in operator

prop

A string or symbol representing a property name (non-symbols will be coerced to strings). Can also be a private property identifier.

object

Object to check if it (or its prototype chain) contains the property with specified name (prop).

TypeError

Thrown if object is not an object (i.e. a primitive).

The in operator tests if a string or symbol property is present in an object or its prototype chain. If you want to check for only non-inherited properties, use Object.hasOwn() instead.

A property may be present in an object but have value undefined. Therefore, x in obj is not the same as obj.x === undefined. To make in return false after a property is added, use the delete operator instead of setting that property's value to undefined.

You can also use the in operator to check whether a particular private class field or method has been defined in an object. The operator returns true if the property is defined, and false otherwise. This is known as a branded check, because it returns true if and only if the object was created with that class constructor, after which you can safely access other private properties as well.

This is a special syntax — the left-hand side of the in operator is a property identifier instead of an expression, but unquoted (because otherwise it's a string property, not a private property).

Because accessing private properties on objects unrelated to the current class throws a TypeError instead of returning undefined, this syntax allows you to shorten:

class C {
  #x;
  static isC(obj) {
    try {
      obj.#x;
      return true;
    } catch {
      return false;
    }
  }
}

To:

class C {
  #x;
  static isC(obj) {
    return #x in obj;
  }
}

It also generally avoids the need for dealing with error handling just to access a private property that may be nonexistent.

However, the in operator still requires the private property to be declared beforehand in the enclosing class — otherwise, it would throw a SyntaxError ("Private field '#x' must be declared in an enclosing class"), the same one as when you try to access an undeclared private property.

class C {
  foo() {
    #x in this;
  }
}

new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class

The following examples show some uses of the in operator.

// Arrays
const trees = ["redwood", "bay", "cedar", "oak", "maple"];
0 in trees; // returns true
3 in trees; // returns true
6 in trees; // returns false
"bay" in trees; // returns false (you must specify the index number, not the value at that index)
"length" in trees; // returns true (length is an Array property)
Symbol.iterator in trees; // returns true

// Predefined objects
"PI" in Math; // returns true

// Custom objects
const mycar = { make: "Honda", model: "Accord", year: 1998 };
"make" in mycar; // returns true
"model" in mycar; // returns true

You must specify an object on the right side of the in operator. For example, you can specify a string created with the String constructor, but you cannot specify a string literal.

const color1 = new String("green");
"length" in color1; // returns true

const color2 = "coral";
// generates an error (color2 is not a String object)
"length" in color2;

If you delete a property with the delete operator, the in operator returns false for that property.

const mycar = { make: "Honda", model: "Accord", year: 1998 };
delete mycar.make;
"make" in mycar; // returns false

const trees = ["redwood", "bay", "cedar", "oak", "maple"];
delete trees[3];
3 in trees; // returns false

If you set a property to undefined but do not delete it, the in operator returns true for that property.

const mycar = { make: "Honda", model: "Accord", year: 1998 };
mycar.make = undefined;
"make" in mycar; // returns true

const trees = ["redwood", "bay", "cedar", "oak", "maple"];
trees[3] = undefined;
3 in trees; // returns true

The in operator will return false for empty array slots, even if accessing it directly returns undefined.

const empties = new Array(3);
empties[2]; // returns undefined
2 in empties; // returns false

To avoid this, make sure a new array is always filled with non-empty values or not write to indexes past the end of array.

const empties = new Array(3).fill(undefined);
2 in empties; // returns true

The in operator returns true for properties in the prototype chain. This may be undesirable if you are using objects to store arbitrary key-value pairs.

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return name in ages;
}

hasPerson("hasOwnProperty"); // true

You can use Object.hasOwn() to check if the object has the key.

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return Object.hasOwn(ages, name);
}

hasPerson("hasOwnProperty"); // false

Alternatively, you should consider using a null prototype object or a Map for storing ages, to avoid other bugs.

const ages = new Map([
  ["alice", 18],
  ["bob", 27],
]);

function hasPerson(name) {
  return ages.has(name);
}

hasPerson("hasOwnProperty"); // false

The code fragment below demonstrates a static function that tells if an object was created with the Person constructor and therefore can perform other methods safely.

class Person {
  #age;
  constructor(age) {
    this.#age = age;
  }
  static isPerson(o) {
    return #age in o;
  }
  ageDifference(other) {
    return this.#age - other.#age;
  }
}

const p1 = new Person(20);
const p2 = new Person(30);
console.log(p1.ageDifference(p2)); // -10
console.log(Person.isPerson(p1)); // true

if (Person.isPerson(p1) && Person.isPerson(p2)) {
  console.log(p1.ageDifference(p2)); // -10
}

It helps to prevent the following case:

const p2 = {};

p1.ageDifference(p2); // TypeError: Cannot read private member #age from an object whose class did not declare it

Without the in operator, you would have to use a try...catch block to check if the object has the private property.

You can also implement this as a @@hasInstance method of the class, so that you can use the instanceof operator to perform the same check (which, by default, only checks for the existence of Person.prototype in the object's prototype chain).

class Person {
  #age;
  constructor(age) {
    this.#age = age;
  }
  static [Symbol.hasInstance](o) {
    // Testing `this` to prevent false-positives when
    // calling `instanceof SubclassOfPerson`
    return this === Person && #age in o;
  }
  ageDifference(other) {
    return this.#age - other.#age;
  }
}

const p1 = new Person(20);
const p2 = new Person(30);

if (p1 instanceof Person && p2 instanceof Person) {
  console.log(p1.ageDifference(p2)); // -10
}

For more examples, see Private class features and the class guide.

• for...in
• delete
• Object.hasOwn()
• Reflect.has()
• Enumerability and ownership of properties