length-prefixed-json-stream

length-prefixed-json-stream

A length-prefixed JSON stream reader that accommodates whitespace padding.

Length Prefixed JSON Streams

Length prefixed JSON streams are streams of JSON values, where each value is preceded by the length-in-bytes of the value. (The length is a decimal integer encoded as a string of digits.) Here's an example:

18{"message": "Hi!"}27{"message": "How are you?"}

Whitespace Padding

This module supports ASCII whitespace characters both before and after the length prefix. This has the effect of allowing literal number values to be sent without necessitating a wrapper object, by simply appending a space after the length prefix string. For example:

2 42 2 84 3 168 4 0.42

Usage

import JSONStreamIterator from 'length-prefixed-json-stream'

async function responseHandler (response) {
  const streamIterator = new JSONStreamIterator(response)

  let value
  while ((value = await streamIterator.readNextValue()) !== undefined) {
    console.log(value)
  }
}

If your JavaScript environment supports it, you can use the more readable for await…of:

async function responseHandler (response) {
  const streamIterator = new JSONStreamIterator(response)
  for await (const value of streamIterator) {
    console.log(value)
  }
}

API

import JSONStreamIterator from 'length-prefixed-json-stream'

new JSONStreamIterator(stream)

Reads values one at a time from stream.

• stream: Node.js readable stream.

.readNextValue()

Returns a promise that resolves to the next value from the input stream. When the stream ends, the promise will resolve to undefined. If there is an error parsing the stream, or some sort of connection error, the promise will be rejected with the error.

.next()

A for await…of (async iterator) compatible method. If you are using for await…of, this is called for you, but you can also use it explicitly if you would like. It wraps the returned values according to the spec: while not complete, returns a promise that resolves to {done: false, value: …}, and when complete, returns a promise that resolves to {done: false}.

.return()

Also a part of the async iterator spec, but you can call it explicitly to end iteration early and free resources.