`in` Keyword

Examples

Basic usage

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

Using `in` with deleted or undefined properties

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

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.hasOwn() 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

in Keyword