How to Check If a Key Exists in a JSON Object Using JavaScript?

In JavaScript, you can check if a key exists in a JSON object in the following ways:

1. Using Object.prototype.hasOwnProperty();
2. Using the in Operator;
3. Checking Against undefined.

Using Object.prototype.hasOwnProperty()

You can use the Object.prototype.hasOwnProperty() method to check if a key exists in a JSON object in the following way:

const jsonObj = {
  key1: 'value1',
};

console.log(jsonObj.hasOwnProperty('key1')); // true
console.log(jsonObj.hasOwnProperty('key2')); // false

You should use the Object.prototype.hasOwnProperty() method when you want to check if a key exists as an own, directly defined, property of an object, ignoring inherited properties (i.e. properties in the prototype chain). For example:

const jsonObj = { key1: 'value1' };
const protoObj = { key2: 'value2' };

// set the prototype of `jsonObj` to `protoObj`
Object.setPrototypeOf(jsonObj, protoObj);

// check for the existence of properties
console.log(jsonObj.hasOwnProperty('key1')); // true (exists in `jsonObj` itself)
console.log(jsonObj.hasOwnProperty('key2')); // false (exists in the prototype chain)

As you can see from the example above, the Object.prototype.hasOwnProperty() method returns true only if the key exists as an own property of the object itself, and not if it exists in the prototype chain.

Using the in Operator

You can use the in operator to check if a key exists in a JSON object in the following way:

const jsonObj = {
  key1: 'value1',
};

console.log('key1' in jsonObj); // true
console.log('key2' in jsonObj); // false

You should use the in operator when you want to check if a key exists in an object, including its prototype chain. For example:

const jsonObj = { key1: 'value1' };
const protoObj = { key2: 'value2' };

// set the prototype of `jsonObj` to `protoObj`
Object.setPrototypeOf(jsonObj, protoObj);

// check for the existence of properties
console.log('key1' in jsonObj); // true (exists in `jsonObj` itself)
console.log('key2' in jsonObj); // true (exists in the prototype chain)

As you can see from the example above, the in operator returns true if the key exists in an object or anywhere in the object's prototype chain.

Checking Against undefined

In JavaScript, when you try to access a non-existent key in an object, it returns undefined. Therefore, you can use the presence of undefined to determine if a key exists in an object, for example, like so:

const jsonObj = {
  key1: 'value1',
};

console.log(typeof jsonObj.key1 !== 'undefined'); // true
console.log(typeof jsonObj.key2 !== 'undefined'); // false

You should check against undefined when you want to check if a key exists in an object, including its prototype chain. For example:

const jsonObj = { key1: 'value1' };
const protoObj = { key2: 'value2' };

// set the prototype of `jsonObj` to `protoObj`
Object.setPrototypeOf(jsonObj, protoObj);

// check for the existence of properties
console.log(typeof jsonObj.key1 !== 'undefined'); // true (exists in `jsonObj` itself)
console.log(typeof jsonObj.key2 !== 'undefined'); // true (exists in the prototype chain)

You should use this method with caution as it is possible to assign undefined as a value to a property of an already parsed JSON object.