Jest: How to get arguments passed to mock constructor?

问题

If I want to create a mock implementation of an instance method of an ES6 Class I would do this

// ExampleClass.js
export class ExampleClass {
    constructor(someValue) {
        this.a = someValue;
    }

    exampleMethod(anotherValue) {
        // do something with 'anotherValue'
    }
}

// OtherModule.js
import {ExampleClass} from './ExampleClass';
export const fooBar = () => {
    const ex = new ExampleClass("hello world");
    ex.exampleMethod("another value");
};

// ExampleClass.test.js
import {fooBar} from './OtherModule';
import {ExampleClass} from './ExampleClass';
jest.mock('./ExampleClass');

it('try to create a mock of ExampleClass', () => {
    ExampleClass.mockClear();

    fooBar();

    // to verify values for of instance method "exampleMethod" of ExampleClass instance
    expect(ExampleClass.mock.instances[0].exampleMethod.calls.length).toBe(1);
    expect(ExampleClass.mock.instances[0].exampleMethod.calls[0][0]).toBe("another value");

    // How to verify values for **constructor** of ExampleClass ?????
    // expect(ExampleClass.mock.instances[0].constructor.calls.length).toBe(1);
    // expect(ExampleClass.mock.instances[0].constructor.calls[0][0]).toBe("another value");
});

What I don't know how to do (and sort of alluded to in the commented code) is how to spy on / access the values of the constructor (not just an instance method).

Any help would be greatly appreciated! ❤

回答1:

ExampleClass is the constructor function and since the entire module is auto-mocked it is already set up as a mock function:

import {fooBar} from './OtherModule';
import {ExampleClass} from './ExampleClass';
jest.mock('./ExampleClass');

it('try to create a mock of ExampleClass', () => {
    ExampleClass.mockClear();

    fooBar();

    // to verify values for of instance method "exampleMethod" of ExampleClass instance
    expect(ExampleClass.mock.instances[0].exampleMethod.mock.calls.length).toBe(1);  // SUCCESS
    expect(ExampleClass.mock.instances[0].exampleMethod.mock.calls[0][0]).toBe("another value");  // SUCCESS

    // Verify values for **constructor** of ExampleClass
    expect(ExampleClass.mock.calls.length).toBe(1);  // SUCCESS
    expect(ExampleClass.mock.calls[0][0]).toBe("hello world");  // SUCCESS
});

来源：https://stackoverflow.com/questions/54384793/jest-how-to-get-arguments-passed-to-mock-constructor

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