series.js

import {_setImmediate} from './_setImmediate'

/**
 * Run `tasks` callback functions in series
 * The function breaks after the first error encountered and calls optional
 * `callback` function
 *
 * @name series
 * @memberOf module:serial
 * @static
 * @method
 * @param {Array} tasks - Array of callback functions of type `function (cb: Function)`
 * @param {Function} [callback] - optional callback function called by last
 * terminating function from `tasks`, needs to be of type
 * `function (err: <Error>, res: Array<any>)`
 * @example
 * series([
 *   (cb) => { cb(null, 1) },
 *   (cb) => { cb('error', 2) }, // breaks on first error
 *   (cb) => { cb(null, 3) },
 * ], (err, res) => {
 *   //> err = 'error'
 *   //> res = [1, 2]
 * })
 */
export default function series (tasks, callback) {
  let length = tasks.length
  let results = []
  let i = 0

  function cb (err, res) {
    results.push(res)
    /* istanbul ignore else */
    if (err || length === i) {
      callback && callback(err, results)
    } else if (i < length) {
      _setImmediate(() => { // prevent RangeError: Maximum call stack size exceeded for sync tasks
        run()
      })
    }
  }

  function run () {
    let fn = tasks[i++]
    fn(cb)
  }

  run()
}