if condition not affecting display of div

I have searched on StackOverflow but no questions matched my specific dilemma. I have a div with a class foo that is set via css to display: inline-block.

I would like foo to be visible when the variable temp is a number or a string, but not display when it is not a number or a string.

var temp;
        if (temp != undefined || temp != null) {
            temp = temp / 1000;
            temp = temp + 'K';
            $('.foo').css("display", "inline-block");
        }
        else {
            $('.foo').css("display", "none");
        }

When I run this code the "foo" does not display at all.

when I change the code to

var temp;
        if (temp != undefined || temp != null) {
            temp = temp / 1000;
            temp = temp + 'K';
            $('.foo').css("display", "inline-block");
        }
        if  (temp == undefined || temp == NaN) {
            $('.foo').css("display", "none");
        }

foo is still all set to display:"none"

I ran console.log on each if, and when temp is a value it does go through the first if and when it is NaN or null it does go through the second if. But why does ONLY the display: "none" take affect?

Thank you in advance. - VS