JavaScript function to partition an array by passing each element through a predicate function

partitionWith.js

/***********************************************************************************************************************
 * Partition an array into two arrays based on passing each element to a predicate function.
 *
 * This function is designed to extend the Array prototype, but only by the explicit action of the consuming library.
 * E.G. Assuming the function below belongs to a "utils" module, you must write something like the following:
 * 
 * const utils = require('./utils.js')
 * Array.prototype.partitionWith = utils.partitionWith
 * 
 * This function must be defined using a traditional "function" declaration instead of an arrow function.  If an arrow
 * function were used, then the reference to "this.reduce" wouldn't work...
 *  
 * partitionWith takes a simple predicate function as an argument and returns an array containing two other arrays.
 * The first array contains all the elements that do not match the predicate, and the second, all the elements that do
 * match the predicate
 * 
 * E.G.
 * 
 * var isEven      = x => x % 2 === 0
 * var numbers     = [1,2,3,4,5,6,7,8,9,10]
 * var [odd, even] = numbers.partitionWith(isEven)
 * 
 * odd  = [1,3,5,7,9]
 * even = [2,4,6,8,10]
 */
const partitionWith = function(predFn) {
  const partitionReducer = (acc, el) =>
    (success => success ? [acc[0], acc[1].concat(el)]
                        : [acc[0].concat(el), acc[1]])
    (predFn(el))

  return this.reduce(partitionReducer, [[],[]])                              
}

Goodness me you wouldn't believe how lovely Ramda's implementation of partition is. Might be worth a beer and a live stream to look at it...

Yes, the Ramda implementation is:

partition = juxt([filter, reject])

Where juxt first applies the filter function to produce the first array, then its logical complement reject to produce the second array