tree.js

class Tree {
  
  constructor(value, children = []) {
    this.value = value
    this.children = children
  }

  isLeaf() {
    return this.children.length === 0
  }

  traverseDepthFirst(fn) {
    fn(this)
    if (!this.isLeaf()) {
      this.children.forEach(n => n.traverseDepthFirst(fn))
    }
  }
  
  traverseBreadthFirst(fn) {
    
    const queue = []
    const visited = new Set()
    fn(this)
    
    function enqueueChildren(queue, node) {
      node.children.forEach(c => queue.push(c) )
      node.children.forEach(c => enqueueChildren(queue, c))
    }
    
    // add all children nodes to queue    
    enqueueChildren(queue, this)
    
    while (queue.length) {
      fn(queue.shift())
    }
    
  }
  
  traverseZigZag(fn) {
        
    function enqueueChildren(nodesByLevel, node, level) {
      nodesByLevel[level] = nodesByLevel[level] || []
      node.children.forEach(c => nodesByLevel[level].push(c))
      node.children.forEach(c => enqueueChildren(nodesByLevel, c, level + 1))
    }
    
    const nodesByLevel = {}
    const level = 1
    enqueueChildren(nodesByLevel, this, level)
    
    fn(this)
    Object.keys(nodesByLevel)
      .map(key => [nodesByLevel[key], key])
      .map(([nodes, level]) => level % 2 !== 0 ? nodes : nodes.reverse())
      .reduce((acc, curr) => acc.concat(curr), [])
      .forEach(fn)
  }
  
}

const t = new Tree(42, [
  new Tree(12, [
    new Tree(5, []),
    new Tree(14, [])
  ]),
  new Tree(32, [
    new Tree(30, []),
    new Tree(36, [])
  ])
])

console.log("TRAVERSE [DEPTH-FIRST]:")
t.traverseDepthFirst(n => console.log(n.value))
console.log("TRAVERSE [BREADTH FIRST]:")
t.traverseBreadthFirst(n => console.log(n.value))
console.log("TRAVERSE [ZIG-ZAG]:")
t.traverseZigZag(n => console.log(n.value))