if some image did not load sho alert javascript code example

Example 1: oneerror javascript image

<img src="imagenotfound.gif" alt="Image not found" onerror="this.onerror=null;this.src='imagefound.gif';" />

Example 2: js detect all images errors

window.addEventListener("error", e => {
  /* query selector images */
  // image element on error => replace source 
})