breadth-first.js

function treeByLevels (rootNode) {
  let sortedArr = []
  if (!rootNode) return sortedArr;

  sortedArr.push(rootNode.value);
  let toReadAfter = [];
  currNode = rootNode;
  while (currNode) {
    currNode.left && sortedArr.push(currNode.left.value);
    currNode.right && sortedArr.push(currNode.right.value);
    if (currNode.left) toReadAfter.push(currNode.left);
    if (currNode.right) toReadAfter.push(currNode.right);
    currNode = toReadAfter.shift()
  }
  return sortedArr;
}

depth-first.js

function treeByLevels (rootNode, sortedArr = []) {
  if (!rootNode) return []
  sortedArr.push(rootNode.value);  
  rootNode.left && treeByLevels(rootNode.left, sortedArr);
  rootNode.right && treeByLevels(rootNode.right, sortedArr);
  return sortedArr;
}

node.js

class Node { 
  constructor(value, left = null, right = null) {
    this.value = value;
    this.left  = left;
    this.right = right;
  }
}