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A through2 to create an Array.prototype.map analog for streams.
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README.md

through2-map

NPM

This is a super thin wrapper around through2 that works like Array.prototype.map but for streams.

For when through2 is just too verbose :wink:

Note you will NOT be able to skip chunks. This is intended for modification only. If you want filter the stream content, use either through2 or through2-filter.

IMPORTANT: If you return null from your function, the stream will end there.

var map = require("through2-map")

var truncate = map(function (chunk) {
  return chunk.slice(0, 10)
})

// vs. with through2:
var truncate = through2(function (chunk, encoding, callback) {
  this.push(chunk.slice(0, 10))
  return callback()
})

// Then use your map:
source.pipe(truncate).pipe(sink)

// Additionally accepts `wantStrings` argument to conver buffers into strings
var stripTags = map({wantStrings: true}, function (str) {
  // OMG don't actually use this
  return str.replace(/<.*?>/g, "")
})

// Works like `Array.prototype.map` meaning you can specify a function that
// takes up to two* arguments: fn(chunk, index)
var spaceout = map({wantStrings: true}, function (chunk, index) {
  return (index % 2 == 0) ? chunk + "\n\n" : chunk
})

// vs. with through2:
var spaceout = through2(function (chunk, encoding, callback) {
  if (this.index == undefined) this.index = 0
  var buf = (this.index++ % 2 == 0) ? Buffer.concat(chunk, new Buffer("\n\n")) : chunk
  this.push(buf)
  return callback()
})

*Differences from Array.prototype.map:

  • Cannot insert null elements into the stream without aborting.
  • No third array callback argument. That would require realizing the entire stream, which is generally counter-productive to stream operations.
  • Array.prototype.map doesn't modify the source Array, which is somewhat nonsensical when applied to streams.

Options

  • wantStrings: Automatically call chunk.toString() for the super lazy.
  • all other through2 options

LICENSE

MIT

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