How to Check if an Object Property is undefined in JavaScript

In JavaScript, you will often have to check if a property is defined on an object. This is important because if you try to access a property that is not defined, you will get an error.

In this post, we will explore the proper way to check if a property is defined on an object.

Check if a property is defined on an object

The best way to check if a property is defined on an object is to use the typeof operator.

This operator will return the type of the property as a string. If the property is not defined, it will return undefined.

From here, we can simply do a string comparison to see if the property is defined.

First, let's see how the typeof operator works by looking at examples.

JAVASCRIPT
const number = 1;
const string = "string";
const boolean = true;
const object = {};
const array = [];
const function_ = function() {};
const list = [1, 2, 3];

typeof number; // 'number'
typeof string; // 'string'
typeof boolean; // 'boolean'
typeof object; // 'object'
typeof array; // 'object'
typeof function_; // 'function'
typeof list; // 'object'
typeof fake; // 'undefined'

Now to check if a property is undefined just use the typeof operator and compare it to undefined.

This also works with objects. Consider this example:

JAVASCRIPT
const object = {
    name: "John",
    age: 30
};

typeof object.name; // 'string'
typeof object.age; // 'number'
typeof object.fake; // 'undefined'

Here's how you can check if any property is undefined:

JAVASCRIPT
if (typeof object.property === "undefined") {
    // do something
}

Conclusion

In this post, we've explored the proper way to check if a property is defined on an object. We also covered the typeof operator.

Remember, the typeof operator will return the type of the property as a string. If the property is not defined, it will return undefined.

Hopefully, you've found this information helpful.

Happy coding!

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