如何使用Jest正确模拟Web3默认导出的构造函数以返回指定模拟对象？

Let's break down why your current setup isn't working first, then fix it:

The Root Cause #

When you use new with a constructor function in JavaScript, if the constructor returns a primitive value (like your string 'Hello, world!'), JavaScript ignores that return value and instead returns the newly created instance object. That's why you're seeing mockConstructor {} instead of your expected string—your mock's return value is being discarded automatically.

Fix 1: Mock the Constructor to Return a Custom Object (Recommended) #

Since Web3 instances are objects by design, you should mock the constructor to return an object with the properties/methods your tests need. Here's how to adjust your code:

import Web3 from 'web3';

jest.mock('web3', () => {
  // Return a mock constructor that creates our custom instance
  return jest.fn().mockImplementation(() => ({
    // Add any Web3 instance properties/methods your test relies on here
    eth: {
      requestAccounts: jest.fn().mockResolvedValue(['0x1234567890abcdef'])
    },
    // Optional: Add a toString method to mimic your original expected output
    toString: () => 'Hello, world!'
  }));
});

describe('app', () => {
  test('mock web3', () => {
    console.log('web3:', Web3);
    const web3 = new Web3(window.ethereum);
    
    console.log("new web3:", web3); // Logs the custom mock object
    console.log("new web3 as string:", web3.toString()); // Logs 'Hello, world!'
    
    // Verify the constructor was called with the correct argument
    expect(Web3).toHaveBeenCalledWith(window.ethereum);
  });
});

Fix 2: Simulate a "Primitive-like" Instance (Edge Case) #

If you absolutely need to mimic a primitive return (though this isn't realistic for actual Web3 usage), you can create an object that behaves like a string:

jest.mock('web3', () => {
  return jest.fn().mockImplementation(() => {
    const mockInstance = Object.create(String.prototype);
    Object.assign(mockInstance, 'Hello, world!');
    return mockInstance;
  });
});

This creates an object that acts like a string (e.g., web3 === 'Hello, world!' will return true) but remains technically an object, which aligns with JavaScript's constructor rules.

Why Your Previous Attempts Didn't Work #

• Moving the import after jest.mock() has no effect because Jest automatically hoists all jest.mock() calls to the top of the module.
• Switching to require() doesn't change JavaScript's core behavior around constructor return values—primitive returns are still ignored.

内容的提问来源于stack exchange，提问作者lmat - Reinstate Monica