JavaScript ReferenceError - Reference to undefined property "x"

Last Updated : 15 Jul, 2025

This JavaScript warning reference to undefined property occurs if a script tries to access an object property that doesn't exist.

Message:

ReferenceError: reference to undefined property "x" (Firefox)

Error Type:

ReferenceError(Only reported by firefox browser)

Cause of the error: The script is trying to access an object property that doesn't exist.

Example 1: In this example, the valid property is accessed, So the error has not occurred.

HTML

<!DOCTYPE html> <html> <head> <script src= "https://code.jquery.com/jquery-3.5.0.js">  </script> </head> <body style="text-align: center;"> <h1 style="color: green;"> GeeksforGeeks </h1> <p> JavaScript ReferenceError Reference to undefined property </p> <button onclick="Geeks();"> click here </button> <p id="GFG_DOWN"></p> <script>  let el_down = document.getElementById("GFG_DOWN");  function GFG() {  "use strict";  let AR_GFG = { prop_1: "Val_1" };  return AR_GFG.prop_1;  }  function Geeks() {  try {  GFG();  el_down.innerHTML =  "'Reference to undefined property'" +  " error has not occurred";  } catch (e) {  el_down.innerHTML =  "'Reference to undefined property'" +  "error has occurred";  }  }  </script> </body> </html>

Output:

Example 2: In this example, the undefined property is accessed, So the error has occurred.

HTML

<!DOCTYPE html> <html> <head> <script src= "https://code.jquery.com/jquery-3.5.0.js">  </script> </head> <body style="text-align: center;"> <h1 style="color: green;"> GeeksforGeeks </h1> <p> JavaScript ReferenceError Reference to undefined property </p> <button onclick="Geeks();"> click here </button> <p id="GFG_DOWN"></p> <script>  let el_down = document.getElementById("GFG_DOWN");  function GFG() {  "use strict";  let GFG = {};  return GFG.prop1;  }  function Geeks() {  try {  GFG();  el_down.innerHTML =  "'Reference to undefined property'" +  "error has not occurred";  } catch (e) {  el_down.innerHTML =  "'Reference to undefined property'" +  "error has occurred";  }  }  </script> </body> </html>