Introduction
One of the most common type errors encountered in JavaScript is the “TypeError: Cannot read property of undefined”. This error occurs when you attempt to access a property or call a function on an undefined variable. In this post, we’ll explore the causes of this error, provide examples, and discuss how to avoid it in your code.
Understanding the Cause
In JavaScript, variables that are declared but not assigned a value are considered undefined. Since undefined is not an object type, attempting to access properties or call functions on such variables will result in the “TypeError: Cannot read property of undefined”.
Example
Let’s consider the following code snippet:
function myFunc(a) {
console.log(a.b);
}
var myVar;
myFunc(myVar);
In this example, myVar
is declared but not initialized, making it undefined. When myVar
is passed to the myFunc
function, the code attempts to access the property b
on a
. However, since a
is undefined at that point, running the code will throw the following error:
TypeError: Cannot read properties of undefined (reading 'b')
Avoiding the Error
To prevent the “TypeError: Cannot read property of undefined”, ensure that the variable in question is properly initialized before accessing its properties or calling its functions.
Here’s an example of initializing the variable:
function myFunc(a) {
console.log(a.b);
}
var myVar = {
b: 'myProperty'
};
myFunc(myVar);
In this updated version, myVar
is initialized as an object with a property b
that holds a string value. Now, when the code is executed, it will run successfully and output the
following in the browser console:
myProperty
Defensive Programming
To further safeguard your code against accidentally accessing undefined variables, you can incorporate conditional checks before working with such variables. Here are a couple of approaches:
if (myVar !== undefined) {
// Code to execute if myVar is defined
}
if (typeof(myVar) !== 'undefined') {
// Code to execute if myVar is defined
}
Let’s update the previous example to include a conditional check:
function myFunc(a) {
if (a !== undefined) {
console.log(a.b);
}
}
var myVar;
myFunc(myVar);
By adding the conditional check, the code avoids the error since the property b
is only accessed if a
is not undefined.
Error Handling with try-catch
Another way to handle potential errors is by using a try-catch
block. Here’s an example:
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);
}
By wrapping your code within a try-catch
block, you can catch and handle any errors that may occur, including the “TypeError: Cannot read property of undefined”.
Conclusion
The “TypeError: Cannot read property of undefined” is a common error in JavaScript that arises when attempting to access properties or call functions on undefined variables. By ensuring proper initialization of variables, employing defensive programming techniques, and utilizing error handling mechanisms like try-catch
blocks, you can effectively avoid and manage this error in your JavaScript code.