Solution To Fix “TypeError: Cannot Set Property ‘0’ Of Undefined” In Javascript

You are trying to access an array element by index, still, suddenly the “TypeError: Cannot set property ‘0’ of undefined“ in Javascript happens. How do you solve when this error appears now? You must first understand what causes this error, and then we will show you how to resolve it.

Why does the “TypeError: Cannot set property ‘0’ of undefined” in Javascript occur?

To understand how the error appears, follow the example below.

Example:

var arrayNew;
console.log(arrayNew[0]);

Output:

Uncaught TypeError: Cannot set properties of undefined (setting '0')

When you create a variable and forget to initialize it, the value is undefined. You try to use a variable with an undefined value and try to access the first element as an array which results in an error. There are many error cases like this, but the core is you try to access the first element of an array while its value is undefined.

How to fix the “TypeError: Cannot set property ‘0’ of undefined” in Javascript?

Use the !== operator

The !== operator is another used to compare differences, including values and data types.

Example:

var arrayNew = undefined;

// Check for arrayNew other than undefined
if (arrayNew !== undefined) {
	console.log(arrayNew[0]);
} else {
	console.log("Error");
}

var arrayOther = [1, 2, 3];

// Check for arrayOther other than undefined
if (arrayOther !== undefined) {
	console.log("The value of the first element: ", arrayOther[0]);
} else {
	console.log("Error");
}

Output:

Error
The value of the first element: 1

To avoid this “TypeError: Cannot set property ‘0’ of undefined” in Javascript, we have to check its value before accessing the first element at index 0. If it is undefined, we disallow it, and if the value is not undefined, then we allow them access.

Use the Array.isArray() method

Array.isArray() method

This method will return true if the value is an array and false if it is not.

Syntax:

Array.isArray(value)

Parameter:

• value: The value you want to check is an array.

Example:

var arrayNew = [1, 2, 3];

// Check if arrayNew is the array
if (Array.isArray(arrayNew)) {
	console.log(arrayNew[0]);
} else {
	console.log("Error");
}

Output:

1

We need the variable to be an array to access the first position and avoid the error in the first example. And we use this method to define it.

Use the optional chaining operator ?.

This operator checks the value before the ‘?.’.It will return undefined if it is null or undefined instead of reporting an error to the user.

Example:

var arrayNew = undefined;
console.log(arrayNew?.[0]);

Output:

undefined

Using operator ||

Example:

var arrayNew = undefined || [1, 2, 3];
console.log(arrayNew[0]);

Output:

1

It is also nice to provide an alternative value if it is undefined. Use the ‘||’  operator so that if its initial value is undefined, it will receive the second value provided by us.

Summary

To solve this, we have shown you many ways to fix the “TypeError: Cannot set property ‘0’ of undefined“ in Javascript. It is easy to understand. Our site has many articles like this. Follow us to read them.

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Tom Joseph

My name is Tom Joseph, and I work as a software engineer. I enjoy programming and passing on my experience. C, C++, JAVA, and Python are my strong programming languages that I can share with everyone. In addition, I have also developed projects using Javascript, html, css.

Job: Developer
Name of the university: UTC
Programming Languages: C, C++, Javascript, JAVA, python, html, css