How to Check if JavaScript Object Property is Undefined

JavaScript objects are popular data structures that allow you to store a lot of different types of data in a compact manner, as key-value pairs. While using JS objects, if you try to access a property that does not exist or is undefined then you will get an error and the J code execution on your page will most likely stop. So it is advisable to check if JavaScript object property is undefined before trying to access it. In this article, we will learn how to check if JavaScript object property is undefined.

How to Check if JavaScript Object Property is Undefined

There are several easy ways to check if JavaScript object property is undefined.

Let us say you have the following JS object.

test = {'a':1, 'b':2}

Let us say you want to check if property ‘c’ is assigned undefined value in JS object, then here is the command syntax on how you can do it.

if(object.property === undefined) {
  alert("property value is the special value `undefined`");
}

Here is how to check if property c is assigned value as undefined in test JS object defined above.

if(test.c === undefined) {
  alert("property value is the special value undefined");
}

If you want to check whether the property exists or not, use the following command.

if(!object.hasOwnProperty('property')) {
  alert("property does not exist");
}

Here is an example to check if property c exists or not in above object.

if(!test.hasOwnProperty('c')) {
  alert("property c does not exist");
}

If you want to check either of the above cases using the same command, that is, either the property is not declared or is set as undefined, use the following command.

if(typeof variable === 'undefined') {
  alert('variable is either the special value `undefined`, or it has not been declared');
}

Here is how to use the above command to check property c in our object test.

if(typeof test.c === 'undefined') {
  alert('test.c is either the special value `undefined`, or it has not been declared');
}

In this article, we have learnt a couple of easy ways to quickly check if an object property is defined or not, and also whether if it is assigned undefined value. Please note, these are two different cases. It is possible that an object property is defined but has been assigned undefined value.

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