The in operator

in

true

Syntax

prop in objectName

Parameters

prop

objectName

Description

in

// Arrays
var 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 ES6+)

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

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

in

String

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

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

Using 

in

 with deleted or undefined properties

delete

in

false

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

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
delete trees[3];
3 in trees; // returns false

undefined

in

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

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
trees[3] = undefined;
3 in trees; // returns true

Inherited properties

in

true

"toString" in {}; // returns true

Specifications

Browser compatibility

See also

delete

Object.prototype.hasOwnProperty()

Reflect.has()