[4 Ways] Check undefined Property in Object


Summary: In this tutorial, you will learn different ways to check if a property is undefined in a JavaScript object.

There are several ways to detect if a property is undefined in a JavaScript object, including:

1. Using typeof operator

One way is to use the typeof operator to check the type of the property. If the property is undefined, the typeof operator will return “undefined”:

if (typeof obj.propertyName === "undefined") {
  // property is undefined

2. Using in operator

Another way to check if a property is undefined is to use the in operator to check if the property exists in the object:

if (!("propertyName" in obj)) {
  // property is undefined

3. Using Object.prototype.hasOwnProperty() method

You can also use the Object.prototype.hasOwnProperty() method which returns a boolean indicating whether the object has the specified property:

if (!obj.hasOwnProperty("propertyName")) {
  // property is undefined

4. Using ! operator

Another way is use the ! operator along with accessing property:

  //property is undefined

It’s important to note that the above methods checks if the property is undefined. It doesn’t check if its value is undefined.

Because JavaScript allows you to add properties to objects even if they don’t exist, you can assign undefined to a property that doesn’t exist, making it defined but having undefined as its value.

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