Capped Array with equivalent API to Array

capped-array.js

'use strict';
/*
  var a = cappedArray(3); //limited to 3 items
  var b = cappedArray(1, 2, 3); //limited to 3 items, populated with [1, 2, 3]
  
  a.concat(b, 10); //deepEqaul to `cappedArray(2, 3, 10);`
*/
module.exports = function cappedArray(size) {
  var self = [];

  if(arguments.length > 1) {
    self = Array.prototype.slice.call(arguments);
    self.size = arguments.length;
  }
  else
    self.size = size;


  self.push = function() {
    var items = Array.prototype.slice.call(arguments);
    Array.prototype.push.apply(self, items);
    if(self.length > self.size) self.splice(0, self.length - size);
    return self.length;
  };

  self.unshift = function() {
    var items = Array.prototype.slice.call(arguments);
    Array.prototype.unshift.apply(self, items);
    if(self.length > self.size) self.splice(self.size, self.length - size);
    return self.length;
  };

  self.concat = function() {
    var arrays = Array.prototype.slice.call(arguments);

    var c = cappedArray(self.size);
    var curA;
    do {
      curA = Array.prototype.slice.call([].concat(arrays.shift()));
      c.push.apply(c, [].concat(curA));
    } while(arrays.length);

    return c;
  };

  return self;
};

test.js

var assert = require('assert');
var cappedArray = require('./capped-array');

var a = cappedArray(3);
assert.strictEqual(a.size, 3);
a.push(10);
a.unshift(0);
assert.deepStrictEqual(Array.prototype.slice.call(a), [0, 10]);

var b = cappedArray(20, 30, 40);
assert.strictEqual(b.size, 3);
assert.deepStrictEqual(Array.prototype.slice.call(b), [20, 30, 40]);

var c = a.concat(b, 50, 60);
assert.strictEqual(c.size, a.size);
assert.deepStrictEqual(Array.prototype.slice.call(c), [40, 50, 60]);

c.push(80);
c.shift();
c.unshift(-10);
assert.deepStrictEqual(Array.prototype.slice.call(c), [-10, 60, 80]);