Recursive tree search in a nested object structure in JavaScript

I’m trying to figure out how to search for a node in this JSON object recursively. I have tried something but cannot get it:

var tree = {
    "id": 1,
    "label": "A",
    "child": [
        {
            "id": 2,
            "label": "B",
            "child": [
                {
                    "id": 5,
                    "label": "E",
                    "child": []
                },
                {
                    "id": 6,
                    "label": "F",
                    "child": []
                },
                {
                    "id": 7,
                    "label": "G",
                    "child": []
                }
            ]
        },
        {
            "id": 3,
            "label": "C",
            "child": []
        },
        {
            "id": 4,
            "label": "D",
            "child": [
                {
                    "id": 8,
                    "label": "H",
                    "child": []
                },
                {
                    "id": 9,
                    "label": "I",
                    "child": []
                }
            ]
        }
    ]
};

Here is my non-working solution, which is probably because the first node is just a value while children are in arrays:

function scan(id, tree) {
    if(tree.id == id) {
        return tree.label;
    }

    if(tree.child == 0) {
        return
    }

    return scan(tree.child);
};

Answer

Your code is just missing a loop to inspect each child of a node in the child array. This recursive function will return the label property of a node or undefined if label not present in tree:

const search = (tree, target) => {
  if (tree.id === target) {
    return tree.label;
  }
  
  for (const child of tree.child) {
    const res = search(child, target);
    
    if (res) {
      return res;
    }
  }
};

const tree = {"id":1,"label":"A","child":[{"id":2,"label":"B","child":[{"id":5,"label":"E","child":[]},{"id":6,"label":"F","child":[]},{"id":7,"label":"G","child":[]}]},{"id":3,"label":"C","child":[]},{"id":4,"label":"D","child":[{"id":8,"label":"H","child":[]},{"id":9,"label":"I","child":[]}]}]};

console.log(search(tree, 1));
console.log(search(tree, 6));
console.log(search(tree, 99));

You can also do it iteratively with an explicit stack which won’t cause a stack overflow (but note that the shorthand stack.push(...curr.child); can overflow the argument size for some JS interpreters due to the spread syntax, so use an explicit loop or concat for massive child arrays):

const search = (tree, target) => {
  for (const stack = [tree]; stack.length;) {
    const curr = stack.pop();
    
    if (curr.id === target) {
      return curr.label;
    }

    stack.push(...curr.child);
  }
};

const tree = {"id":1,"label":"A","child":[{"id":2,"label":"B","child":[{"id":5,"label":"E","child":[]},{"id":6,"label":"F","child":[]},{"id":7,"label":"G","child":[]}]},{"id":3,"label":"C","child":[]},{"id":4,"label":"D","child":[{"id":8,"label":"H","child":[]},{"id":9,"label":"I","child":[]}]}]};

for (let i = 0; ++i < 12; console.log(search(tree, i)));

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