JS undefined and NULL checker not working [duplicate]

Question

     for(const id of this.extraRooms)
            {
              let doesRoomExist = document.getElementById('newRoomAddress' + newRoomId);
              //if(typeof(doesRoomExist) !== "null")              
               if(typeof(doesRoomExist) !== "undefined")
              {
                  const newRoomLabel = document.getElementById('newRoomAddress' + newRoomId).value        
             {

Probably a simple fix but if doesRoomExist is NULL it is still going inside the if statement...

let doesRoomExist = document.getElementById('newRoomAddress' + newRoomId);

will show doesRoomExist as NULL when stepping through the code in VS code, but show as undefined if used in developer tools...why is it going inside the if?

any help please?

Run typeof null in a Javascript debugger to understand your issue. All you need to do is if(doesRoomExist) {...} - research Javascript truthiness. — Andy Ray
– Andy Ray, Commented Aug 29, 2023 at 2:12
Because typoeof something will never be "null" use if (doesRoomExist !== null) or simply if (doesRoomExist) instead — derpirscher
– derpirscher, Commented Aug 29, 2023 at 2:19

Abdel · Accepted Answer · 2023-08-29 02:31:58Z

2

You should check the value of doesRoomExist directly not its type, you can either check if (doesRoomExist !== null) or just doing if (doesRoomExist) will be enough;

if(doesRoomExist !== null)
    const newRoomLabel = document.getElementById('newRoomAddress' + newRoomId).value

answered Aug 29, 2023 at 2:31

Abdel

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msrumon · Accepted Answer · 2023-08-29 02:33:57Z

0

Just use if (doesRoomExist) { /*...*/ }.

for (const id of [1, 2, 3, 4, 5]) {
  const doesExist = document.getElementById('item' + id);
  if (doesExist) {
    const newLabel = document.getElementById('item' + id).value;
    console.log(newLabel);
  }
}

<input id="item1" value="value1">
<input id="item2" value="value2">
<input id="item4" value="value4">
<input id="item5" value="value5">

answered Aug 29, 2023 at 2:33

msrumon

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