in operator

The in operator returns true if the specified property is in the specified object or its prototype chain.

Syntax

prop in object

Parameters

prop

A string or symbol representing a property name or array index (non-symbols will be coerced to strings).

object

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

Examples

Basic usage

The following examples show some uses of the in operator.

// Arrays
let 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 (arrays are iterable, works only in ES2015+)

// Predefined objects
'PI' in Math          // returns true

// Custom objects
let 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. 

let color1 = new String('green')
'length' in color1 // returns true

let color2 = 'coral'
// generates an error (color2 is not a String object)
'length' in color2

Using in with deleted or undefined properties

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

let mycar = {make: 'Honda', model: 'Accord', year: 1998}
delete mycar.make
'make' in mycar   // returns false

let trees = new Array('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. 

let mycar = {make: 'Honda', model: 'Accord', year: 1998}
mycar.make = undefined
'make' in mycar   // returns true

let trees = new Array('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. 

let 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. 

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

Inherited properties

The in operator returns true for properties in the prototype chain. (If you want to check for only non-inherited properties, use Object.prototype.hasOwnProperty() instead.) 

'toString' in {}  // returns true

Private fields and methods

You can also use the in operator to check whether a particular private class field or method has been defined in a class. The operator returns true if the method is defined, and false otherwise.

Note: Code will throw if you attempt to access a private class field/method that has not been defined. Using the in operator to check for potentially missing private fields is more compact than using try/catch.

The code fragment below demonstrates a static function that checks whether a specified class has particular private methods and fields.

  class ClassWithPrivateFeatures {
    #a;
    #b = null;
    #c() {}
    get #d() {}
    static f(o) {
      return #a in o && #b in o && #c in o && #d in o;
    }
  }
  ClassWithPrivateFeatures.f(new ClassWithPrivateFeatures()) // returns true
  ClassWithPrivateFeatures.f({}) // returns false

Specifications

Specification
ECMAScript Language Specification
# sec-relational-operators

Browser compatibility

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See also