Wrap a function to have a specify "arity" (`.length`)

arity.js

/**
 * Wrap a function so that it has `.length` (a.k.a "arity") n.
 *
 * Any additional parameters are dropped before the wrapped function is called.
 *
 * @param {int} n number of parameters on the wrapper.
 * @param {function} fn function with arbitrary signature.
 * @return {function} wrapper
 */
exports.arity = function(n, fn) {
  switch(n) {
  case 0: return function() {
    return fn.call(this);
  };
  case 1: return function(a) {
    return fn.call(this,a);
  };
  case 2: return function(a,b) {
    return fn.call(this,a,b);
  };
  case 3: return function(a,b,c) {
    return fn.call(this,a,b,c);
  };
  case 4: return function(a,b,c,d) {
    return fn.call(this,a,b,c,d);
  };
  case 5: return function(a,b,c,d,e) {
    return fn.call(this,a,b,c,d,e);
  };
  default:
    throw new TypeError('Only up to airity 5 is implemented');
  }
};