thread.js

const
  numberOfCPUs = require('os').cpus().length,
  cluster = require('cluster');

let activeCPUS = 0;

function chunk( array, number ) {

  const
    result = [],
    arrayLength = array.length;

  for ( let index = 0; index < arrayLength; ++index ) {

    if ( !result[ Math.floor( index / number ) ] ) {
      result[ Math.floor( index / number ) ] = [];
    }

    result[ Math.floor( index / number ) ].push( array[ index ] );

  }

  return result;

}

function flatten( array ) {

  return [].concat.apply( [], array );

}

function map( array, func = identity ) {

  const
    arrayLength = array.length,
    result = [];

  for ( let index = 0; index < arrayLength; ++index ) {

    result[index] = func( array[ index ], index, array );

  }

  return result;

}

function times( length, func = identity ) {
  return map( new Array( length ).fill( 0 ), ( _, i ) => func( i, length ) );
}

function thread( array, func ) {

  const
    usableCPUS = numberOfCPUs - activeCPUS,
    chunkData = chunk( array, array.length / usableCPUS ),
    chunks = chunkData.length;

  if (cluster.isMaster) {
    const workers = times( chunks, () => cluster.fork() );
    return Promise.all(
        workers.map( ( worker, i ) => createThread( worker, chunkData, i, func ) )
      )
      .then( results => flatten( results ) )
      .catch( error => {
        throw error;
      } )
  } else if ( cluster.isWorker ) {
    let index = 0;
    process.on('message', ( data ) => {

      let results;

      try {
        results = map( data, data => func( data ) );
      } catch ( error ) {
        process.exit(1);
      }

      if ( !!results.then ) {
        results.then( data => process.send( results, () => process.exit(0) ) )
      } else {
        process.send( results, () => process.exit(0) )
      }
    });
    return { then: () => {} };
  }

}

function createThread( worker, chunkData, index, func ) {

  return new Promise( ( resolve, reject ) => {

    worker.on( 'message', function workerTask( msg ) {

      resolve( msg );

    } );

    worker.send( chunkData[ index ] );

  } );

}

thread.isMaster = cluster.isMaster;
thread.isWorker = cluster.isWorker;

module.exports = thread;