localStorage and boolean 'string'

Question

Storing boolean value in localStorage, this value is converted to string. Now trying to converting back this value from localStorage to boolean, i need to use JSON.parse() method, the more handy !! doesn't work.

Code sample:

var test = false;
localStorage['test'] = test;
console.log("JSON.parse returns: ", JSON.parse(localStorage['test']), "expected: ", test);
console.log("'!!' returns: ", !! localStorage['test'], "expected: ", test);

-jsFiddle-

I'm quite confused why this behaviour. Any explaination?

PS: using getter/setter localStorage methods doesn't matter here, same result.

Answer 1

Local storage stores strings , I'm afraid, whatever the input (if you feed it with an object, it will be converted automatically with its standard toString() method)... So you're doing !! test on a string, which is always true.

You should always use JSON.stringify() and JSON.parse() when dealing with what you store in DOM storage

Answer 2

Use JSON.stringify() when save the object. As you know it will convert JavaScript value to a JSON string so when using JSON.parse() its converted back properly.

localStorage['test'] = JSON.stringify(test);

DEMO

Answer 3

This happens because any stored value in localstorage is a string. So you've perofming !!"false" and !! is always true for non-empty string. To be able to store non-string values in localStorage you have to always use JSON.