Question

Particularly for localStorage.foo

For Safari it is set to:

undefined

For Firefox it is set to:

null

Does anyone know the values for Chrome and IE?

Why is it different? Just random choices by browser programmers?

Answer 1

It’s always undefined. Perhaps your observation method is what led you to believe the values are different in different browsers.

Oh, I’ll qualify that statement for old versions of IE, which might do some other thing for all I know. I bet they use undefined also.

Ah – Mr. Protagonist has an interesting point. On any normal object, a non-existent property will be null. However, Firefox does indeed seem to report null as the value of a non-existent property specifically of localStorage. Hmm… My vote would be that that’s a bug, but I’ll check the w3c spec (or proto-spec or whatever it is).

The “value” undefined isn’t really a value; it’s more like the Buddhist mu — it’s kinda like saying, “what you asked for doesn’t make sense”. The value null in JavaScript is treated differently than undefined. Thus:

var a = {};
var b = a.banana;

The variable “b” will be undefined. It’s weird, but it lets you tell the difference between a property being present but null and a property being missing. (Of course, the in operator lets you figure that out too.)