depth first.js

  let tree = {
    name: 'root',
    children: [
      {
        name: 'a',
        children: [
          { name: 'd' },
          { name: 'e' }
        ]
      },
      {
        name: 'b',
        children: [
          { name: 'f'},
          { name: 'g'},
          { name: 'h'},
        ]
      },
      {
        name: 'c'
      }
    ]
  }

  function depthFirstFlat(node) {
    node.level = node.level || 0;
    console.log(`name: ${node.name}, level: ${node.level}`);
    if (node.children && node.children.length > 0 && (node.name === 'root' || node.name === 'b')) {
      const modifiedChildren = node.children.map(child => {
          return Object.assign({}, child, {level: node.level + 1})
        })
        .map(depthFirstFlat)
        .reduce((prev, item) => [...prev, ...item], []);
      return [node, ...modifiedChildren]
    } else {
      return [node];
    }
  }

  depthFirstFlat(tree);