istreesymmetric.js

 // recursive function wrapper
 function isTreeSymmetric(t) {
    // empty tree is automatically symmetric
    if (!t) return true;
    return isTreeSymmetricRecursive(t.left, t.right);
  }
  function isTreeSymmetricRecursive(left, right) {
      // implementation detail
      // if we dont have children, we are symmetric ourselves
      if (!(left || right)) return true;
      // if we only have one child, we cannot be symmetric
      if (!left || !right) return false;
      
      return (left.value === right.value) 
      && isTreeSymmetricRecursive(left.left, right.right)
      && isTreeSymmetricRecursive(left.right, right.left);
  }