javascript - 'Object.assign' mutates state in deeper levels -

i have sample jsbin code @ http://jsbin.com/givolafala/edit?html,js,console,output

var oldobject = { name: 'john', address: { city: 'new york'}};    var newobject = object.assign({}, oldobject);    console.log(oldobject);  console.log(newobject);    newobject.name = 'mathew';  newobject.address.city = 'los angeles';    console.log(oldobject);  console.log(newobject);

in example, i'm changing name @ top level john mathew. object.assign returns new state new name mathew. oldobject keeps previous value john.

if change value in level other root, not work. in example change city los angeles, , notice newobject , oldobject have same city los angeles. oldobject state mutated.

is bug?

this not bug, called reference. there's difference between how primitive values , objects (references objectes actually) behaves.

var oldobject = { name: 'john', address: { city: 'new york'}}; 

oldobject consist of:

• name string value (primitive)
• addres object, oldobject has reference address object

now:

var newobject = object.assign({}, oldobject); 

after copying newobject has name value , address reference

newobject.name = 'mathew'; 

here change value of newobject's property.

newobject.address.city = 'los angeles'; 

but here change property of object newobject references. reference in oldobject change oldobject too.

basically both newobject , oldobject have reference same same address object.

what expected achieve called deep copy.