JSFiddle - React, Tailwind, and code Playground

JavaScript

const tree = {
  value: 1,
  next: [
    {
      value: 4,
      next: [
        {
          value: 5,
          next: []
        },
        {
          value: 3,
          next: [
            {
              value: 6,
              next: []
            }
          ]
        }
      ]
    },
    {
      value: 10,
      next: []
    }
  ]
};


function sum(node) {
	return node.value + node.next.reduce((s, child) => s + sum(child), 0);
} 


console.log(sum(tree));