How to Check if Key Exists in Object in JavaScript

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Moataz Farid Feb 02, 2024
JavaScript JavaScript Object
  1. Use the Element Direct Access Method to Check if the Object Key Exists in JavaScript
  2. Use the in Operator to Check if the Object Key Exists or Not in JavaScript
  3. Use the hasOwnProperty Method to Check if the Object Key Exists or Not in JavaScript
  4. Use the underscore Library to Check if the Object Key Exists or Not in JavaScript
How to Check if Key Exists in Object in JavaScript

In this tutorial, we will learn how to check if the key exists in JavaScript using multiple ways.

Use the Element Direct Access Method to Check if the Object Key Exists in JavaScript

If a key exists, it should not return undefined. To check if it returns undefined or not, we have direct access to the keys, and it can be done in two styles, the object style, and the brackets access style.

In the following example, we will show how to check if the key exists by direct access to the key using the brackets style.

let myObject = {'mykey1': 'My Value 1', 'mykey2': 'My Value 2'};

function isKeyExists(obj, key) {
  if (obj[key] == undefined) {
    return false;
  } else {
    return true;
  }
}
let result0 = isKeyExists(myObject, 'mykey0')
console.log('Check for the non-existing key, is key exists > ' + result0)

let result1 = isKeyExists(myObject, 'mykey1')
console.log('Check for the existing key, is key exists > ' + result1)

Output:

Check for the non-existing key, is key exists > false
Check for the existing key, is key exists > true

The Other style to check for the key by direct access is using the object style. In the following example, we can see how to check for that key existence in JavaScript.

let myObject = {mykey1: 'My Value 1', mykey2: 'My Value 2'};

let result = myObject.mykey0 != undefined
console.log('Is myKey0 exists ? ' + result)

let result1 = myObject.mykey1 != undefined
console.log('Is myKey1 exists ? ' + result1)

Output:

Is myKey0 exists ? false
Is myKey1 exists ? true

One of the drawbacks of checking for the undefined using direct key access is that the key’s value can be equal to undefined. Let’s see the following example to show the issue with that method.

let myObject = {mykey0: undefined, mykey1: 'My Value 1'};

let result = myObject.mykey0 != undefined
console.log('Is myKey0 exists? ' + result)

let result1 = myObject.mykey1 != undefined
console.log('Is myKey1 exists? ' + result1)

Output:

Is myKey0 exists? false
Is myKey1 exists? true

The above case’s solution is never to assign undefined to a property when you want to initialize it with an undefined value, but initialize it with null instead.

If we insist on using the undefined value to initialize the keys, we should use the in operator, the hasOwnProperty method, or the underscore library _.has method.

Use the in Operator to Check if the Object Key Exists or Not in JavaScript

The in operator is simply used to return false if the key was not found in the target object and returns true if found.

let myObject =
    {favoriteDish: 'Spaghetti', language: 'English'}

    function isKeyExists(obj, key) {
      return key in obj;
    }

    console.log(
        'Does language key exists? ' + isKeyExists(myObject, 'language'))
console.log(
    'Does nationality key exists? ' + isKeyExists(myObject, 'nationality'))

Output:

Does language key exists? true
Does nationality key exists? false

Use the hasOwnProperty Method to Check if the Object Key Exists or Not in JavaScript

Another way to check if the object contains a specific property key or not is to use the hasOwnProperty method. In the following example, we will show how we can use the hasOwnProperty method.

let myObject =
    {favoriteDish: 'Spaghetti', language: 'English'}

    function isKeyExists(obj, key) {
      return obj.hasOwnProperty(key);
    }

    console.log(
        'Does the object have language key? ' +
        isKeyExists(myObject, 'language'))
console.log(
    'Does the object have nationality key? ' +
    isKeyExists(myObject, 'nationality'))

Output:

Does the object have language key? true
Does the object have nationality key? false

Use the underscore Library to Check if the Object Key Exists or Not in JavaScript

If we are already using any of the underscore library methods, we can use the _.has() method, as it returns true if that object has the provided key and returns false if not.

let myObject =
    {favoriteDish: 'Spaghetti', language: 'English'}

    console.log(
        'Check using underscore library if the object has language key? ' +
        _.has(myObject, 'language'))

console.log(
    'Check using underscore library if the object has nationality key? ' +
    _.has(myObject, 'nationality'))

Output:

Check using underscore library if the object has language key? true
Check using underscore library if the object has nationality key? false

We can import the library from here.

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