Top 10 Most Common JavaScript Errors and How to Prevent Them

1. Uncaught TypeError: Cannot Read Property

This error appears when a script tries to read a property or call a method on an undefined object, often because component state is not initialized before rendering.

class Quiz extends Component {
  constructor(props) {
    super(props);
    this.state = { items: [] };
  }
  componentWillMount() {
    axios.get('/thedata').then(res => {
      this.setState({ items: res.data });
    });
  }
  render() {
    return (
      <ul>
        {this.state.items.map(item => <li key={item.id}>{item.name}</li>)}
      </ul>
    );
  }
}

2. TypeError: ‘undefined’ Is Not an Object (evaluating…)

Safari throws this when accessing a property on an undefined value; it mirrors Chrome’s Cannot read property error but with a different message.

3. TypeError: Null Is Not an Object (evaluating…)

Occurs when code attempts to use a null reference, such as accessing DOM elements before they exist. Using strict equality checks ( ===) helps differentiate null from undefined.

4. (unknown): Script Error

Cross‑origin script errors are masked as “Script error” for security reasons. To see the real message, configure the server to send Access‑Control‑Allow‑Origin headers and add crossorigin="anonymous" to script tags.

5. TypeError: Object Doesn’t Support Property

In Internet Explorer, calling a method that does not exist on an object results in this error. Using fully‑qualified namespace calls (e.g., Rollbar.isAwesome()) avoids the issue.

6. TypeError: ‘undefined’ Is Not a Function

This Chrome error is triggered by invoking a function that is undefined, often due to incorrect this binding inside callbacks. Solutions include storing this in a variable (e.g., var self = this;) or using bind():

function testFunction() {
  this.clearLocalStorage();
  this.timer = setTimeout(this.reset.bind(this), 0);
}

7. Uncaught RangeError: Maximum Call Stack Size Exceeded

Caused by infinite recursion or passing out‑of‑range arguments to functions like Number.toExponential() or Number.toFixed(). Keeping recursion terminating and using valid numeric ranges prevents this error.

8. TypeError: Cannot Read Property ‘length’

Happens when trying to read .length of an undefined variable, often because a function parameter shadows a globally defined array. Removing the parameter or passing the array explicitly resolves the issue.

9. Uncaught TypeError: Cannot Set Property

Attempting to assign a property to undefined throws this error. Ensure the target object is created before setting its properties.

10. ReferenceError: Event Is Not Defined

This error appears when an event handler references the event object without receiving it as a parameter. Always declare the handler as function (event) { … } and avoid relying on legacy global event variables.

Conclusion

Most of these null or undefined errors are pervasive. Using a static type checker like TypeScript with strict options can catch many of them at compile time, helping developers write more robust JavaScript code.