImplementation?

When setting up my test suite, I used the following approach to mock a class method:

beforeEach(() => {
    jest
      .spyOn(MyClass.prototype, "loadVars")
      .mockImplementation(async () => {

        const file:string = `MyClass.${
          this.prefix  // <---- how can I address this?
        }.data.mock.json`;
        logger.info(`Mocking all parameters from ${file}`);

        return JSON.parse(
          fs.readFileSync(
            process.cwd() + `/../data/${file}`,
            "utf-8"
          )
        );
      });
  });

Is there a way to reference the current instance of the class within this mock function?

Answer №1

Using arrow functions () => {} allows them to capture the value of this from their surrounding scope. Therefore, when replacing a prototype method, it is necessary to utilize a regular function declaration.

Furthermore, in Typescript, it is essential to provide a hint regarding the expected context for this, which can be achieved by specifying a type for this.

The following code snippet should meet your expectations:

jest
  .spyOn(MyClass.prototype, 'loadVars')
  .mockImplementation(function (this: MyClass) {
    console.log(this.prefix) // this will work as intended
  })

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