Javascript bitmap data structure

Bitmap.js

//------------------------------------------
//Compact bitmap datastructure
//Memory efficient array of bool flags
var Bitmap = function(size){
	this._cols  = 8;
	this._shift = 3;
	this._rows  = (size>>this._shift)+1;
	this._buf   = new ArrayBuffer(this._rows);
	this._bin   = new Uint8Array(this._buf);
};

//Gets the bool at offset
Bitmap.prototype.get = function(off){
	var row = off>>this._shift;
	var col = off%this._cols;
	var bit = 1<<col;
	return (this._bin[row]&bit)>0;
};

//Sets a bit at offset to bool
Bitmap.prototype.set = function(off,bool){
	var row = off>>this._shift;
	var col = off%this._cols;
	var bit = 1<<col;
	if (bool) {
		this._bin[row] |= bit;
	} else {
		bit = 255 ^ bit;
		this._bin[row] &= bit;
	}
};

//Flip a single bit at offset
Bitmap.prototype.flip = function(off){
	var row = Math.floor(off/this._cols);
	var col = off%this._cols;
	var bit = 1<<col;
	this._bin[row] ^= bit;
};

//Reset to all 1's
Bitmap.prototype.fill = function() {
	for(var i=0;i<this._rows;i++) {
		this._bin[i] = 255;
	}
};

//Reset to all 0's
Bitmap.prototype.clear = function() {
	for(var i=0;i<this._rows;i++) {
		this._bin[i] = 0;
	}
};

Eratosthenes.js

//Example - gets a list of Primes using the Sieve of Eratosthenes
//Returns a function to check if a number is prime or not
var getSieveOfEratosthenes = function(size) {

	var bits = new Bitmap(size);
	bits.fill();
	bits.set(0,0);
	var lim = Math.ceil(Math.sqrt(size));
	for(var i=2;i<=lim;i++) {
		if(bits.get(i)){
			for(var j=i*i;j<=size;j+=i) {
				bits.set(j,0);
			}
		}
	};

	return function(n){ return bits.get(n); };
};