object_mutation.js

var a = [1, 2, 3];
var c = a; 
function doSomething(b){ 
  // b is a copy of a reference to the array a
  b.push(4); 
  console.log(b) // [1, 2, 3, 4] 
}

doSomething(a); 

console.log(a) // [1, 2, 3, 4]

// mutate a direct reference to a
c.push(5);
console.log(c) // [1, 2, 3, 4, 5]
console.log(a) // [1, 2, 3, 4, 5]