How to check whether an object exists in javascript ?

28 Jul, 2020
An object can be used to check if it exists using 2 approaches:
Method 1: Using the typeof operatorThe 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 existselse    // 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 errorAccessing 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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