sets the length property of a given function to the given arity

setArity.js

function setArity (arity, fn) {
  if (typeof setArity.cache === 'undefined') {
    setArity.cache = {}
  }
  if (typeof setArity.cache[arity] === 'undefined') {
    setArity.cache[arity] = (fn) => {
      switch (arity) {
        case 0: return function () { return fn.apply(this, arguments) }
        case 1: return function (a) { return fn.apply(this, arguments) }
        case 2: return function (a, b) { return fn.apply(this, arguments) }
        case 3: return function (a, b, c) { return fn.apply(this, arguments) }
        case 4: return function (a, b, c, d) { return fn.apply(this, arguments) }
        case 5: return function (a, b, c, d, e) { return fn.apply(this, arguments) }
        case 6: return function (a, b, c, d, e, f) { return fn.apply(this, arguments) }
        case 7: return function (a, b, c, d, e, f, g) { return fn.apply(this, arguments) }
        case 8: return function (a, b, c, d, e, f, g, h) { return fn.apply(this, arguments) }
        case 9: return function (a, b, c, d, e, f, g, h, i) { return fn.apply(this, arguments) }
        case 10: return function (a, b, c, d, e, f, g, h, i, j) { return fn.apply(this, arguments) }
        default:
          try {
            const proxy = function () { return fn.apply(this, arguments) }
            Object.defineProperty(proxy, 'length', { value: arity })
            return proxy
          } catch (e) {
            const params = Array(arity).join(', _').substr(2)
            const proxy = new Function(
              'fn',
              `return function (${params}) { return fn.apply(this, arguments); }`
            )
            return proxy(fn)
          }
      }
    }
  }
  return setArity.cache[arity](fn)
}