Checking if a key exists in a JavaScript object

We are required to illustrate the correct way to check whether a particular key exists in an object or not. Before moving on to the correct way let's first examine an incorrect way and see how it's actually incorrect.

Way 1: Checking for undefined value (incorrect way)

Due to the volatile nature of JavaScript, we might want to check for the existence of key in an object like this:

const obj = {
    name: 'Rahul'
};

// Incorrect approaches
console.log(!obj['fName']);                    // true (misleading)
console.log(obj['fName'] === undefined);      // true (misleading)

true
true

These both are incorrect ways. Why?

Because in this case there happens to be no 'fName' key, but suppose there existed a 'fName' which was deliberately set to false or undefined:

const objWithFalsy = {
    name: 'Rahul',
    fName: false,
    age: undefined
};

// These fail when value is falsy
console.log(!objWithFalsy['fName']);               // true (wrong - key exists!)
console.log(objWithFalsy['age'] === undefined);   // true (wrong - key exists!)

true
true

Our function should have returned that the key exists, but these methods fail when values are falsy.

Way 2: Using the in operator (Correct Way)

The 'in' keyword checks for an entry in an object, including inherited properties. Therefore, to check for the existence of a key:

const obj = {
    name: 'Rahul',
    fName: false,
    age: undefined
};

console.log('fName' in obj);    // true - key exists
console.log('age' in obj);      // true - key exists  
console.log('email' in obj);    // false - key doesn't exist

true
true
false

Way 3: Using hasOwnProperty() method (Correct Way)

Using the Object.prototype.hasOwnProperty() method we can determine whether or not an object contains a key as its own property (not inherited).

Its syntax is:

const obj = {
    name: 'Rahul',
    fName: false
};

console.log(obj.hasOwnProperty('fName'));     // true - own property
console.log(obj.hasOwnProperty('name'));      // true - own property
console.log(obj.hasOwnProperty('toString'));  // false - inherited property
console.log('toString' in obj);               // true - inherited property exists

true
true
false
true

Comparison

The difference between Way 2 and Way 3 is that Way 3 only checks for the property of the Object instance it's called upon, whereas 'in' checks for Object instance properties as well as inherited properties (if there are any).

Conclusion

Use 'in' operator or hasOwnProperty() to reliably check key existence. Choose 'in' if you need inherited properties, or hasOwnProperty() for own properties only.