stdlib-js
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Transform Stream
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[Transform stream][transform-stream].
bash
npm install @stdlib/streams-node-transform
javascript
var transformStream = require( '@stdlib/streams-node-transform' );
#### transformStream( [options] )
Creates a [transform stream][transform-stream].
javascript
var stdout = require( '@stdlib/streams-node-stdout' );
function transform( chunk, enc, clbk ) {
clbk( null, chunk.toString()+'\n' );
}
var stream = transformStream({
'transform': transform
});
stream.pipe( stdout );
stream.write( '1' );
stream.write( '2' );
stream.write( '3' );
stream.end();
// e.g., => '1\n2\n3\n'
The function accepts the following options:
- transform: callback to invoke upon receiving a new chunk.
- flush: callback to invoke after receiving all chunks and prior to a stream closing.
- objectMode: specifies whether a [stream][stream] should operate in [objectMode][object-mode]. Default: false.
- encoding: specifies how Buffer objects should be decoded to strings. Default: null.
- highWaterMark: specifies the Buffer level at which write() calls start returning false.
- allowHalfOpen: specifies whether a [stream][stream] should remain open even if one side ends. Default: false.
- decodeStrings: specifies whether to decode strings into Buffer objects when writing. Default: true.
To set [stream][stream] options,
javascript
var opts = {
'objectMode': true,
'encoding': 'utf8',
'highWaterMark': 64,
'allowHalfOpen': true,
'decodeStrings': false
};
var stream = transformStream( opts );
The transform option should be a function having the following signature:
javascript
function transform( chunk, enc, clbk ) {
var multipleData;
var err;
if ( multipleData ) {
// Push as many chunks as required...
this.push( chunk );
this.push( chunk );
// ...
if ( err ) {
return clbk( err );
}
return clbk();
}
if ( err ) {
return clbk( err );
}
clbk( null, chunk );
}
The flush option should be a function which performs any remaining tasks before the stream closes, such as unfinished data processing.
javascript
function flush( clbk ) {
var err;
// Push any remaining chunks...
this.push( '...' );
this.push( '...' );
// ...
if ( err ) {
return clbk( err );
}
clbk();
}
If no transform is provided, the returned stream will be a simple pass through stream.
#### transformStream.factory( [options] )
Returns a function for creating [streams][transform-stream] which are identically configured according to provided options.
javascript
var opts = {
'objectMode': true,
'encoding': 'utf8',
'highWaterMark': 64
};
var factory = transformStream.factory( opts );
This method accepts the same options as transformStream(), except for the transform and flush options which must be provided explicitly, as shown below.
##### factory( transform[, flush] )
Creates a [transform stream][transform-stream].
javascript
function transform1( chunk, enc, clbk ) {
clbk( null, chunk.toString()+'\n' );
}
function transform2( chunk, enc, clbk ) {
clbk( null, chunk.toString()+'\t' );
}
function flush( clbk ) {
clbk();
}
var factory = transformStream.factory();
var s1 = factory( transform1 );
var s2 = factory( transform2, flush );
#### transformStream.objectMode( [options] )
This method is a convenience function to create [streams][stream] which always operate in [objectMode][object-mode].
javascript
var stdout = require( '@stdlib/streams-node-stdout' );
function stringify( chunk, enc, clbk ) {
clbk( null, JSON.stringify( chunk ) );
}
function newline( chunk, enc, clbk ) {
clbk( null, chunk+'\n' );
}
var s1 = transformStream.objectMode({
'transform': stringify
});
var s2 = transformStream.objectMode({
'transform': newline
});
s1.pipe( s2 ).pipe( stdout );
s1.write( { 'value': 'a' } );
s1.write( { 'value': 'b' } );
s1.write( { 'value': 'c' } );
s1.end();
// e.g., => '{"value":"a"}\n{"value":"b"}\n{"value":"c"}\n'
This method accepts the same options as transformStream(); however, the method will always override the [objectMode][object-mode] option in options.
#### transformStream.ctor( [options] )
Instead of returning a [transform stream][transform-stream] instance, this method returns a custom [transform stream][transform-stream] constructor. If provided transform and flush options, these methods are bound to the constructor prototype. If not provided a transform, the returned constructor creates simple pass through streams.
javascript
function transform( chunk, enc, clbk ) {
clbk( null, chunk.toString()+'\n' );
}
function flush( clbk ) {
this.push( 'beep' );
clbk();
}
var opts = {
'transform': transform,
'flush': flush
};
var MyStream = transformStream.ctor( opts );
var bool = ( MyStream.prototype._transform === transform );
// returns true
bool = ( MyStream.prototype._flush === flush );
// returns true
The returned constructor accepts the same options as documented above, except for the transform and flush options, which are not supported. Any options provided to the constructor override options provided to the constructor factory.
javascript
var stdout = require( '@stdlib/streams-node-stdout' );
var factory = require( '@stdlib/streams-node-transform' ).factory;
function parse( chunk, enc, clbk ) {
clbk( null, JSON.parse( chunk ) );
}
function pluck( chunk, enc, clbk ) {
clbk( null, chunk.value );
}
function square( chunk, enc, clbk ) {
var v = +chunk;
clbk( null, v*v );
}
function toStr( chunk, enc, clbk ) {
clbk( null, chunk.toString() );
}
function join( chunk, enc, clbk ) {
clbk( null, chunk+'\n' );
}
// Create a factory which generates streams running in `objectMode`:
var tStream = factory({
'objectMode': true
});
// Create streams for each transform:
var s1 = tStream( parse );
var s2 = tStream( pluck );
var s3 = tStream( square );
var s4 = tStream( toStr );
var s5 = tStream( join );
// Create the pipeline:
s1.pipe( s2 )
.pipe( s3 )
.pipe( s4 )
.pipe( s5 )
.pipe( stdout );
// Write data to the pipeline...
var v;
var i;
for ( i = 0; i < 100; i++ ) {
v = '{"value":'+i+'}';
s1.write( v, 'utf8' );
}
s1.end();
