Mimicking a ChildProcess in a nodeJS unit test

ChildProcess.test.ts

/**
 * This is the function that we'll be testing. It takes a ChildProcess as
 * an input arg, collects all the messages from the 'stderr' stream, then
 * throws an Error with all the error messages concatenated once the
 * ChildProcess ends
 */
function collectChildProcessErrors(cp: ChildProcess) {
    const errorMessages: string[] = [];

    cp.on('stderr', (data: Buffer) => {
        errorMessages.push(data.toString());
    });

    return new Promise<void>((resolve, reject) => {
        cp.on('close', () => {
            if (errorMessages.length > 0) {
                resolve();
                // reject(new Error(errorMessages.join('; ')));
                return;
            } else {
                resolve();
                return;
            }
        });
    });
}

describe.('ChildProcess', () => {
    it('mock a ChildProcess', async () => {
        // This EventEmitter is masquerading as a ChildProcess by emitting events on the 'stderr' channel instead of writing to the 'stderr' stream
        const childProcessWithErrors = new EventEmitter() as ChildProcess;
        setTimeout(() => {
            childProcessWithErrors.emit('stderr', 'This is error 1');
            childProcessWithErrors.emit('stderr', 'This is error 2');
            childProcessWithErrors.emit('close');
        }, 0);

        try {
            await collectChildProcessErrors(childProcessWithErrors);
            assert.fail('Expected the function to throw an Error, but it didnt');
        } catch (error) {
            expect(error.message).to.have.string('This is error 1');
            expect(error.message).to.have.string('This is error 2');
        }
    });
});