line240-243 why write like that

[screepinail]

    return function (zIndex) {
        if (+zIndex == +zIndex) {
            f.zIndex = +zIndex;
        }
    };

I'm curious about why did U write like that？ Isn't the condition ‘if’ always be true?

[DmitryBaranovskiy]

Check what it will be for zIndex = undefined or "blah" or {}