Streams & Performance — Series 2

Preview — 3 of 10 questions

What is eventually logged?

javascript

const { Transform } = require('stream');

class UppercaseTransform extends Transform {
  _transform(chunk, encoding, callback) {
    this.push(chunk.toString().toUpperCase());
    callback();
  }
}

const upper = new UppercaseTransform();
let output = '';
upper.on('data', (chunk) => {
  output += chunk;
});
upper.on('end', () => {
  console.log(output);
});

upper.write('hello ');
upper.write('world');
upper.end();

A"hello world"

B"HELLO WORLD"

C"HELLO " then "WORLD" logged as two separate lines

DTypeError: _transform is not a valid stream method

What does a false return value from writable.write(chunk) signal, and what should well-behaved producer code do in response?

AIt signals the write permanently failed and the chunk was discarded; the same chunk should never be retried.

BIt signals the stream has been closed; no further writes are possible under any circumstances.

Cwrite() never returns false for Writable streams; only Readable streams can signal backpressure.

DIt signals "backpressure" — the stream's internal buffer has reached or exceeded its highWaterMark, meaning the consumer can't keep up; well-behaved code should pause writing further data and wait for the stream's 'drain' event before writing again, to avoid unbounded memory growth.

What is the logging order?

javascript

console.log('start');

setImmediate(() => console.log('setImmediate'));

process.nextTick(() => console.log('nextTick'));

Promise.resolve().then(() => console.log('promise'));

console.log('end');

A"start", "end", "promise", "nextTick", "setImmediate"

B"start", "end", "setImmediate", "nextTick", "promise"

C"start", "end", "nextTick", "promise", "setImmediate"

D"start", "nextTick", "end", "promise", "setImmediate"

Answer all 10 questions (7 more), see explanations for every answer, and track your score.