Question

Can not quite understand the reasoning for this. In the following code the localStorage of an item is alerted as undefined, but if I use an if(x==undefined) syntax it does not work. Can somebody explain what is the problem. Thank you.

alert(localStorage["x"]);

if(localStorage["x"]=="undefined"){alert("y");}

The top line alerts undefined

The bottom line does not alert y for me.

Answer 1

It doesn’t contain the string "undefined", it contains a value of the type undefined:

if (localStorage["x"] == undefined) { alert("y"); }

The value undefined is possible to change in older browsers, so good practice is to check the type instead:

if (typeof localStorage["x"] == 'undefined') { alert("y"); }