How can I convert an array to a binary tree with JavaScript?

I need a refresher on traversing trees.

I'm about to start interviewing, and tech interviewers LOVE to give binary tree questions. This one is a practice question from Hired.com. (I wouldn't dare ask for this on an actual interview question posed to me - this is pure study. But it stumped me, and I know it shouldn't have.)

Here's an example question:

Given a binary tree represented as an array, (where a node of -1 is non-existent) write a function that determines whether the left or right branch of the tree is larger. The size of each branch is the sum of the node values. The function should return "Left" if the left side is larger, and "Right" if the right side is larger.

Here's the problem. The sample array they've given is:

[3, 6, 2, 9, -1, 10]

which is supposed to represent a binary tree of:

     3
   /   \
  6     2
 /     /
9    10

I just couldn't figure out how to basically turn that array into the binary tree. I think I can create a binary tree just fine, but I'm not sure how to create a tree from an array like that. It's one of those things I "used to know" but just for the life f me can't figure out.

Here's the code I'm working with (Javascript);

// went with a class, know I could probably do this FP, but f'it.
class TreeNode {
  constructor(value){
    this.value = value;
    this.left = null;
    this.right = null;
  }
  setLeft(node){
    this.left = node;
  }
  setRight(node){
    this.right = node;
  }
}

const buildNode = (root, valLeft, valRight){
  if(valLeft > 0){
    root.setLeft(new TreeNode(valLeft));
  }
  if(valRight > 0){}
    root.setRight(new TreeNode(valRight));
  }
  return root;
}

const solution = (arr) => {
  const rootNode = buildNode(new Node(arr.shift()), arr.shift(), arr.shift());
  /* this is where I run into trouble */
  if(arr.length){
    buildNode(rootNode.left, arr.shift(), arr.shift());
    buildNode(rootNode.right, arr.shift(), arr.shift());
    // I know I have to make a recursive function here,
    // and just can't wrap my head around it.  
  }
}

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