traverse-level-order.js

function traverseLevelOrder(thing, predicate, maxDepth=20) {
  function traverse(thing, predicate, targetDepth, depth=0) {
    if (targetDepth !== depth) {
      for (const key in thing) {
        const val = traverse(thing[key], predicate, targetDepth, depth + 1);
        if (val) {
          return val;
        }
      }
      return;
    }

    for (const key in thing) {
      if (predicate(thing[key])) {
        return thing[key];
      }
    }
  }

  for (let i = 0; i < maxDepth; i++) {
    const val = traverse(thing, predicate, i);
    if (val) {
      return val;
    }
  }
}