generateRecursiveObj.js

function generateRecursiveObj(input){
    let output = {};
    input.forEach(item => {
        output[item.title] = {};
        if(item.children && item.children.length){
            output[item.title] = generateRecursiveObj(item.children);
        }    
    })   
    return output; 
}

var arr = [
    {
      title: "Section 1",
      children: []
    },
    {
      title: "Section 2",
      children: [
        {
          title: "Section 2.1",
          children: []
        },
        {
          title: "Section 2.2",
          children: []
        },
        {
          title: "Section 2.3",
          children: []
        }
      ]
    }];

generateRecursiveObj(arr);
//output
//"{
//  "Section 1": {},
//  "Section 2": {
//   "Section 2.1": {},
//    "Section 2.2": {},
//    "Section 2.3": {}
//  }
//}"