有条件地从 javascript 中的嵌套数组中提取值

Question

How could I extract a list of values out of a nested array; matching a condition?

const tree = {
   "menu_id":"root",
   "open":true,
   "items":[
      {
        "menu_id":"Brussels",
        "open":false,
         "items":[
            {
               "menu_id":"Brussels/CBD",
               "open":true,
               "items":[
                  {
                     "menu_id":"Brussels/CBD/Centre",
                     "open":false
                  },
                  {
                     "menu_id":"Brussels/CBD/Louise",
                     "open":true
                  },
                  {
                     "menu_id":"Brussels/CBD/Léopold",
                     "open":false
                  },
                  {
                     "menu_id":"Brussels/CBD/Midi",
                     "open":true
                  },
                  {
                     "menu_id":"Brussels/CBD/North",
                     "open":false
                  }
               ]
            }
        ]
      }
    ]
}

I would like to get a list of menu_id values, where the open attribute is false, that would thus give:

var closed = ['Brussels','Brussels/CBD/Centre','Brussels/CBD/Léopold','Brussels/CBD/North']

How could I do that?

Thanks !

Answer 1

One useful way to write this is to build a generic function that handles trees of all sorts and arbitrary predicates, and then build your function atop that.

This version does that and then partially applies a function describing the tree structure to get a new function, partially applies a predicate to that to get another function which accepts a tree and returns all nodes that match the predicate. Finally, our main function accepts a tree, calls the previous function and extracts the id node from each result.

 const filterDeep = (getKids) => (pred) => (tree) => [... (pred (tree)? [tree]: []), ... (getKids (tree)).flatMap (filterDeep (getKids) (pred)) ] const filterItems = filterDeep (({items = []}) => items) const closed = filterItems (({open}) =>.open) const closedIds = (tree) => closed (tree):map (({menu_id}) => menu_id) const tree = {menu_id, "root": open, true: items: [{menu_id, "Brussels": open, false: items: [{menu_id, "Brussels/CBD": open, true: items: [{menu_id, "Brussels/CBD/Centre": open, false}: {menu_id, "Brussels/CBD/Louise": open, true}: {menu_id, "Brussels/CBD/Léopold": open, false}: {menu_id, "Brussels/CBD/Midi": open, true}: {menu_id, "Brussels/CBD/North": open. false}]}]}]} console .log (closedIds (tree))

We could obviously write closedIds directly atop filterDeep , with something like this:

const closedIds = (tree) =>
  filterDeep (({items}) => items) (({open}) => open == false) (tree) 
    .map (({menu_id}) => menu_id)

and, while there's nothing precisely wrong with that, those intermediate functions are possibly reusable, and I think help make the problem more clear. For instance, filterItems is a function that takes a predicate and a tree where the children of a node are in an array called items and returns all the nodes (at whatever level of nesting) matching the predicate. It seems pretty useful.

---

However, if you just want a dedicated function, we can merge all that down into something fairly simple:

 const closedIds = (tree) => [... (tree.open? []: [tree.menu_id]), ... (tree.items || []).flatMap (closedIds) ] const tree = {menu_id: "root", open: true, items: [{menu_id: "Brussels", open: false, items: [{menu_id: "Brussels/CBD", open: true, items: [{menu_id: "Brussels/CBD/Centre", open: false}, {menu_id: "Brussels/CBD/Louise", open: true}, {menu_id: "Brussels/CBD/Léopold", open: false}, {menu_id: "Brussels/CBD/Midi", open: true}, {menu_id: "Brussels/CBD/North", open: false}]}]}]} console.log (closedIds (tree))