index.js

// @flow
'use strict';

const path = require('path');
const child = require('child_process');

const CHILD_SCRIPT = path.join(__dirname, 'child.js');

/*::
type Serializeable =
  | null
  | number
  | boolean
  | string
  | Array<Serializeable>
  | { [key: string]: Serializeable };

type Projector =
  & ((script: string, args?: Array<Serializeable>) => Promise<Serializeable>)
  & ((script: string, exportName?: string, args?: Array<Serializeable>) => Promise<Serializeable>);
*/

function projector(script, exportName, args) {
  if (arguments.length === 1) {
    exportName = 'default';
    args = [];
  } else if (arguments.length === 2) {
    if (typeof exportName === 'object') {
      args = exportName;
      exportName = 'default';
    } else {
      args = [];
    }
  } else if (typeof exportName === 'undefined') {
    exportName = 'default';
  }

  if (typeof script !== 'string') {
    throw new Error('Projector script must be a string');
  }

  if (typeof exportName !== 'string') {
    throw new Error('Projector export name must be a string');
  }

  if (!Array.isArray(args)) {
    throw new Error('Projector `args` must be an array');
  }

  return new Promise((resolve, reject) => {
    let proc = child.fork(CHILD_SCRIPT, {
      silent: false
    });

    proc.on('message', ({ status, payload }) => {
      if (status === 'ready') {
        proc.send({ script, exportName, args });
      } else if (status === 'complete') {
        resolve(payload);
      } else if (status === 'error') {
        let err = new Error(payload.message);
        err.stack = payload.stack;
        reject(err);
      } else {
        reject(new Error('Recieved unknown message from child process.'));
      }
    });

    proc.on('close', () => {
      reject(new Error('Never recieved a response from child process.'));
    });
  });
}

module.exports = (projector /*: Projector */);