Breadth-First.js

let identity = x => x
let createBF = (getChildren) => {
  let handler = (stack = [], results = [], f) => {
    if (stack.length === 0) return results;
    let [head, ...tail] = stack;
    let nextStack = tail.concat(getChildren(head));
    return handler(nextStack, [...results, f(head)], f);
  };
  return (tree, f = identity) => handler([tree], [], f);
};

let tree = [
  0, 
    [1, 
      [2], 
      [3], 
      [4, 
        [5]
      ]
    ], 
    [6], 
    [7]
];

let bfForTree = createBF((tree) => tree.slice(1));
let bfForDOM = createBF(tree => Array.from(tree.children))

console.log('bfForTree', bfForTree(tree, node => node[0]));
console.log('bfForDOM', bfForDOM(document.documentElement, node => node.tagName.toLowerCase()));