Why this does not work ?
if(typeof(localStorage.getItem("username"))=='undefined'){ alert('no'); }; The goal is to redirect the user from the index page to the login page if not already logged. Here the localStorage.getItem("username")) variable is not defined for the moment.
It's for an ios phonegap app.
Quoting from the specification:
The getItem(key) method must return the current value associated with the given key. If the given key does not exist in the list associated with the object then this method must return null.
You should actually check against null.
if (localStorage.getItem("username") === null) { //... } 
null is "null", not null (hope I'm making sense).
typeof(..). Check my answer again.
CREATE TABLE webappsstore2 (scope TEXT, key TEXT, value TEXT, secure INTEGER, owner TEXT) while Chrome does not: CREATE TABLE ItemTable (key TEXT UNIQUE ON CONFLICT REPLACE, value BLOB NOT NULL ON CONFLICT FAIL)This method worked for me:
if ("username" in localStorage) { alert('yes'); } else { alert('no'); } 
length. The accepted answer is the correct way to solve this problem.
localStorage.__proto__ in the development console and you will see the properties that will conflict.Update:
if (localStorage.hasOwnProperty("username")) { // } Another way, relevant when value is not expected to be empty string, null or any other falsy value:
if (localStorage["username"]) { // } 
if( localStorage["username"]==undefined )
if (localStorage["username"]==undefined || localStorage["username"]==0 || localStorage["username"]==null || localStorage["username"]==false || localStorage["username"]=='') @AllanRuin
if (localStorage["username"]!==undefined && localStorage["username"]!==0 && localStorage["username"]!==null && localStorage["username"]!==false && localStorage["username"]!=='')
The MDN documentation shows how the getItem method is implementated:
Object.defineProperty(oStorage, "getItem", { value: function (sKey) { return sKey ? this[sKey] : null; }, writable: false, configurable: false, enumerable: false }); If the value isn't set, it returns null. You are testing to see if it is undefined. Check to see if it is null instead.
if(localStorage.getItem("username") === null){ 
typeof the value to null. You need to compare the actual value.== instead of === because (for some weird reason) null==undefined is true. Then again, you don't need anything, like @Derin answered, you can simply if (localStorage["username"])
localStorage['username']='admin'then mess with your website.