FreeGratefulOstracod created by anonymous - https://repl.it/repls/FreeGratefulOstracod

FreeGratefulOstracod.js

function astParse(a) {
  const arr = a.split(',')
  let ast = []
  let currentLevel = 0
  let currentNode = null
  arr.forEach(tag => {
    const taglevel = tag.search(/[^_]/) // gives how many underscores
    const tagNode = {
      name: tag.slice(taglevel),
      children: []
    }
    if (taglevel > currentLevel) {
      currentNode.children.push(tagNode)
    } else {
      ast.push(tagNode)
    }
    currentNode = tagNode
  })
  return ast
}
function astPrint(a) {
  let ans = ''
  a.forEach(x => {
    ans += `<${x.name}>`
    if (x.children.length) ans += astPrint(x.children)
    ans += `</${x.name}>`
  })
  return ans
}
function parse(a) {
  return astPrint(astParse(a))
}

console.log('test2')
testStr = 'div,span,h1,_span,__a'
console.log('should return')
console.log('<div></div><span></span><h1><span><a></a></span></h1>')
console.log(parse(testStr))
console.log('-------')


console.log('test3')
testStr = "div,_p,__h1,'foo',_p,'bar'"
console.log('should return')
console.log('<div><p><h1>foo</h1></p><p>bar</p></div>')
console.log(parse(testStr))
console.log('-------')