How to find the size of localStorage

Question

I am currently developing a site that will make use of HTML5's localStorage. I've read all about the size limitations for different browsers. However, I haven't seen anything on how to find out the current size of a localStorage instance. This question seems to indicate that JavaScript doesn't have a built in way of showing the size for a given variable. Does localStorage have a memory size property that I haven't seen? Is there an easy way to do this that I'm missing?

My site is meant to allow users to enter information in an 'offline' mode, so being able to give them a warning when the storage is almost full is very important.

Answer 1

Execute this snippet in JavaScript console (one line version):

var _lsTotal=0,_xLen,_x;for(_x in localStorage){ if(!localStorage.hasOwnProperty(_x)){continue;} _xLen= ((localStorage[_x].length + _x.length)* 2);_lsTotal+=_xLen; console.log(_x.substr(0,50)+" = "+ (_xLen/1024).toFixed(2)+" KB")};console.log("Total = " + (_lsTotal / 1024).toFixed(2) + " KB");

The same code in multiple lines for reading sake

var _lsTotal = 0,
    _xLen, _x;
for (_x in localStorage) {
    if (!localStorage.hasOwnProperty(_x)) {
        continue;
    }
    _xLen = ((localStorage[_x].length + _x.length) * 2);
    _lsTotal += _xLen;
    console.log(_x.substr(0, 50) + " = " + (_xLen / 1024).toFixed(2) + " KB")
};
console.log("Total = " + (_lsTotal / 1024).toFixed(2) + " KB");

or add this text in the field 'location' of a bookmark for convenient usage

javascript: var x, xLen, log=[],total=0;for (x in localStorage){if(!localStorage.hasOwnProperty(x)){continue;} xLen =  ((localStorage[x].length * 2 + x.length * 2)/1024); log.push(x.substr(0,30) + " = " +  xLen.toFixed(2) + " KB"); total+= xLen}; if (total > 1024){log.unshift("Total = " + (total/1024).toFixed(2)+ " MB");}else{log.unshift("Total = " + total.toFixed(2)+ " KB");}; alert(log.join("\n")); 

P.S. Snippets are updated according to request in the comment. Now the calculation includes the length of the key itself. Each length is multiplied by 2 because the char in javascript stores as UTF-16 (occupies 2 bytes)

P.P.S. Should work both in Chrome and Firefox.