saveParentGetAllChildren.js

var data = [
	{
		id: 2,
		name: 'c',
		parent: 0
	},
	{
		id: 3,
		name: 'd',
		parent: 1
	},
	{
		id: 0,
		name: 'a',
		parent: null
	},
	{
		id: 1,
		name: 'b',
		parent: 0
	},
	{
		id: 4,
		name: 'e',
		parent: 1
	},
	{
		id: 5,
		name: 'f',
		parent: null
	}
]

var dataMap = {}

for (var i = 0; i < data.length; i++) {
	dataMap[data[i].id] = []
	for (var j = 0; j < data.length; j++) {
		if (i !== j && data[j].parent === data[i].id) {
			dataMap[data[i].id].push(data[j])
		}
	}
}

for (var k = 0; k < data.length; k++) {
	data[k].children = dataMap[data[k].id]
}

data = data.filter((item) => {
	return item.parent === null;
})

// console.log(dataMap);
console.log(JSON.stringify(data, null, 4))