Solution.js

var isBalanced = function (root) {
    if(!root) return true
    return Math.abs(depth(root.left) - depth(root.right)) <= 1
          && isBalanced(root.left)
          && isBalanced(root.right)
  }
  var depth = function (node) {
      if(!node) return -1
      return 1 + Math.max(depth(node.left), depth(node.right))
  }