tree.js

class Tree {
  constructor() {
    this.children = []
  }
  
  add(value, parentValue) {
    let newNode = { value, children: [] }
    
    if (parentValue === null) {
      this.children.push(newNode)
      return
    }
    
    for (let node of this.traverse()) {
      if (node.value === parentValue) {
        node.children.push(newNode)
        break
      }
    }
  }
  
  *traverse() {
    let queue = [...this.children]

    while (queue.length) {
      let node = queue.shift()
      yield node
      queue = [...node.children, ...queue]
    }
  }
}

let tree = new Tree()

tree.add("a", null)
tree.add("a.a", "a")
tree.add("a.b", "a")
tree.add("b", null)
tree.add("b.a", "b")
tree.add("b.b", "b")

for (let node of tree.traverse()) {
  console.log(node.value)
}

Thanks! — right, didn’t click that I hadn’t wrapped it in anything and it was just an ordinary array there.