example-binary-tree-iterator.js

(() => {
class BinaryTree {
  constructor(value, left = null, right = null) {
    this.value = value
    this.left = left
    this.right = right
  }

  [Symbol.iterator]() {
    var tree = [this.value, this.left, this.right]
    
    return {
      next() {
        let node = tree.shift()
        if (node instanceof BinaryTree) {
          tree.splice(0, 0, node.value, node.left, node.right)
          return this.next()
        }
        else if (node === null) {
         return this.next()
        }
        else if (node !== undefined) {
          return { value: node }
        }
        return { done: true }
      }
    }  
  }

}

let tree = new BinaryTree('a', 
            new BinaryTree('b', 
             new BinaryTree('c'), 
             new BinaryTree('d')
            ), 
            new BinaryTree('e')
           );
    
    for (let x of tree) {
        console.log(x);
    }
    // Output:
    // a
    // b
    // c
    // d
    // e
    
})()