To solve the "Cannot read property 'querySelector' of undefined" error, make sure you're calling the method on a valid DOM element. The error often occurs after passing an invalid class name to the `getElementsByClassName` method.

Cannot read property 'querySelector' of Undefined in JS

## Cannot read property 'querySelector' of Undefined

**The "Cannot read property 'querySelector' of undefined" error occurs for 2
reasons:**

1. Calling the `querySelector()` method on an `undefined` value (DOM element
   that doesn't exist).
2. Inserting the JS script tag above the HTML where the DOM elements are
   declared.

```javascript:index.js
const el = undefined;

// ⛔️ Cannot read properties of undefined (reading 'querySelector')
const box = el.querySelector('#box');
```

**To solve the "Cannot read property 'querySelector' of undefined" error, make
sure you're calling the method on a valid DOM element. The error often occurs
after passing an invalid class name to the `getElementsByClassName` method.**

```javascript:index.js
const elements = document.getElementsByClassName('does-not-exist');
console.log(elements); // 👉️ []

// ⛔️ Cannot read properties of undefined (reading 'querySelector')
const box = elements[0].querySelector('#box');
```

We passed a non-existent class name to the `getElementsByClassName` method, so
we got an empty array-like object back.

Trying to access an array at an index out of bounds, returns an `undefined`
value and calling the `querySelector` method on an `undefined` value causes the
method.

**To solve the "Cannot read property 'querySelector' of undefined" error, insert
the JS script tag at the bottom of the body tag. The script should run after the
DOM elements have been created.**

```html:index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    
    <script src="index.js"></script>

    <div class="container">
      <div id="box">Text</div>
    </div>
  </body>
</html>
```

We placed the JS script tag above the code that declares the DOM elements. This
means that the `div` element won't be accessible inside the `index.js` file.

```javascript:index.js
const elements = document.getElementsByClassName('container');
console.log(elements); // 👉️ []

// ⛔️ Cannot read properties of undefined (reading 'querySelector')
const box = elements[0].querySelector('#box');
```

Instead, place the JS script tag at the bottom of the body tag, after the DOM
elements it needs access to.

```html:index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <div class="container">
      <div id="box">Text</div>
    </div>

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

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

```javascript:index.js
const elements = document.getElementsByClassName('container');
console.log(elements); // 👉️ [div.container]

// ✅ Works
const box = elements[0].querySelector('#box');
console.log(box); // 👉️ div#box
```

## Conclusion

The "Cannot read property 'querySelector' of undefined" error occurs when trying
to call the `querySelector` method on an `undefined` value.

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

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Cannot read property 'querySelector' of Undefined in JS

Cannot read property 'querySelector' of Undefined #

Conclusion #

Borislav Hadzhiev

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Cannot read property 'querySelector' of Undefined in JS