Basic mockito-style mocking and stubbing in JavaScript

mocking.js

const mockField = Symbol("mockField");

class Stub {
    constructor(mockTarget, property, receiver, matchers = []) {
        this.mockTarget = mockTarget;
        this.property = property;
        this.receiver = receiver;
        this.matchers = matchers;

        this.mocked = () => null;
    }

    matches(targetSecondary, propertySecondary, receiverSecondary) {
        return propertySecondary === this.property;
    }

    getMocked(args) {
        return this.mocked(args);
    }

    thenAnwser(answer) {
        this.mocked = answer;
    }

    thenReturn(val) {
        this.mocked = () => val;
    }

    thenThrow(throwable) {
        this.mocked = () => throw throwable;
    }


    thenCallActual() {
        this.mocked = (args) => this.mockTarget[this.property](args);
    }
}

const when = (mockTarget) => new Proxy(mockTarget, {
    get(mockTarget, property, receiver) {
        const mockInstance = mockTarget[mockField];

        if (!(property in mockInstance.actual)) {
            throw new Error("Property does not exist in mock!");
        }

        return (...matchers) => {
            const propMock = new Stub(mockTarget, property, receiver, matchers);
            mockInstance.stubs.push(propMock);
            return propMock;
        };
    }
});

const mock = (target) => {
    const mockInstance = {
        stubs: [],
        actual: target
    };

    return new Proxy(target, {
        get(target, property, receiver) {
            if (property === mockField) {
                return mockInstance;
            }

            for (let stub of mockInstance.stubs) {
                // TODO add matcher checks
                if (stub.matches(target, property, receiver)) {
                    return (...args) => stub.getMocked(args);
                }
            }

            if (property in target) {
                return () => null;
            }
        }
    });
};



/**
 * Example
 */

// Example class
class Foo {
    getNumber() {
        return 0;
    }
}

const fooMock = mock(new Foo()); // Create plain mock.
when(fooMock).getNumber().thenReturn(100); // Mock getNumber() to return 100

console.log(fooMock.getNumber()); // Logs 100