Conversion between flat array and JSON tree

Flat arrays -> JSON Trees

let flatArr = [
	{
     id: 1, title: "title1", parent_id: 0 },
	{
     id: 2, title: "title2", parent_id: 0 },
	{
     id: 3, title: "title2-1", parent_id: 2 },
	{
     id: 4, title: "title3-1", parent_id: 3 },
	{
     id: 5, title: "title4-1", parent_id: 4 },
	{
     id: 6, title: "title3-2", parent_id: 3 }
]

const flatArrToJSONTree = arr => {
    
	let map = flatArr.reduce((prev, item) => {
    
	    // prev  The value returned by the last call callback , Or initial value 
		prev[item.id] = item
		return prev
	}, {
    })
	let result = []
	for(let item of flatArr) {
    
		if(!item.parent_id) {
     //  Class A 
			result.push(item)
			continue
		}
		//  Can get here   Definitely not the first level 
		//  Find parent first 
		if(item.parent_id in map) {
    
			let parent = map[item.parent_id] //  Parent 
			//  Classic and concise value taking , No value initialization 
			;(parent.children || (parent.children = [])).push(item)
		}
	}
	return result
}

JSON Trees -> Flat arrays

const JSONThreeToFlat = tree => {
    
	return tree.reduce((prev, item) => {
    
		const {
     children = [], ...rest } = item
		return prev.concat( [{
    ...rest}], JSONThreeToFlat(children) )
	}, [])
}