Node.js: splitting stream content for n-parts

Question

I\'m trying to understand node streams and their life-cycle. So, I want to split the content of a stream for n-parts. The code below is just to explain my intentions and to

Answer 1

Using ReadableStream you could use a single function to check if elements of current chunk of data is equal to expected result.

Create variables, CHUNK and N, where CHUNK is the number of elements to slice or splice from original array, N is the variable incremented by CHUNK at each .enqueue() call within pull() call.

const [data, CHUNK, result] = [[1,2,3,4,5,6], 3, []];

let N = 0;

const stream = new ReadableStream({
  pull(controller) {
    if (N < data.length)
      // slice `N, N += CHUNK` elements from `data`
      controller.enqueue(data.slice(N, N += CHUNK))
    else
      // if `N` is equal to `data.length` call `.close()` on stream
      controller.close()
  }
});

const reader = stream.getReader();

const processData = ({value, done}) => {
  // if stream is closed return `result`; `reader.closed` returns a `Promise`
  if (done) return reader.closed.then(() => result);
  if (data.slice(N - CHUNK, N).every((n, index) => n === value[index])) {
    console.log(`N: ${N}, value: [${value}]`)
    result.push(...value);
    return reader.read().then(data => processData(data))
  }
}

const readComplete = res => console.log(`result: [${res}]`);

reader.read()
.then(processData)
.then(readComplete)
.catch(err => console.log(err));

Using chained .then()

const [data, CHUNK, result] = [[1,2,3,4,5,6], 3, []];

let N = 0;

const stream = new ReadableStream({
  pull(controller) {
    if (N < data.length)
      // slice `N, N += CHUNK` elements from `data`
      controller.enqueue(data.slice(N, N += CHUNK))
    else
      // if `N` is equal to `data.length` call `.close()` on stream
      controller.close()
  }
});

const reader = stream.getReader();

const processData = ({value, done}) => {
  // if stream is closed return `result`; `reader.closed` returns a `Promise`
  if (done) return reader.closed.then(() => result);
  if (data.slice(N - CHUNK, N).every((n, index) => n === value[index])) {
    console.log(`N: ${N}, value: [${value}]`)
    result.push(...value);
    return reader.read().then(data => processData(data))
  }
}

const readComplete = res => console.log(`result: [${res}]`);

reader.read()
.then(({value, done}) => {
  if ([1,2,3].every((n, index) => n === value[index])) {
    console.log(`N: ${N}, value: [${value}]`)
    result.push(...value);
    return reader.read()
  }
})
.then(({value, done}) => {
  if ([4,5,6].every((n, index) => n === value[index])) {
    console.log(`N: ${N}, value: [${value}]`)
    result.push(...value);
    // return `result`; `reader.closed` returns a `Promise`
    return reader.closed.then(() => result);
  }
})
.then(readComplete)
.catch(err => console.log(err));

See also Chrome memory issue - File API + AngularJS