Solve - Cannot read property 'offsetHeight' of Null in JS

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Borislav Hadzhiev

Thu Oct 21 20212 min read

Solve - Cannot read property 'offsetHeight' of Null #

The "Cannot read property 'offsetHeight' of null" error occurs for 2 reasons:

  1. Accessing the offsetHeight property on a null value (DOM element that doesn't exist).
  2. Placing the JS script tag above the HTML where the DOM elements are declared.
index.js
const el = null; // โ›”๏ธ Cannot read properties of null (reading 'offsetHeight') console.log(el.offsetHeight);

To solve the "Cannot read property 'offsetHeight' of null" error, make sure the id you're using to get the element is present in the DOM. The error often occurs after providing a non-existent id to the getElementById method.

index.js
const el = document.getElementById('does-not-exist'); console.log(el); // ๐Ÿ‘‰๏ธ null // โ›”๏ธ Cannot read properties of null (reading 'offsetHeight') console.log(el.offsetHeight);

We passed a non-existent id to the getElementById() method and got a null value back.

Accessing the offsetHeight property on a null value causes the error.

To solve the "Cannot read property 'offsetHeight' of null" error, place the JS script tag at the bottom of the body tag. The script should run after the DOM elements have been created.

index.html
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8" /> </head> <body> <!-- โŒ BAD - script runs before div exists โŒ --> <script src="index.js"></script> <div id="box">Test</div> </body> </html>

The JS script tag is placed above the code that creates the div element, therefore the element won't be accessible in the index.js file.

index.js
const el = document.getElementById('box'); console.log(el); // ๐Ÿ‘‰๏ธ null // โ›”๏ธ Cannot read properties of null (reading 'offsetHeight') console.log(el.offsetHeight);

Instead, we should place the JS script tag at the bottom of the body tag, after the elements we need to access.

index.html
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8" /> </head> <body> <div id="box">Test</div> <!-- โœ… GOOD - div already exists โœ… --> <script src="index.js"></script> </body> </html>

Now we can access the div inside of the index.js file.

index.js
const el = document.getElementById('box'); console.log(el); // ๐Ÿ‘‰๏ธ div#box // โœ… Works console.log(el.offsetHeight); // ๐Ÿ‘‰๏ธ 18

Conclusion #

The "Cannot read property 'offsetHeight' of null" error occurs when trying to access the offsetHeight property on a null value.

To solve the error, run the JS script after the DOM elements are available and make sure you only access the property on valid DOM elements.

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