“Cannot set properties of Undefined” in JavaScript is a common error that shows up in many ways. What is the cause of it and how to fix it? Read on this article to find the answers.

How Does The Error “Cannot set properties of Undefined” In JavaScript Happen?

In JavaScript, practically everything is an object with the exception of null and undefined. Undefined always returns when you attempt to access a variable that isn’t defined, and we are unable to get or set any of the variable’s properties. An application will throw the error “Cannot set properties of Undefined” in JavaScript in that scenario.

Cannot set properties of Undefined

How To Solve The Error “Cannot set properties of Undefined” In JavaScript?

Here are some solutions you can apply to resolve the error “Cannot set properties of Undefined” in JavaScript:

Solution 1: Ensure you are setting a property on an object

First, you must ensure you are setting a property on an object. For example, you can assign d to an object by writing as shown below:

d = { 
  [a]: { 
    greetings: b, 
    data: c, 
  }, 
}; 
d[a]["greeting"] = "hello, welcome to Ittutoria";

Because d[a] is an object with the greetings property, we may set d[a][“greeting”] to “hello, welcome to Ittutoria”.

Solution 2: Initialize the array

If a variable is declared in JavaScript but not given a value, it is automatically assigned an undefined value. Therefore, it will cause the issue “Cannot set properties of Undefined” in JavaScript if you attempt to retrieve the value of such a variable. For instance:

var example_array
example_array[1] = 1878
console.log(example_array);

Output:

Uncaught TypeError: Cannot set properties of undefined (setting '1')

The null is a special assignment value that can be used to represent no value by assigning it to a variable. It also will throw the issue “Cannot set properties of Undefined” in JavaScript if you attempt to retrieve the value of such a variable.

var example_test = null
example_test[0] = 1902

Output:

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

You don’t have the array initialization in the situations mentioned above. Therefore, you must first initialize the array.

var example_array = []

Your code will work well after you add the following line to it.

var example_array = []
example_array[1] = 1878
console.log(example_array);

Output:

[empty, 1878]

In example 2, we mentioned “null”. Do you know how to determine if a variable is ‘null’ or ‘undefined’? If you don’t know, let’s follow the next part.

How Can We Determine If A Variable Is ‘Null’ Or ‘Undefined’?

A null value is the absence of a value or no value, while an undefined variable is that has been declared but has not yet been given a value. One of the important causes of runtime issues in JavaScript is undefined and null. This occurs because you do not check the value of any return variables that are unknown first. The recommended practice is to check a variable’s value for null or undefined before you use it if you are unsure that it will always have some value. If all you want is to see if there is any value, you can execute as the following:

if( value ) { 
  //do something 
}

If the value is not null, undefined, the empty string (“”), 0, NaN, false, it will evaluate to true.

All ECMA-/Javascript-compatible false values are represented in the list above.

Conclusion

Individual solutions provided in these tools are some of the most fundamental for anyone faced with the question “Cannot set properties of Undefined” in JavaScript. If you still need assistance or have basic Python questions, a growing community of people are usually willing to assist you. In addition, we anticipate a more creative day filled with new ideas and code.

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