MergeTwoBinaryTrees.js

const mergeTrees = (t1, t2) => {
  //when there is null given tree
  if(!t1 || !t2) return t1 || t2

  //both of trees are not null,
  //we sum the value, and transform t1,
  //we can also choose t2, and return t2 at the end
  t1.val += t2.val
  
  //recursively traverse left path  
  t1.left = mergeTrees(t1.left, t2.left)
  //recursively traverse right path
  t1.right = mergeTrees(t1.right, t2.right)
  
  //return merged tree
  return t1
     
    
}