Storing Objects in HTML5 localStorage
I'd like to store a JavaScript object in HTML5 localStorage , but my object is apparently being converted to a string.
I can store and retrieve primitive JavaScript types and arrays using localStorage , but objects don't seem to work. Should they?
Here's my code:
var testObject = { 'one': 1, 'two': 2, 'three': 3 };
console.log('typeof testObject: ' + typeof testObject);
console.log('testObject properties:');
for (var prop in testObject) {
console.log(' ' + prop + ': ' + testObject[prop]);
}
// Put the object into storage
localStorage.setItem('testObject', testObject);
// Retrieve the object from storage
var retrievedObject = localStorage.getItem('testObject');
console.log('typeof retrievedObject: ' + typeof retrievedObject);
console.log('Value of retrievedObject: ' + retrievedObject);
The console output is
typeof testObject: object
testObject properties:
one: 1
two: 2
three: 3
typeof retrievedObject: string
Value of retrievedObject: [object Object]
It looks to me like the setItem method is converting the input to a string before storing it.
I see this behavior in Safari, Chrome, and Firefox, so I assume it's my misunderstanding of the HTML5 Web Storage spec, not a browser-specific bug or limitation.
I've tried to make sense of the structured clone algorithm described in http://www.w3.org/TR/html5/infrastructure.html. I don't fully understand what it's saying, but maybe my problem has to do with my object's properties not being enumerable (???)
Is there an easy workaround?
Update: The W3C eventually changed their minds about the structured-clone specification, and decided to change the spec to match the implementations. See https://www.w3.org/Bugs/Public/show_bug.cgi?id=12111. So this question is no longer 100% valid, but the answers still may be of interest.
Looking at the Apple, Mozilla and Microsoft documentation, the functionality seems to be limited to handle only string key/value pairs.
A workaround can be to stringify your object before storing it, and later parse it when you retrieve it:
var testObject = { 'one': 1, 'two': 2, 'three': 3 };
// Put the object into storage
localStorage.setItem('testObject', JSON.stringify(testObject));
// Retrieve the object from storage
var retrievedObject = localStorage.getItem('testObject');
console.log('retrievedObject: ', JSON.parse(retrievedObject));
A minor improvement on a variant:
Storage.prototype.setObject = function(key, value) {
this.setItem(key, JSON.stringify(value));
}
Storage.prototype.getObject = function(key) {
var value = this.getItem(key);
return value && JSON.parse(value);
}
Because of short-circuit evaluation, getObject() will immediately return null if key is not in Storage. It also will not throw a SyntaxError exception if value is "" (the empty string; JSON.parse() cannot handle that).
You might find it useful to extend the Storage object with these handy methods:
Storage.prototype.setObject = function(key, value) {
this.setItem(key, JSON.stringify(value));
}
Storage.prototype.getObject = function(key) {
return JSON.parse(this.getItem(key));
}
This way you get the functionality that you really wanted even though underneath the API only supports strings.
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