Cannot read properties of null (reading 'getContext') in JS

# Cannot read properties of null (reading 'getContext') in JS

The "Cannot read properties of null (reading 'getContext')" error occurs for 2 reasons:

Calling the getContext method on a non-existent canvas element.
Inserting the JS script tag above the HTML where the canvas element is declared.

cannot read property getcontext of null

Here is an example of how the error occurs.

index.js

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const canvas = document.getElementById('does-not-exist');
console.log(canvas); // 👉️ null

// ⛔️ Uncaught TypeError: Cannot read properties of null (reading 'getContext')
const ctx = canvas.getContext('2d');

The code for this article is available on GitHub

We attempted to call the getContext() method on a null value which caused the error.

# Make sure the DOM element exists before calling `getContext()`

Make sure you're providing the correct id when trying to get the canvas element.

The error often occurs when providing an invalid id to the getElementById method.

index.html

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <!-- ✅ id should match your JS code -->
    <canvas id="canvas" width="500" height="500"></canvas>

    <script src="index.js"></script>
  </body>
</html>

The id in your HTML code should be the same as the id you pass in the call to the document.getElementById() method.

# Place the JS script tag at the bottom of the body tag

Place the JS script tag at the bottom of the body tag, after the canvas element has been created.

index.html

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <!-- ❌ BAD - script is run before canvas is created ❌ -->
    <script src="index.js"></script>

    <canvas id="canvas" width="500" height="500"></canvas>
  </body>
</html>

The code for this article is available on GitHub

The index.js script is run before the canvas element is created, so you can't access the element inside the index.js file.

index.js

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const canvas = document.getElementById('canvas');
console.log(canvas); // 👉️ null

// ⛔️ Cannot read properties of null (reading 'getContext')
const ctx = canvas.getContext('2d');

You should place the JS script tag at the bottom of the body, after the DOM elements have been declared.

index.html

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <canvas id="canvas" width="500" height="500"></canvas>

    <h2>bobbyhadz.com</h2>

    <!-- ✅ GOOD - canvas is already created ✅ -->
    <script src="index.js"></script>
  </body>
</html>

place js script tag at bottom of body tag

The code for this article is available on GitHub

Now you can access the canvas element inside of the index.js file.

index.js

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const canvas = document.getElementById('canvas');
console.log(canvas); // 👉️ canvas#canvas

// ✅ works
const ctx = canvas.getContext('2d');

console.log(ctx); // 👉️ CanvasRenderingContext2D

access canvas element inside index js successfully

The code for this article is available on GitHub

HTML code is parsed from top to bottom, so the script tag has to be placed at the bottom of the body tag, after all of the DOM elements it needs to access.

# Conclusion

The "Cannot read properties of null (reading 'getContext')" error occurs when calling the getContext method on a null value.

To solve the error, make sure the JS script tag is loaded after the HTML is declared and the id of the element exists in the DOM.

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Cannot read properties of null (reading 'getContext') in JS