Store/retrieve bits in JavaScript number arrays

bitArray.js

function BitArray(options) {
  var USABLE_BITS = 32;

  if (options.data) {
    this.data = options.data;
  } else if (options.bits) {
    var count = Math.ceil(options.bits / USABLE_BITS);
    this.data = new Array(count);
    for (var i = 0; i < count; i++)
      this.data[i] = 0;
  } else {
    this.data = [0];
  }

  this.get = function(pos) {
    var idx = Math.floor((pos - 1) / USABLE_BITS);
    var offset = pos - (idx * USABLE_BITS) - 1;
    var mask = 1 << offset;

    return +!!(this.data[idx] & mask);
  };

  this.set = function(pos, on) {
    var idx = Math.floor((pos - 1) / USABLE_BITS);
    var offset = pos - (idx * USABLE_BITS) - 1;
    var mask = 1 << offset;

    if (on)
      this.data[idx] |= mask;
    else
      this.data[idx] &= ~mask;
  };
}