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JavaScript Check For Undefined

Last Updated : Mar 11, 2024

JavaScript Check For Undefined

In this article we will show you the solution of JavaScript check for undefined, to determine if a variable/object is undefined in JavaScript, the keyword undefined is available.

The JavaScript compiler will always return "undefined" for variables that aren't defined. Although both imply an empty state, it differs from null.

To find out if a variable is undefined in JavaScript, you can use several methods and approaches.

An undefined variable should not be used when performing an operation if you wish to prevent code-throwing errors.

The direct comparison comes to mind as one of the first methods. To determine if the output is undefined, you compare the output.

The undefined datatype is the data type for undefined values, which is a great thing for us.

There are no values in JavaScript that are null, which means they are empty, unknown, or missing. A null value indicates that an object value is intended to be absent.

It can also be treated as false for Boolean operations, like the undefined in JavaScript. An undefined property cannot be configured in a non-legacy browser or written to.

Overriding it should be avoided even when it isn't necessary. Variables without values are known as undefinable variables.

If a variable is being evaluated but does not have a value, the method or statement returns undefined as well. If no value is returned from the function, it returns undefined.

If you want to find out whether a variable has a value, you can use the undefined and strict equality operators.

Types of throws a Reference Error if x is undefined, but x === undefined doesn't if x hasn't been declared.

For x === undefined to be effective, the declared variable must be in the undefined range.

It is not possible to define obj.propName in JavaScript. Obj's property 'propName' will be strictly equal to undefined if either obj has a property 'propName' that has a value of 'undefined', or obj is missing a property 'propName'.

A property whose strict value is undefined should be checked by the in operator if it is found on the obj.

Step By Step Guide On JavaScript Check For Undefined :-

let my_var_1;
let my_var_2 = 10;
if (my_var_1 === undefined) {
    console.log("my_var_1 is undefined")
} else {
    console.log("my_var_1 holds some value")
if (my_var_2 === undefined) {
    console.log("my_var_2 is undefined")
} else {
    console.log("my_var_2 holds some value")
  1. Firstly, we create variables for creating values.
  2. Then we create an if and else statement for creating an undefined function.
  3. Then we use console.log for executing the function.
  4. After that we again create if and else statements.
  5. Then the program is run and we get output from either variable 1 or variable 2.

Conclusion :-

In this article, we hope that we have helped you understand what is meant by checking if a variable is defined or not.

As we have seen, the "==", "===", and type of operators, which possess some minor advantages and disadvantages, can be used in JavaScript for checking if a variable is undefined, null, or undefined.

I hope this article on JavaScript check for undefined helps you and the steps and method mentioned above are easy to follow and implement.

Author Image About Dikshita

Passionate Electronics and Communication Engineering student with expertise in web development (HTML, CSS, JS,PHP, Bootstrap, React.js) and content writing. Eager problem solver and tech enthusiast, adept at creating engaging web experiences.

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