How do I store an array in localStorage?

Question

If I didn't need localStorage, my code would look like this:

var names=new Array(); 
names[0]=prompt("New member name?");

This works. However, I need to store this variable in localStorage and it's proving quite stubborn. I've tried:

var localStorage[names] = new Array();
localStorage.names[0] = prompt("New member name?");

Where am I going wrong?

Solution 1

localStorage only supports strings. Use JSON.stringify() and JSON.parse().

var names = [];
names[0] = prompt("New member name?");
localStorage.setItem("names", JSON.stringify(names));

//...
var storedNames = JSON.parse(localStorage.getItem("names"));

You can also use direct access to set/get item:

localstorage.names = JSON.stringify(names);
var storedNames = JSON.parse(localStorage.names);

Solution 2

The localStorage and sessionStorage can only handle strings. You can extend the default storage-objects to handle arrays and objects. Just include this script and use the new methods:

Storage.prototype.setObj = function(key, obj) {
    return this.setItem(key, JSON.stringify(obj))
}
Storage.prototype.getObj = function(key) {
    return JSON.parse(this.getItem(key))
}

Use localStorage.setObj(key, value) to save an array or object and localStorage.getObj(key) to retrieve it. The same methods work with the sessionStorage object.

If you just use the new methods to access the storage, every value will be converted to a JSON-string before saving and parsed before it is returned by the getter.

Solution 3

Use JSON.stringify() and JSON.parse() as suggested by no! This prevents the maybe rare but possible problem of a member name which includes the delimiter (e.g. member name three|||bars).

Solution 4

Just created this:

https://gist.github.com/3854049

//Setter
Storage.setObj('users.albums.sexPistols',"blah");
Storage.setObj('users.albums.sexPistols',{ sid : "My Way", nancy : "Bitch" });
Storage.setObj('users.albums.sexPistols.sid',"Other songs");

//Getters
Storage.getObj('users');
Storage.getObj('users.albums');
Storage.getObj('users.albums.sexPistols');
Storage.getObj('users.albums.sexPistols.sid');
Storage.getObj('users.albums.sexPistols.nancy');

Solution 5

The JSON approach works, on ie 7 you need json2.js, with it it works perfectly and despite the one comment saying otherwise there is localStorage on it. it really seems like the best solution with the least hassle. Of course one could write scripts to do essentially the same thing as json2 does but there is little point in that.

at least with the following version string there is localStorage, but as said you need to include json2.js because that isn't included by the browser itself: 4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; BRI/2; NP06; .NET4.0C; .NET4.0E; Zune 4.7) (I would have made this a comment on the reply, but can't).

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Comments

[Saif Bechan]

JSON is not supported in IE7 and earlier.

[tungd]

@SaifBechan Don't worry about IE 6/7, we are talking about localStorage

[Camilo Martin]

Kills the performance gain of not JSONifying.

[Howdy_McGee]

Can somebody explain why you have to do this and what is going on please?

[James Coyle]

@Howdy_McGee As localStorage only supports strings, arrays must be converted to a string format. The best way to do this is to convert it into a JSON array as it is easy to convert between the two states (JSON is just a long piece of string data). You may want to read up on JSON if that is what you don't understand. :)

[Howdy_McGee]

Didn't realize localStorage only holds string. That's fairly limiting but I suppose anything more complicated would also need to be more secure and probably last long. @jimjimmy1995 Thanks!

[Jonathan Tonge]

I would test whether localStorage['names'] returns a value before parsing it or an error will be thrown.

[Dagg Nabbit]

@JonathanTonge, you could do something like JSON.parse(localStorage["names"] || null) if you wanted.

[Phil]

Can you show us a better alternative?

[PiTheNumber]

Instead of names_length use names_keys and you don't need to re-index! This would also allow you to use string keys. Of course this makes only sense if the array elements are kind of hugh.

[Znarkus]

@PiTheNumber If names_keys contains an array of indices, isn't that a chicken/egg situation?

[PiTheNumber]

@Znarkus As I said, this makes only sense for hugh array elements. If you have for example 100KB per element it would be better to use a small entry with keys.

[Dvid Silva]

if you needed all of them how would you retrieve them?

[Sebastian]

The storage objects provide the property "length" to determine the count of saved objects and the method "key(int index)" to get the current key. So you can try: for (var i = 0; i < localStorage.length; i++) console.log( localStorage.key(i) +" has value " + localStorage[localStorage.key(i)] )

[Alix Axel]

@PiTheNumber What is hugh?

[PiTheNumber]

@AlixAxel I don't have an exact number for you but if things are slowing down because of reindexing or you just don't like to rebuild the index you might want to consider storing the keys instead. Or the other way around: If you store 10'000 integer values your index in names_keys would be larger than you actually data.

[Parag Gangil]

@Sebastian using localStorage.length could be wrong in some cases if the website is also localStorage to store something.

[ChiefTwoPencils]

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post.

[J0HN]

@ChiefTwoPencils have you read comments before flagging?

[ChiefTwoPencils]

@J0HN: yes I did. Since when is a members reputation good reason to break the rules? In fact, doesn't the auto-messaging on recommended deletions specifically mention not posting commentary as answers and suggests the poster wait until they have enough reputation to comment and up-vote until then? Poster says it's not an answer; what more evidence needs to exist?

[jayeff]

@ChiefTwoPencils: Some historical context: When I posted this comment the accepted answer proposed doing a join and split with "|||" by Ian (Note: this answer no longer exists). Because I considered this answer actually harmful I posted my answer. Today my non-answer is no longer necessary (the correct answer is accepted and has a healthy dose of upvotes). I'm not too familiar with the guidelines: Is it ok/recommended to delete my "answer"?

[Sebastian]

You may prefix your module specific values and delete everything with that prefix while iterating all elements of localStorage.

[twobob]

This is the sort of thing I was looking for, except it would walk the keys of the provided object and save the (single) linked value in each. Much obliged for the inspire.

[David Wihl]

If you compress the resulting string, up to 4x times as much data can be stored. See jsfiddle.net/davidwihl/n12rogcx (uses the LZ JavaScript library: pieroxy.net/blog/pages/lz-string/index.html)

[Michaël van de Weerd]

@SaifBechan if(typeof JSON == "undefined") { alert("Upgrade your browser."); }

[zevero]

I used the same approch in my solution, which provides a minimalistic interface to how localStorage should work :) github.com/zevero/simpleWebstorage

[ali_wetrill]

You could also do JSON.parse(localStorage.getItem("names") || '[]') (typescript doesn't complain then)

[albertrw]

ali_wetrill has a good pointer, if you use var storedNames = JSON.parse(localStorage.names || '[]'); or var storedNames = JSON.parse(localStorage.names || null) it'll default to either the empty string or null, as desired, in the case where the localStorage item has not been defined yet.

[Kapil Soni]

@DaggNabbit: sir i received Converting circular structure to JSON --> starting at object with constructor 'TView this error after store array in the localstorage.can you tell me me why i get this error?