in

prop in object

// 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 ES2015+)

// 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

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

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

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

'toString' in {}; // returns true