Unhandled Runtime Error nextjs – TypeError: Cannot read properties of null (reading ‘tagName’)

  47 | const oldTags = [];
  48 | for(let i = 0, j = headCountEl.previousElementSibling; i < headCount; i++, j = j.previousElementSibling){
> 49 |     if (j.tagName.toLowerCase() === type) {
     |          ^
  50 |         oldTags.push(j);
  51 |     }
  52 | }` 

Answer

First Solution

use j?.tagName.toLowerCase() instead of j.tagName.toLowerCase()

?. is the Optional Chaining concept in JavaScript

for more information you can check these links:

• reference
• JavaScript Optional Chaining

Second Solution

also if you don’t want to use ?., you must add some expressions in your condition

if (j && j.tagName && j.tagName.toLowerCase() === type) {
        oldTags.push(j);
}