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Resolving TypeError: Cannot Read Property of Undefined in JavaScript

Resolving TypeError: Cannot Read Property of Undefined in JavaScript
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The TypeError: Cannot read property of undefined is one of the most common type errors in JavaScript. It occurs when a property is read or a function is called on an undefined variable.

Error message:

TypeError: Cannot read properties of undefined (reading x)

Error Type:

TypeError

 

What Causes TypeError: Cannot Read Property of Undefined

Undefined means that a variable has been declared but has not been assigned a value.

In JavaScript, properties and functions can only belong to objects. Since undefined is not an object type, calling a function or a property on such a variable causes the TypeError: Cannot read property of undefined.

 

TypeError: Cannot Read Property of Undefined Example

Here’s an example of a JavaScript TypeError: Cannot read property of undefined thrown when a property is attempted to be read on an undefined variable:

function myFunc(a) {
console.log(a.b);
}

var myVar;
myFunc(myVar);

Since the variable myVar is declared but not initialized, it is undefined. When it is passed to the myFunc function, the property b is attempted to be accessed. Since a is undefined at that point, running the code causes the following error:

TypeError: Cannot read properties of undefined (reading 'b')

 

How to Avoid TypeError: Cannot Read Property of Undefined

When such an error is encountered, it should be ensured that the variable causing the error is assigned a value:

function myFunc(a) {
    console.log(a.b);
}

var myVar = {
    b: 'myProperty'
};

myFunc(myVar);

In the above example, the myVar variable is initialized as an object with a property b that is a string. The above code runs successfully and produces the following output on the browser console:

myProperty

To avoid coming across situations where undefined variables may be accessed accidentally, an if check should be added before dealing with such variables:

if (myVar !== undefined) {
    ...
}

if (typeof(myVar) !== 'undefined') {
    ...
}

Updating the previous example to include an if check:

function myFunc(a) {
    if (a !== undefined) {
        console.log(a.b);
    }
}

var myVar;
myFunc(myVar);

Running the above code avoids the error since the property b is only accessed if a is not undefined.

Here is how you can handle errors using a try { } catch (e) { } block.

// Caught errors
try {

  //Place your code inside this try, catch block
  //Any error can now be caught and managed

} catch (e) {
  Rollbar.error("Something went wrong", e);
  console.log("Something went wrong", e);
}

Here is how you can setup a JavaScript Error handler: Setup JavaScript Error Handler

 

Where TypeError Resides in the JavaScript Exception Hierarchy

JavaScript provides a number of core objects that allow for simple exception and error management. Error handling is typically done through the generic Error object or from a number of built-in core error objects, shown below:

  • Error
  • InternalError
  • RangeError
  • ReferenceError
  • SyntaxError
  • TypeError
    • Cannot read property of undefined

As seen from the hierarchy above, TypeError is a built-in JavaScript error object that allows for the administration of such errors. The “Cannot read property of undefined” TypeError is a descendant of the TypeError object.

 

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