Call a function only if a value is neither null nor undefined

When a button is clicked I check if something exists in a localstorage key as such:

var a = localStorage.getItem('foo');
if (typeof a != 'undefined') {
    // Function
}

but if the key doesn't exists at all, it return null. How can I call if not undefined and not null do function, else return true(?) or continue?

Solution

JavaScript has a concept of falsy values... i.e. 0, null, undefined and an empty string.

Because of this, you should be able to just check if a is "truthy" (i.e. not one of the values I've mentioned above), by doing this:

var a = localStorage.getItem('foo');
if (a) {
    // Function
}