nodejs/test/parallel/test-readline-async-iterators-backpressure.js

'use strict';

const common = require('../common');
const assert = require('assert');
const { Readable } = require('stream');
const readline = require('readline');

const CONTENT = 'content';
const TOTAL_LINES = 18;

(async () => {
  const readable = new Readable({ read() {} });
  readable.push(`${CONTENT}\n`.repeat(TOTAL_LINES));

  const rli = readline.createInterface({
    input: readable,
    crlfDelay: Infinity
  });

  const it = rli[Symbol.asyncIterator]();
  const highWaterMark = it.stream.readableHighWaterMark;

  // For this test to work, we have to queue up more than the number of
  // highWaterMark items in rli. Make sure that is the case.
  assert(TOTAL_LINES > highWaterMark);

  let iterations = 0;
  let readableEnded = false;
  for await (const line of it) {
    assert.strictEqual(readableEnded, false);

    assert.strictEqual(line, CONTENT);

    const expectedPaused = TOTAL_LINES - iterations > highWaterMark;
    assert.strictEqual(readable.isPaused(), expectedPaused);

    iterations += 1;

    // We have to end the input stream asynchronously for back pressure to work.
    // Only end when we have reached the final line.
    if (iterations === TOTAL_LINES) {
      readable.push(null);
      readableEnded = true;
    }
  }

  assert.strictEqual(iterations, TOTAL_LINES);
})().then(common.mustCall());