javascript undefined nodelist返回true

Question

I have two sets of divs. Fist set is assigned classname tD , while the other has a class tU . tD / tU could be undefined. In cases where tU is undefined, tD !== null returns true in chrome. It actually returns []. Why is this happening?

var tD = document.getElementsByClassName("t-d"); // undefined
var tDLength = tD.length;
var tU = document.getElementsByClassName("tU"); //defined
var tULength = tU.length;

while (tU !== null && triangle_up_length > 0) {
        tU[tULength - 1].style.borderLeftWidth = 128 + "px";
        tU[tULength - 1].style.borderRightWidth = 128 + "px";
        tULength--;
}
while (tD !== null && triangle_down_length > 0) {
    tD[tDLength - 1].style.borderLeftWidth = 128 + "px";
    tD[tDLength - 1].style.borderRightWidth = 128 + "px";
    tULength--;
}

Answer 1

while (tU.length > 0) will do what you want.

document.getElementsByClassName returns an empty nodelist (almost array), if nothing is found, so you shouldn't check for null .

Answer 2

Compete reedit:

if( tU ) {
   if( tU.length > 0 ) {
      //do the stuff
   }
{

Answer 3

This is the standard behavior of GetElementByClassName

"..Returns an array-like object of all child elements which have all of the given class names..". Then you must check for

 if( tD.length != 0 ){}