无需传参的Jest模拟函数：_recordError监视失败问题求解

Absolutely! This is a common issue when dealing with pseudo-private methods or internal functions in Jest, and there are solid ways to mock and verify _recordError without passing it as a parameter. Let’s break down the solutions based on how your _recordError is implemented:

1. If _recordError is a pseudo-private instance method (underscore convention) #

If your code looks like this (a class with an underscore-prefixed method):

class ErrorHandler {
  _recordError(error) {
    // Your error logging logic here
  }

  processRequest(data) {
    try {
      // Risky operation that might throw
      if (!data) throw new Error("Invalid data");
    } catch (err) {
      this._recordError(err);
    }
  }
}

You can spy on the instance method directly, or on the class prototype to cover all instances. Here’s how:

Spy on a specific instance #

test("calls _recordError when processing invalid data", () => {
  const handler = new ErrorHandler();
  // Spy on the instance's _recordError method and mock it
  const recordErrorSpy = jest.spyOn(handler, "_recordError").mockImplementation(() => {});

  // Trigger the error path
  handler.processRequest(null);

  // Verify the spy was called correctly
  expect(recordErrorSpy).toHaveBeenCalled();
  expect(recordErrorSpy).toHaveBeenCalledWith(expect.any(Error));
  expect(recordErrorSpy).toHaveBeenCalledTimes(1);

  // Clean up the spy to avoid affecting other tests
  recordErrorSpy.mockRestore();
});

Spy on the class prototype (for all instances) #

If you create multiple instances in your test, spying on the prototype is more efficient:

test("calls _recordError for any instance on failure", () => {
  // Spy on the prototype method
  const recordErrorSpy = jest.spyOn(ErrorHandler.prototype, "_recordError").mockImplementation(() => {});

  const handler1 = new ErrorHandler();
  handler1.processRequest(null);

  const handler2 = new ErrorHandler();
  handler2.processRequest(undefined);

  expect(recordErrorSpy).toHaveBeenCalledTimes(2);
  recordErrorSpy.mockRestore();
});

Note: If you still get the "undefined given instead" error, check the this context in processRequest. If processRequest is used as a callback (e.g., in an event listener), this might not point to the instance. Fix this by binding this in the constructor or using an arrow function for processRequest.

2. If _recordError is a private module function (not exported) #

If _recordError is a function inside a module that’s only used internally (not exported):

// myModule.js
function _recordError(error) {
  // Log error to service
}

export function doWork() {
  try {
    // Work logic
  } catch (err) {
    _recordError(err);
  }
}

You can use Jest’s module mocking or a tool like rewire to access and spy on the private function:

Using rewire #

First install rewire:

npm install --save-dev rewire

Then in your test:

const rewire = require("rewire");
const myModule = rewire("./myModule");

test("calls _recordError when doWork fails", () => {
  // Mock the private _recordError function
  const mockRecordError = jest.fn();
  myModule.__set__("_recordError", mockRecordError);

  // Trigger the failure
  myModule.doWork(); // Assume this throws an error

  expect(mockRecordError).toHaveBeenCalled();
  myModule.__restore__(); // Reset the module
});

Using Jest module mocking #

If you prefer not to use external tools, you can mock the entire module and override the private function:

jest.mock("./myModule", () => {
  const originalModule = jest.requireActual("./myModule");
  const mockRecordError = jest.fn();
  
  return {
    ...originalModule,
    doWork: () => {
      try {
        // Replicate the original failure path
        throw new Error("Test error");
      } catch (err) {
        mockRecordError(err);
      }
    },
    // Expose the mock for verification
    __mockRecordError: mockRecordError
  };
});

test("calls _recordError on failure", () => {
  const myModule = require("./myModule");
  myModule.doWork();
  expect(myModule.__mockRecordError).toHaveBeenCalled();
});

3. If _recordError is a true private field (using # syntax) #

If you’re using JavaScript’s true private fields (#recordError instead of _recordError), Jest can’t directly spy on these since they’re inaccessible outside the class. In this case, your best options are:

• Refactor slightly: Add a test-only hook (e.g., a getter that exposes the mock) or inject a logger dependency (but you said you don’t want to pass it as a parameter, so this might not be ideal).
• Mock the entire class: Override the class implementation in your test to replace the private method’s behavior:

  jest.mock("./ErrorHandler", () => {
    return {
      ErrorHandler: jest.fn().mockImplementation(() => {
        let _recordErrorCalled = false;
        return {
          processRequest: (data) => {
            if (!data) {
              _recordErrorCalled = true;
            }
          },
          // Expose a way to check the call
          wasErrorRecorded: () => _recordErrorCalled
        };
      })
    };
  });
  
  test("triggers error recording for invalid data", () => {
    const handler = new ErrorHandler();
    handler.processRequest(null);
    expect(handler.wasErrorRecorded()).toBe(true);
  });
  

Why you got the original error #

The "Cannot spy the _recordError property because it is not a function; undefined given instead" error usually means:

• You’re trying to spy on an object that doesn’t have the _recordError property (e.g., spying on the class instead of an instance).
• The _recordError property exists but isn’t a function (e.g., it’s a variable instead of a method).
• The this context in the calling method is incorrect, so this._recordError is undefined when the code runs.

内容的提问来源于stack exchange，提问作者Jarede