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in Keyword

Site: Saylor Academy
Course: PRDV401: Introduction to JavaScript I
Book: in Keyword
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Date: Friday, October 4, 2024, 1:24 AM

Table of contents

in Keyword

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

Try It

Click here to try an example using Code Runner.

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
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