Javascript Array Recursion

Question

So I'm trying to get some practice in javascript by building up a tree class. I'm having some weird issues with a function in which I try to recursively get the leaves of the tree.

For example,

  function Tree(root, branches){ 
      this.root = root;
      this.branches = branches;
    }


    t = new Tree(2, [new Tree(6), new Tree(5)]); 

    Tree.prototype.is_leaf = function(){
      return !(this.branches)
    }

    Tree.prototype.get_leaves = function(){
      mainTree = this;
      root = this.root;
      branches = this.branches
      list_of_leaves = []

      if(mainTree.is_leaf()){
        list_of_leaves.push(root);
      } else {
          for(i = 0; i < branches.length; i++){ //go through each branch and run get_leaves
            console.log(branches); //Branches logs correctly here
            list_of_leaves.push.apply(list_of_leaves, branches[i].get_leaves()); 
             /*extend the main list_of_leaves with the list of leaves of each branch (THIS LINE CAUSES THE PROBLEM)*/
            console.log(branches); //Branches is set to undefined here
          }
      }
      return list_of_leaves;
    }

    t.get_leaves();

When I try to run this function I get a "length of branches undefined" error. For some reason branches is getting mutated through the recursive calls, I don't understand why that is happening. Is the list_of_leaves in the array shared across all instances? So should I define get_leaves as a method within the Tree object as opposed to in it's prototype? (it seems inefficient to do this, so I was hoping there was a better way). Thanks!

Eric Gopak · Accepted Answer · 2016-12-18 07:35:27Z

1

For some reason you are not using var for variable declaration, this has an effect of branches not being a local but rather a global variable. A quick fix to your code would be to add var to branches like this:

  function Tree(root, branches){ 
      this.root = root;
      this.branches = branches;
    }


    t = new Tree(2, [new Tree(6), new Tree(5)]); 

    Tree.prototype.is_leaf = function(){
      return !(this.branches)
    }

    Tree.prototype.get_leaves = function(){
      mainTree = this;
      root = this.root;
      var branches = this.branches; // <--- This line ;)
      list_of_leaves = []

      if(mainTree.is_leaf()){
        list_of_leaves.push(root);
      } else {
          for(i = 0; i < branches.length; i++){ //go through each branch and run get_leaves
            console.log(branches); //Branches logs correctly here
            list_of_leaves.push.apply(list_of_leaves, branches[i].get_leaves()); 
             /*extend the main list_of_leaves with the list of leaves of each branch (THIS LINE CAUSES THE PROBLEM)*/
            console.log(branches); //Branches is set to undefined here
          }
      }
      return list_of_leaves;
    }

    t.get_leaves();

answered Dec 18, 2016 at 7:35

Eric Gopak

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2 Comments

Raghav Anand Over a year ago

Oops, that worked perfectly now. Python spoiled me with no variable declaration haha. Thanks!

Eric Gopak Over a year ago

Haha, indeed, languages like Python are waaay to convenient :) and develop not-so-nice coding habits. Happy to help :)

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