为何Jest中使用requireActual/genMockFromModule需访问default属性？

Let’s break down exactly why you hit that error and why adding .default fixes the problem:

1. ES Modules vs. CommonJS: The Interop Quirk #

Your original module uses ES module default export (export default class {...}). When Jest (which relies on Node.js’s module system) loads an ES module via require.requireActual or jest.genMockFromModule, it wraps the ES module in a CommonJS-compatible object. The class you exported as the default ends up attached to the .default property of this wrapper object.

The Jest example you looked at uses CommonJS export (module.exports = class User {...}) instead. With CommonJS, require directly returns the class itself—no wrapper, no .default needed. That’s why the example never mentions this step: it’s tailored to CommonJS, not ES modules.

2. Why Your First Mock Failed #

When you wrote this:

const RandomNumber = require.requireActual('../RandomNumber');
RandomNumber.prototype.generate = function() { return this.min; };

RandomNumber wasn’t your class—it was the CommonJS wrapper object for your ES module. This wrapper isn’t a class constructor, so it doesn’t have a prototype property. Trying to set generate on undefined (since RandomNumber.prototype doesn’t exist) threw that TypeError.

Adding .default pulls out the actual class constructor from the wrapper:

const RandomNumber = require.requireActual('../RandomNumber').default;
// Now RandomNumber is your class, so prototype is accessible

3. Why jest.genMockFromModule Needs .default Too #

The same logic applies here. jest.genMockFromModule processes ES modules the same way, returning that CommonJS wrapper. To get the mockable class, you have to grab the .default property first:

const RandomNumberMock = jest.genMockFromModule('../RandomNumber').default;
RandomNumberMock.prototype.generate.mockReturnValue(10);

This is a gotcha from how Node.js handles interop between ES modules and CommonJS. Since many Jest examples stick to CommonJS for simplicity, they don’t cover this ES module-specific edge case.

内容的提问来源于stack exchange，提问作者dragonfly