数组转树

const arr = [{
    id: 1,
    parent: null
}, {
    id: 2,
    parent: null
}, {
    id: 3,
    parent: 1
}, {
    id: 4,
    parent: 2
}, {
    id: 5,
    parent: 3
}, {
    id: 6,
    parent: 1
}];

function tree(parent, arr) {
    for (let j = arr.length - 1; j >= 0; j--) {
        if (!parent) {
            parent = [];
        }
        if (parent.id) {
            if (arr[j].parent === parent.id) {
                if (!parent.children) {
                    parent.children = [];
                }
                parent.children.push(tree(arr[j], arr));
            }
        } else {
            if(!arr[j].parent){
                parent.push(tree(arr[j], arr));
            }
        }
    }
    return parent;
}
const res = tree(null, arr);
// const res = tree({
//     id: null
// }, arr);
console.log(JSON.stringify(res));