Anil Singh | Code-sample.com

var z = {}; var x = { id: 10 }; y = { id: 20 }; z[x] = 'Anil' z[y] = 'Singh' console.log(z[x]); console.log(z[y]); Answer is   -  In Detailed Explanation: Because you are adding an object key to the z object. So when you do z[x] = 'Anil' the z object becomes z: { [object object]: 'Anil' } KEEP IN MIND : When you use objects as keys the key will be [ object object ]. So when you do z[y] it overrides the key "[object object] " with 'Singh'. So in the end the object z has a key "[ object object ]" and a value 'Singh'. Finally, when you do console.log you log twice the key. With what in mind, if you create another object another = {hello: ‘world’} and you console.log ( z[another ]) you will again get ''Singh".