An example of a nasty stream test

example-stream.spec.ts

import { expect } from 'chai';
import { stub } from 'sinon';
import * as zlib from 'zlib';
import { streamExample } from '../src/stream-example';
import { Readable, Writable } from 'stream';

describe('streamExample()', () => {
	let data: any[];
	let writable: Writable;

	beforeEach(() => {
		data = [];
		writable = new Writable({
			write(chunk: Buffer, _encoding: string, next: Function) {
				data.push(chunk.toString());
				next();
			},
		});
		stub(zlib, 'createGzip').returns(writable as any);
	});

	it('should pipe the given stream to a gzip writer', async () => {
		let index = 0;
		const stream = new Readable({
			read() {
				if (index < 3) {
					index++;
					this.push(`value${index}`);
				} else {
					this.push(null);
				}
			}
		});
		stub(stream, 'pipe').callThrough();

		const result = streamExample(stream);
		await new Promise((resolve, reject) => {
			result.on('finish', resolve);
			result.on('error', reject);
		});

		expect(zlib.createGzip).to.have.been.calledOnceWithExactly();
		expect(stream.pipe).to.have.been.calledOnceWithExactly(writable);
		expect(data).to.be.eql(['value1', 'value2', 'value3']);
	});
})