How to get json result with parent - child relation?(see details) - node.js

Question

I have two json one is parent and another is child, I want to merge both. Please help. See the example below
Two tables are as below.
var parent = [{id : 1, name : 'India'},
{id : 2, name : 'USA'},
{id : 3, name : 'Japan'},
{id : 4, name : 'UK'}]
var child = [{id: 1, parentId: 1, city : 'Ahmedabad', population:100},
{id: 2, parentId: 1, city : 'Mumbai', population:200},
{id: 3, parentId: 2, city : 'NewYork', population:300},
{id: 4, parentId: 2, city : 'Chicago', population:400},
{id: 5, parentId: 3, city : 'Tokyo', population:500}]
I want to result like below
var result =
[{id : 1, name : 'India', city:[{name : 'Ahmedabad', population:100},{name : 'Mumbai', population:200}]},
{id : 2, name : 'USA', city:[{name : 'NewYork', population:300},{name : 'Chicago', population:400}]},
{id : 3, name : 'Japan', city:[{name : 'Tokyo', population:500}]},
{id : 4, name : 'UK', city:[]}
]

Answer 1

var result =JSON.parse(JSON.stringify(parent));
result.map(function (d, i) {
d.city = (child.filter(function (d1) {
if (d1.parentId == d.id) {
return d1;
}
})).map(function(d2){
return {city:d2.city,population:d2.population}
});
})
Now result variable contains your expected result.