The in operator

JavaScriptJunior~1 min read

The in operator in JavaScript is used to check whether a given property exists in an object or an index exists in an array.

This is one of the basic, but often misunderstood, operators. Let's go through it in detail.

Syntax

javascript

propName in object

Where:

propName is a string (or an expression coerced to a string),
object is an object, array, or another structure with keys.

Example with an object

javascript

const user = {
  name: 'Tim',
  age: 25
};

console.log('name' in user); // true
console.log('age' in user);  // true
console.log('email' in user); // false

The in operator returns true if the property exists in the object (even if its value is undefined).

Features

Even if the property's value is undefined, in will still return true:

javascript

const obj = { a: undefined };

console.log('a' in obj); // true
console.log(obj.a === undefined); // true

In other words, in checks for the presence of a key, not its value.

Example with an array

javascript

const arr = ['a', 'b', 'c'];

console.log(0 in arr); // true  - index 0 exists
console.log(2 in arr); // true  - index 2 exists
console.log(3 in arr); // false - no such index

JS treats array indexes as properties: arr[0] is equivalent to arr['0'].

Example with inheritance (via the prototype)

The in operator checks not only own properties, but also ones inherited through the prototype.

javascript

const person = { name: 'Tim' };
console.log('toString' in person); // true, because it's inherited from Object.prototype

To check only own properties, use obj.hasOwnProperty('key'):

javascript
person.hasOwnProperty('toString'); // false person.hasOwnProperty('name'); // true

Example with a class

javascript

class User {
  constructor() {
    this.name = 'Alex';
  }
}

User.prototype.age = 25;

const u = new User();

console.log('name' in u); // true (an own property)
console.log('age' in u);  // true (inherited from the prototype)

Error when used with a non-object

If the right-hand side is not an object (and cannot be coerced to one), JS will throw an error:

javascript

'length' in null; // TypeError
'length' in undefined; // TypeError

So before checking, make sure the variable is not null or undefined.

Summary

Checks	The presence of a key (property / index)
Works with	Objects, arrays, functions
Returns	`true` / `false`
Considers the prototype	Yes
Checks the value	No

Comparison examples

Check	Result	Explanation
`'name' in { name: 'Tim' }`	true	the key exists
`'email' in { name: 'Tim' }`	false	the key does not exist
`0 in ['a', 'b']`	true	index 0 exists
`2 in ['a', 'b']`	false	index 2 does not exist
`'toString' in {}`	true	inherited from `Object.prototype`

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