tree.js

const nodes = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J'];

const tree = { node: nodes[0], children: [{}, {}] };

addNodes(tree.children[0], nodes[1], 1);
addNodes(tree.children[1], nodes[2], 2);


function addNodes(theRoot, node, index) {
  if (!nodes[index]) {
    return;
  }
  theRoot.node = node;
  const firstChildIndex = index * 2 + 1;
  const secondChildIndex = index * 2 + 2;
  if (firstChildIndex > nodes.length - 1) {
    return;
  }
  else if (secondChildIndex > nodes.length - 1) {
    theRoot.children = [{}];
  } else {
    theRoot.children = [{}, {}];
  }
  addNodes(theRoot.children[0], nodes[firstChildIndex], firstChildIndex);
  addNodes(theRoot.children[1], nodes[secondChildIndex], secondChildIndex);
}


console.log(JSON.stringify(tree, null, 2));