TypeScript构建Express服务器时Jest模拟类方法遇TS2339错误求助

Let’s break down why you’re hitting the TS2339 error and walk through actionable fixes:

The Root Cause #

TypeScript enforces type checking based on the original Orig class definition in src/orig.ts—it has no awareness of the __fake static property you added in your mock file. When you try to access Orig.__fake, TypeScript throws TS2339 because that property doesn’t exist in the original type schema.

Solution 1: Augment the Original Class Type #

You can extend the type of the Orig class to include the __fake property using TypeScript’s module augmentation. This tells TypeScript to recognize the mock-only property as valid:

1. Create a type declaration file (e.g., src/orig.mock.d.ts) to augment the original module:

// src/orig.mock.d.ts
import { Orig } from './orig';

declare module './orig' {
  interface Orig {
    /** Static fake property for Jest mocking */
    __fake: string;
  }
}

2. Ensure your tsconfig.json includes this declaration file. Most standard configs automatically pick up .d.ts files in the source directory, but you can explicitly add it to the include array if needed.

Solution 2: Use Type Assertion in Tests #

If you only need to access __fake in test files, a quick fix is to use type assertion to bypass TypeScript’s strict checking for the mock property:

// In your test file
import { Orig } from '../src/orig';

jest.mock('../src/orig');

// Assert that Orig includes the __fake property
const mockedOrig = Orig as typeof Orig & { __fake: string };

// Now you can modify or access __fake without TS errors
mockedOrig.__fake = 'custom fake value';
expect(mockedOrig.testFunc()).toBe('custom fake value');

Solution 3: Avoid Extra Properties with Jest Mock Functions #

Instead of adding a custom __fake property, use Jest’s built-in mock functions to control return values. This eliminates type mismatches entirely:

Update your mock file to use a Jest mock function:

// src/__mocks__/orig.ts
export class Orig {
  static testFunc = jest.fn(() => 'fake');
}

Then in your tests, adjust the mock return value like this:

// In your test file
import { Orig } from '../src/orig';

jest.mock('../src/orig');

// Type assertion to access the mock function's methods
const mockedTestFunc = Orig.testFunc as jest.Mock;

mockedTestFunc.mockReturnValue('custom fake');
expect(Orig.testFunc()).toBe('custom fake');

Quick Check: Verify Jest Mock Resolution #

Ensure your jest.config.js (or jest.config.ts) is set to recognize mocks in the __mocks__ directory—standard setups handle this automatically, but if you’re using custom import paths, confirm the mock path aligns with the original module’s import path.

内容的提问来源于stack exchange，提问作者Tomer Amir