@stdlib/streams/node/transform/lib/ctor.js

Streams.

/**
* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

'use strict';

// MODULES //

var logger = require( 'debug' );
var Transform = require( 'readable-stream' ).Transform;
var inherit = require( '@stdlib/utils/inherit' );
var assign = require( '@stdlib/object/assign' );
var DEFAULTS = require( './defaults.json' );
var validate = require( './validate.js' );
var destroy = require( './destroy.js' );
var _transform = require( './_transform.js' ); // eslint-disable-line no-underscore-dangle


// VARIABLES //

var debug = logger( 'transform-stream:ctor' );


// MAIN //

/**
* Transform stream constructor factory.
*
* @param {Options} [options] - stream options
* @param {Function} [options.transform] - callback to invoke upon receiving a new chunk
* @param {Function} [options.flush] - callback to invoke after receiving all chunks and prior to the stream closing
* @param {boolean} [options.objectMode=false] - specifies whether a stream should operate in object mode
* @param {(string|null)} [options.encoding=null] - specifies how `Buffer` objects should be decoded to `strings`
* @param {NonNegativeNumber} [options.highWaterMark] - specifies the `Buffer` level for when `write()` starts returning `false`
* @param {boolean} [options.allowHalfOpen=false] - specifies whether the stream should remain open even if one side ends
* @param {boolean} [options.decodeStrings=true] - specifies whether to decode `strings` into `Buffer` objects when writing
* @throws {TypeError} options argument must be an object
* @throws {TypeError} must provide valid options
* @returns {Function} Transform stream constructor
*
* @example
* var stdout = require( '@stdlib/streams/node/stdout' );
*
* function transform( chunk, enc, clbk ) {
*     clbk( null, chunk.toString()+'\n' );
* }
*
* var opts = {
*     'transform': transform
* };
*
* var TransformStream = ctor( opts );
*
* var stream = new TransformStream();
*
* stream.pipe( stdout );
*
* stream.write( '1' );
* stream.write( '2' );
* stream.write( '3' );
*
* stream.end();
*
* // prints: '1\n2\n3\n'
*/
function ctor( options ) {
	var transform;
	var copts;
	var err;

	copts = assign( {}, DEFAULTS );
	if ( arguments.length ) {
		err = validate( copts, options );
		if ( err ) {
			throw err;
		}
	}
	if ( copts.transform ) {
		transform = copts.transform;
	} else {
		transform = _transform;
	}
	/**
	* Transform stream constructor.
	*
	* @private
	* @constructor
	* @param {Options} [options] - stream options
	* @param {boolean} [options.objectMode=false] - specifies whether a stream should operate in object mode
	* @param {(string|null)} [options.encoding=null] - specifies how `Buffer` objects should be decoded to `strings`
	* @param {NonNegativeNumber} [options.highWaterMark] - specifies the `Buffer` level for when `write()` starts returning `false`
	* @param {boolean} [options.allowHalfOpen=false] - specifies whether the stream should remain open even if one side ends
	* @param {boolean} [options.decodeStrings=true] - specifies whether to decode `strings` into `Buffer` objects when writing
	* @throws {TypeError} options argument must be an object
	* @throws {TypeError} must provide valid options
	* @returns {TransformStream} transform stream
	*
	* @example
	* var stdout = require( './../../../node/stdout' );
	*
	* var stream = new TransformStream();
	*
	* stream.pipe( stdout );
	*
	* stream.write( '1' );
	* stream.write( '2' );
	* stream.write( '3' );
	*
	* stream.end();
	*
	* // prints: '1\n2\n3\n'
	*/
	function TransformStream( options ) {
		var opts;
		var err;
		if ( !( this instanceof TransformStream ) ) {
			if ( arguments.length ) {
				return new TransformStream( options );
			}
			return new TransformStream();
		}
		opts = assign( {}, copts );
		if ( arguments.length ) {
			err = validate( opts, options );
			if ( err ) {
				throw err;
			}
		}
		debug( 'Creating a transform stream configured with the following options: %s.', JSON.stringify( opts ) );
		Transform.call( this, opts );
		this._destroyed = false;
		return this;
	}

	/**
	* Inherit from the `Transform` prototype.
	*/
	inherit( TransformStream, Transform );

	/**
	* Implements the `_transform` method.
	*
	* @private
	* @name _transform
	* @memberof TransformStream.prototype
	* @type {Function}
	* @param {(Buffer|string)} chunk - streamed chunk
	* @param {string} encoding - Buffer encoding
	* @param {Callback} clbk - callback to invoke after transforming the streamed chunk
	*/
	TransformStream.prototype._transform = transform; // eslint-disable-line no-underscore-dangle

	if ( copts.flush ) {
		/**
		* Implements the `_flush` method.
		*
		* @private
		* @name _flush
		* @memberof TransformStream.prototype
		* @type {Function}
		* @param {Callback} callback to invoke after performing flush tasks
		*/
		TransformStream.prototype._flush = copts.flush; // eslint-disable-line no-underscore-dangle
	}

	/**
	* Gracefully destroys a stream, providing backward compatibility.
	*
	* @private
	* @name destroy
	* @memberof TransformStream.prototype
	* @type {Function}
	* @param {Object} [error] - optional error message
	* @returns {TransformStream} stream instance
	*/
	TransformStream.prototype.destroy = destroy;

	return TransformStream;
}


// EXPORTS //

module.exports = ctor;