JavaScript TypeError: cannot set properties of undefined [Solved]

Submitted by oretnom23 on Tuesday, August 29, 2023 - 17:05.

This article explores the reasons underlying the JavaScript TypeError: cannot set properties of undefined and presents viable solutions for addressing it. Moreover, I will present code snippets that exemplify situations giving rise to this error and provide corresponding code snippets that offer solutions to fix the problem.

If you are new in JavaScript and currently facing the 'TypeError: cannot set properties of undefined', this article will help you to understand about it and give you knowledge to fix it and prevent it to happen again to your future projects.

What is TypeError in JavaScript?

In JavaScript, TypeError is an error that occurs when performing an operation whereas the value provided is not the expected type. This kind of error is thrown when the argument type passed on a certain function is not the type that it was expected, attempting to use inappropriate value, or attempting to modify the value that cannot be changed. The said error is one of the common error messages that even other programming languages have such as the Python TypeError:'int' object is not callable

Why does the TypeError: cannot set properties of undefined in JavaScript?

The TypeError: cannot set properties of undefined error message appears when there is an endeavor to assign a property to an object or array that has not been defined. In simple terms, this error arises when trying to create a new property along with/without its associated value without first ensuring the proper initialization.

Here's an example snippet that throws the said error message:

let data = [
    {
        'id': 1,
        'name': 'John Smith',
    }
]
data[data.length].id = data.length
data[data.length].name = 'Mark Cooper'

On the sample snippet, we have an object data with default value named data. Then, after the initialization of the object, I attempt to set a new object to the data without initializing wether the data with the given key is defined as array or object.

Solution #1:

In the presented snippet that triggers the error, we can address the issue by defining the property and its associated value as an object, as exemplified in the following code.

let data = [
    {
        'id': 1,
        'name': 'John Smith',
    }
]
data[data.length] = {'id': data.length , name: 'Mark Cooper'}
console.log(data)

Solution #2:

Furthermore, JavaScript provides a range of built-in functions and methods, among which the push() function holds significance. This function is employed to append instances or items to an array, a capability that proves valuable in the context under consideration.

let data = [
    {
        'id': 1,
        'name': 'John Smith',
    }
]
data.push({'id': data.length , name: 'Mark Cooper'})
console.log(data)

Conclusion

In simpler terms, the 'TypeError: cannot set property of undefined' error message within JavaScript occurs when there is an absence of proper initialization for an array or object. In essence, rectifying this issue involves ensuring appropriate initialization of the variable or constant as an object or array. Moreover, JavaScript provides access to built-in functions that prove useful when encountering this error, particularly the push() function.

There you go! I hope this JavaScript Bug Fixing article help you with what you are looking for. Explore more on this website for more Free Source Codes, Tutorials, and articles about programming.

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