Simple shallow copy of global will not work as expected with runInNewContext

vm_node.js

function clone(obj) { var nobj={}, key; for(key in obj) nobj[key]=obj[key]; return nobj; }

var local = null

var g = clone(global);

local = 3
console.assert(local === 3, 'not initialized to 3')
console.assert(null == g.local, 'g.local should be null or undefined') 

vm.runInNewContext("console.log(local=4)", g)

console.assert(local === 3, 'local unchanged')
console.assert(4 == g.local, 'g.local should be 4')

vm.runInThisContext("console.log(local=5)")
console.assert(local === 5, 'local should be changed to 5 by vm.runInThisContext')