Streams in JavaScript

Question

I am using gulp and would like to create a function that can be piped into that passes on the input to the output and writes a single line to the console.

For example:

   this.src('/foo')
        .pipe(myFunction)
        .pipe(...)

Can someone help me understand what myFunction needs to look like?

The following code, stops the stream in its tracks. How can I ensure the stream data is passed onwards?

function myFunction(obj) {
  var stream = new Stream.Transform({objectMode: true});

  stream._transform = function () {
    console.log('foo bar bam baz');
  };

  return stream;
}

Juho · Accepted Answer · 2015-06-21 11:49:39Z

2

Transformation stream is meant - unsuprisingly - for transformation of data. An example of this is zlib stream, which compresses the data written to it, and the compressed bytes can be read out from the same stream.

You are creating a transformation stream that doesn't transform anything - that's quite ok. But you still need to pass the "transformed value" (in this case the same value) along. In it's current form, your stream only consumes what's put in. The _transform function recieves 3 arguments: data, encoding and callback, which has to be called for example like this:

function myFunction(obj) {
    var stream = new Stream.Transform({objectMode: true});

    stream._transform = function (data, encoding, callback) {
        console.log('foo bar bam baz');
        callback(null, data);
    };

    return stream;
}

Source: https://nodejs.org/api/stream.html#stream_transform_transform_chunk_encoding_callback

Excellent guide on Node streams: https://github.com/substack/stream-handbook

answered Jun 21, 2015 at 11:49

Juho

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2 Comments

Juho Over a year ago

Also, _transform gets called every time a new chunk is recieved, so you'll have to work around that, if you only want one line in your console.

Ben Aston Over a year ago

Is there a word to describe these stream steps that are piped together?

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