How to push elements of an object into another object?

You could add some properties of an object simply like this :

obj = {a : "1", b : "2"};

myObj = {c: "3", d : "4"};
myObj.a = obj.a;
myObj.b = obj.b;

Update:

In that case just do this :

for(var prop in obj) myObj[prop] = obj[prop];

And to filter out the unwanted properties inside the loop body you could also do this :

for (var prop in obj) {
    if (obj.hasOwnProperty(prop)) {
        myObj[prop] = obj[prop];
    }
}

To copy all elements of one object to another object, use Object.assign:

var myObject = { apple: "a", orange: "o" };
var anothObject = Object.assign( { lemon: "l" }, myObject );

Or, more elegantly ES6 style using spread ... operator:

let myObject = { apple: "a", orange: "o" };
let anothObject = { lemon: "l", ...myObject };

You can use jQuery's extend function: http://api.jquery.com/jquery.extend/

var object1 = {
  apple: 0,
  banana: { weight: 52, price: 100 },
  cherry: 97
};
var object2 = {
  banana: { price: 200 },
  durian: 100
};

// Merge object2 into object1
$.extend( object1, object2 );