How to check whether an object exists in javascript ?
An object can be used to check if it exists using 2 approaches:
Method 1: Using the typeof operator
The typeof operator returns the type of the variable on which it is called as a string. The return string for any object that does not exist is “undefined”. This can be used to check if an object exists or not, as a non-existing object will always return “undefined”.
Syntax:
if (typeof objectToBeTested != "undefined") // object exists else // object does not exist |
Example:
<!DOCTYPE html> <html> <head> <title>How to check whether an object exists in javascript</title> </head> <body> <h1 style="color: green"> GeeksforGeeks </h1> <b>How to check whether an object exists in javascript</b> <p>Click on the button to check if the object exists</p> <p>Output for existing object: <span class="outputExist"></span></p> <p>Output for non existing object: <span class="outputNonExist"></span></p> <button onclick="checkObjectExists()"> Click here </button> <script type="text/javascript"> function checkObjectExists() { // create an existing object for comparison let existingObject = {}; if (typeof existingObject != "undefined") { ans = true; } else { ans = false } document.querySelector( '.outputExist').textContent = ans; if (typeof nonExistingObject != "undefined") { ans = true; } else { ans = false; } document.querySelector( '.outputNonExist').textContent = ans; } </script> </body> </html> |
Output:
- Before clicking the button:
- After clicking the button:
Method 2: Using a try-catch statement to catch a Reference error
Accessing a non-existing object will always throw a Reference error. A try-catch block can be used to determine this error. Any random property of the element can be accessed for this error to be thrown.
Syntax:
try { objectToBeTested.prop; // object exists } catch { // object does not exist } |
Example:
<!DOCTYPE html> <html> <head> <title>How to check whether an object exists in javascript</title> </head> <body> <h1 style="color: green"> GeeksforGeeks </h1> <b>How to check whether an object exists in javascript</b> <p>Click on the button to check if the object exists</p> <p>Output for existing object: <span class="outputExist"></span></p> <p>Output for non existing object: <span class="outputNonExist"></span></p> <button onclick="checkObjectExists()">Click here</button> <script type="text/javascript"> function checkObjectExists() { // create an existing object for comparison let existingObject = {}; try { // accessing a random property existingObject.prop; ans = true; } catch { ans = false; } document.querySelector( '.outputExist').textContent = ans; try { // accessing a random property nonExistingObject.prop; ans = true; } catch { ans = false; } document.querySelector( '.outputNonExist').textContent = ans; } </script> </body> </html> |
Output:
- Before clicking the button:
- After clicking the button:
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