Weight equalizer (repl: https://repl.it/@alieksandrdubinin/Equalizer)

dataset.json

const data =
  [
    {
      "val": 30,
      "children": [
        {
          "val": 123,
          "children": [
            {
              "val": 0.2,
            },
            {
              "val": 0.56,
            },
            {
              "val": 0.21,
            },
          ]
        },
        {
          "val": 456,
          "children": [
            {
              "val": 123124,
            },
            {
              "val": 412231,
            },
            {
              "val": 640283,
            },
          ]
        },
        {
          "val": 789,
          "children": [
            {
              "val": 2,
            },
            {
              "val": 3,
            },
            {
              "val": 7,
            },
          ]
        }
      ]
    }
  ];

equalizer.js

import data from './dataset.json';

/**
 * Weight equalizer
 */
function Equalizer(setup = {}) {
  const options = {
    val: 'val',
    children: 'children',
    weight: 'weight',
    maxDeep: 100,
    ...setup
  };

  function getWeights(parent, _deep = 0) {
    if (!parent[options.children]) return parent;
    const summ = parent[options.children].reduce((s, c) => s + (c[options.weight] || c[options.val]), 0);

    parent[options.children].forEach(child => {
      child[options.weight] = parent[options.val] * child[options.val] / summ;
      if (_deep >= options.maxDeep) {
        console.warn(`[Equalizer]: Max deep reached (${options.maxDeep});`);
        return;
      }
      if (child[options.children]) getWeights(child, _deep + 1);
    });
    return parent
  }

  return parent => getWeights(parent, 0);
}

const equalizer = Equalizer();
equalizer(data[0]);
console.log(data[0]);