A version of _.partial that caches functions so that `partial(fn, 1, 2, 3) === partial(fn, 1, 2, 3)`.

partial.js

var partial = (function () {
  'use strict';

  const caches = new WeakMap()
  
  return function partial(fn, ...args) {
    let toApply = fn
    
    while (args.length) {
      if (!caches.has(toApply)) caches.set(toApply, [])
      const cache = caches.get(toApply)
      const arg = args.shift()
      let cached = cache.find(cache => cache[0] === arg)
      if (!cached) {
        const applied = toApply
        cached = [arg, (...args) => applied(arg, ...args)]
        cache.push(cached)
      }
      toApply = cached[1]
    }
    
    return toApply
  }
}())

test.js

var log = (...args) => console.log(...args)
log(1, 2, 3)

var part1 = partial(log, 1, 2, 3)
part1()

var part11 = partial(log, 1, 2, 3)
console.assert(part1 === part11, 'part 1 and part 11 should be the same function')

var part2 = partial(part1, 4)
part2()
console.assert(part1 !== part2, 'part 1 and part 2 should NOT be the same function')