catchment
Collect Node.JS Stream Data Into Catchment: Stream With Promise Property Resolved On Finish.
Last updated 2 years ago by zvr .
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catchment

npm version

A Node.JS package to collect stream data into a catchment and return it either as a buffer or a string.

yarn add -E catchment
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Table of Contents

API

The package exports the default Catchment class, and the collect method.

import Catchment, { collect } from 'catchment'

The types and externs for Google Closure Compiler via Depack are defined in the _catchment namespace.

Catchment Class

Catchment extends Writable, and pushes incoming data into an internal array. When the stream finishes, a promise referenced in the promise property is fulfilled with concatenated data. If an error occurred, the promise is rejected.

A new Catchment can be created with a constructor, which accepts optional options.

/* yarn example/catchment.js */
import { Readable } from 'stream'
import Catchment from 'catchment'

const DATA = 'test-data'

// creating a readable stream to use in the example
const rs = new Readable({
  read() {
    for (let i = 0; i < DATA.length; i++) {
      const c = DATA.charAt(i)
      this.push(c)
    }
    this.push(null)
  },
})

;(async () => {
  try {
    const catchment = new Catchment()
    rs.pipe(catchment)
    const res = await catchment.promise
    console.log(res)
  } catch (err) {
    console.log(err)
  }
})()
test-data

constructor(
  options?: Options,
): Catchment

Options

An optional options object can be passed to the constructor.

import('stream').Readable stream.Readable

_catchment.Options: Options to pass to the Writable super constructor, and others shown below.

Name Type Description Default
rs !stream.Readable A readable stream to automatically pipe into the catchment. If an error occurs during reading of this stream, the catchment promise will be rejected with it. -
binary boolean Whether to return a raw buffer instead of a string. The string is created by joining all incoming chunks together with .join('') method. false

Collect Buffer

To receive a buffer, the binary option should be set to true:

/* yarn example/binary.js */
import Catchment from 'catchment'
import { createReadable } from './lib'

(async () => {
  try {
    const rs = createReadable('test-data')
    const catchment = new Catchment({ binary: true })
    rs.pipe(catchment)

    const res = await catchment.promise
    console.log(res)
  } catch (err) {
    console.log(err)
  }
})()
<Buffer 74 65 73 74 2d 64 61 74 61>

Pipe Readable

To automatically pipe a Readable, and reject the promise if an error occurs there, the rs option can be passed:

/* yarn example/rs.js */
import Catchment from 'catchment'
import { createReadStream } from 'fs'

(async () => {
  try {
    const rs = createReadStream('missing-file.txt')
    const { promise } = new Catchment({ rs })

    const res = await promise
    console.log(res)
  } catch ({ message }) {
    console.log(message)
  }
})()
ENOENT: no such file or directory, open 'missing-file.txt'

async collect(
  readable: Readable,
  options?: CollectOptions,
): string|Buffer

The collect method is a shorthand for creating a new catchment, and piping a readable stream into it. It will accumulate all data from the read stream, and asynchronously return when the stream finishes. If an error occurs in the stream, the promise will be rejected.

Some options can be passed to the collect method. The proxyError option is described in the Proxy Error section.

_catchment.CollectOptions: Options when collecting data into a catchment. They can extend Writable options which will be passed to the Catchment constructor.

Name Type Description Default
binary boolean Whether to return a raw buffer instead of a string. The string is created by joining all incoming chunks together with .join('') method. false
proxyError boolean Sets whether an error emitted by the stream with have its stack start at the line where the collect was called rather than inside of the stream. In other words, hides the implementation of the stream. false

Errors Handling

Whenever an error is encountered during reading a readable stream, either piped into a Catchment via the rs option, or passed as an argument to the collect method, it will result in a rejected promise.

If the error has a stack, it will be modified to clean it from internal Node.js lines, such as _module.

import { Readable } from 'stream'
import Catchment from 'catchment'

const rs = new Readable({
  read() {
    const er = new Error('example-error')
    this.emit('error', er) // emit an error to reject catchment
    this.push(null)
  },
})

;(async () => {
  try {
    const catchment = new Catchment({
      rs,
    })
    rs.pipe(catchment)
    await catchment.promise
  } catch ({ stack }) {
    console.log(stack)
  }
})()
Error: example-error
    at Readable.read [as _read] (/Users/zavr/adc/catchment/example/error-catchment.js:6:16)

If the error does not have the stack (which can happen when using createReadStream from the fs module), it will appear as thrown at the point of either creating an instance of Catchment, or calling the collect method.

import { createReadStream } from 'fs'
import { collect } from 'catchment'

(async () => {
  try {
    const rs = createReadStream('missing-file.txt')
    await collect(rs)
  } catch ({ stack }) {
    console.log(stack)
  }
})()
Error: ENOENT: no such file or directory, open 'missing-file.txt'
    at /Users/zavr/adc/catchment/example/error-collect.js:7:11
    at Object.<anonymous> (/Users/zavr/adc/catchment/example/error-collect.js:11:3)

Proxy Error

The collect method can throw an error with its stack updated to when it was called. This can be useful when using 3-rd party streams without the need to look into details of their internal stack. By setting the proxyError option, all internal lines of the stream will be hidden, and the error will appear to be thrown by the call to the collect method.

import { collect } from 'catchment'
import { createReadable } from './lib'
import frame from 'frame-of-mind'

/** 0. Prepare a read function in a stream that emits an error. */
function read() {
  const err = new Error('Whatever error happens')
  setTimeout(() => {
    this.emit('error', err)
    this.push(null)
  }, 10)
}

const Collect = async ({ proxyError } = {}) => {
  try {
    const rs = createReadable(read)
    await collect(rs, { proxyError })
  } catch ({ stack }) {
    console.log('COLLECT %s \n%s', proxyError ? 'WITH PROXY' : '', frame(stack))
  }
}

const Listeners = async () => {
  try {
    const rs = createReadable(read)
    const p = collect(rs).catch(() => {})
    await new Promise((r, j) => {
      rs.on('finish', r)
      rs.on('error', j)
    })
    await p
  } catch ({ stack }) {
    console.log('LISTENERS:\n%s', frame(stack))
  }
}

(async () => {
  await Collect()
  await Listeners()
  await Collect({ proxyError: true })
})()
COLLECT  
┌────────────────────────────────────────────────────────────────────────────────────────┐
│ Error: Whatever error happens                                                          │
│     at Readable.read (/Users/zavr/adc/catchment/example/error-collect2.js:8:15)        │
│     at Readable.read [as _read] (/Users/zavr/adc/catchment/example/lib/index.js:11:16) │
└────────────────────────────────────────────────────────────────────────────────────────┘
LISTENERS:
┌────────────────────────────────────────────────────────────────────────────────────────┐
│ Error: Whatever error happens                                                          │
│     at Readable.read (/Users/zavr/adc/catchment/example/error-collect2.js:8:15)        │
│     at Readable.read [as _read] (/Users/zavr/adc/catchment/example/lib/index.js:11:16) │
└────────────────────────────────────────────────────────────────────────────────────────┘
COLLECT WITH PROXY 
┌────────────────────────────────────────────────────────────────────────────┐
│ Error: Whatever error happens                                              │
│     at Collect (/Users/zavr/adc/catchment/example/error-collect2.js:18:11) │
│     at /Users/zavr/adc/catchment/example/error-collect2.js:41:9            │
│     at <anonymous>                                                         │
└────────────────────────────────────────────────────────────────────────────┘

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