How can I add an element to a json object in JavaScript?

var obj={"student":[{"id":1,"name":"mark"}]}

For example I have this object I want to add another element to it such as this:

var teacher=  "teacher":[{"id":1,"name":"Stacy"}] 

to make this new object:

var objnew= {"student":[{"id":1,"name":"mark"}], "teacher":[{"id":1,"name":"Stacy"}] }