July 4, 2020

How to Check if Key is Present in JSON Object in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if a key is present in a JSON object in javascript. By JSON object, we simply mean JSON string. It is extremely important to verify if JSON object is missing any property before we start using it blindly. Accessing any property without performing any checks can lead to unforeseen errors in production.

We have also covered in the past how to validate JSON string in Javascript. We suggest you have a look over that article and include that validation code as part of your error handling mechanism.

In the following example, we have a global JSON object. We simply want to verify if this JSON object has a name property or not. After verification, we will display a Boolean value on the screen. Please have a look over the code example and steps given below.

HTML & CSS

We have 3 elements in the HTML file (div, button, and h1). The div element is just a wrapper for the rest of the elements.
The inner text for the button element is “Check” and for the h1 element is “Result”.
We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
We have also included our javascript file script.js with a script tag at the bottom.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="style.css">
    <title>Document</title>
</head>
<body>
    
    <div>
        <button>Check</button>
        <h1>Result</h1>
    </div>

    <script src="script.js"></script>
</body>
</html>

body {
    text-align: center;
}

div {
    display: inline-block;
}

button {
    display: inline-block;
    padding: 10px 20px;
}

Javascript

We have selected two elements button and h1 using the document.querySelector() method and stored them in btnCheck and result variables respectively.
We have a global variable jsonObj and it holds a JSON object.
We have attached the click event listener to the button element.
In the click event handler function, we are calling JSON.parse() method and passing our global variable jsonObj as a parameter. This method simply parses JSON object and returns a javascript object. We assigned returned value to the obj variable.
We are using the hasOwnProperty() method to check if there is a property with a key name.
We are setting the inner text of the h1 element to the Boolean value returned by the hasOwnProperty() method.

let btnCheck = document.querySelector('button');
let result = document.querySelector('h1');

let jsonObj = '{"name":"Peter","age":"30"}';


btnCheck.addEventListener('click', () => {
    let obj = JSON.parse(jsonObj);
    result.innerText = obj.hasOwnProperty('name') ? 'True' : 'False';
});

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