@stdlib/streams-node-debug

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Debug Stream

Transform stream for debugging stream pipelines.

Installation

npm install @stdlib/streams-node-debug

Usage

var debugStream = require( '@stdlib/streams-node-debug' );

debugStream( [options,] [clbk] )

Creates a transform stream for debugging stream pipelines.

var ENV = require( '@stdlib/process-env' );

// Set the `DEBUG` environment variable...
ENV.DEBUG = '*';

var stream = debugStream({
    'name': 'my-stream'
});

stream.write( 'a' );
stream.write( 'b' );
stream.write( 'c' );
stream.end();

The function accepts the following options:

• name: debug namespace.
• objectMode: specifies whether a stream should operate in objectMode. Default: false.
• highWaterMark: specifies the Buffer level at which write() calls start returning false.
• allowHalfOpen: specifies whether a stream should remain open even if one side ends. Default: false.
• readableObjectMode: specifies whether the readable side should be in objectMode. Default: false.

To set stream options,

var opts = {
    'name': 'my-app',
    'objectMode': true,
    'highWaterMark': 64,
    'allowHalfOpen': true,
    'readableObjectMode': false // overridden by `objectMode` option when `objectMode=true`
};

var stream = debugStream( opts );

By default, each chunk is logged as a JSON stringified string, along with the chunk index. For more control over logging behavior, provide a callback.

function logger( debug, chunk, idx ) {
    debug( 'Received a new chunk...' );
    debug( 'Beep: %s', chunk.beep );
    debug( 'Boop: %s', chunk.boop );
}

var opts = {
    'name': 'my-pipeline'
};

var stream = debugStream( opts, logger );

debugStream.factory( [options] )

Returns a function for creating streams which are identically configured according to provided options.

var opts = {
    'objectMode': true,
    'highWaterMark': 64
};

var factory = debugStream.factory( opts );

This method accepts the same options as debugStream(), except for name, which must be provided explicitly.

factory( name[, clbk] )

Creates a debug stream.

var factory = debugStream.factory();

var streams = [];
var i;

// Assign each stream to a separate debug namespace...
for ( i = 0; i < 10; i++ ) {
    streams.push( factory( 'stream '+i ) );
}

debugStream.objectMode( [options,] [clbk] )

This method is a convenience function to create streams which always operate in objectMode.

var stream = debugStream.objectMode({
    'name': 'beep-boop'
});

stream.write({
    'value': 'a'
});
stream.write({
    'value': 'b'
});
stream.write({
    'value': 'c'
});
stream.end();

This method accepts the same options as debugStream(); however, the method will always override the objectMode option in options.

Notes

• If the DEBUG environment variable is not set, no data is logged.
• Providing a name option is strongly encouraged, as the DEBUG environment variable can be used to filter debuggers.

Examples

var parseJSON = require( '@stdlib/utils-parse-json' );
var stdout = require( '@stdlib/streams-node-stdout' );
var transformFactory = require( '@stdlib/streams-node-transform' ).factory;
var debug = require( '@stdlib/streams-node-debug' ).objectMode;

function parse( chunk, enc, clbk ) {
    clbk( null, parseJSON( 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 for generating streams running in `objectMode`:
var tStream = transformFactory({
    'objectMode': true
});

// Create streams for each transform:
var s1 = tStream( parse );
var d1 = debug({
    'name': 'parse'
});
var s2 = tStream( pluck );
var d2 = debug({
    'name': 'pluck'
});
var s3 = tStream( square );
var d3 = debug({
    'name': 'square'
});
var s4 = tStream( toStr );
var d4 = debug({
    'name': 'toString'
});
var s5 = tStream( join );
var d5 = debug({
    'name': 'join'
});

// Create the pipeline:
s1.pipe( d1 )
    .pipe( s2 )
    .pipe( d2 )
    .pipe( s3 )
    .pipe( d3 )
    .pipe( s4 )
    .pipe( d4 )
    .pipe( s5 )
    .pipe( d5 )
    .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();

See Also

• @stdlib/streams-node/debug-sink: writable stream for debugging stream pipelines.
• @stdlib/streams-node/inspect: transform stream for inspecting streamed data.

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.