Insert start/stop markers into a string

mark_substrings.js

#! /usr/bin/env node
'use strict'


let inp_str = 'Hello, world!'
let inp_markers = [ [6, 10], [0, 2] ]


String.from_iterator = (str_iter) => ''.concat(...str_iter)

const mark_str_segments = (str, markers) => String.from_iterator(chunk_generator(str, markers))


function* chunk_generator(str, markers) {
  `Emit chunks for new string with marked regions.

  str (String): source string
  markers (Array): unordered list of start-end region markers pairs
  `
  const start_mark = '<M>'
  const end_mark = '</M>'
  const _markers = [...markers]
        .sort((m1, m2) => m1[0] - m2[0])  // sort markers by start index
  const str_len = str.length  // cache the str length

  let cur_ind = 0

  for (let m of _markers) {
    let m_start = m[0]
    let m_end = m[1] + 1
  
    yield cur_ind < m_start  // emit raw chunk before the marked region
          ? str.substring(cur_ind, m_start)
          : ''

    if (m_start >= str_len) return // start marker is out of bound

    yield start_mark

    yield str.substring(m_start, m_end)  // emit marked chunk

    yield end_mark

    if (m_end >= str_len) return // end marker is out of bound

    cur_ind = m_end
  }

  yield cur_ind < str.length  // emit the rest of input string, unmarked
        ? str.substring(cur_ind)
        : ''
}

console.log(`IN: '${inp_str}'`)
console.log(mark_str_segments(inp_str, inp_markers))
console.log("IN: ''")
console.log(mark_str_segments('', inp_markers))
console.log("IN: 'hey'")
console.log(mark_str_segments('hey', inp_markers))
console.log("IN: 'heyyaaaaaaaaaaaaaaaaaaaaaahh'")
console.log(mark_str_segments('heyyaaaaaaaaaaaaaaaaaaaaaahh', inp_markers))
console.log("IN: 'heyyaa'")
console.log(mark_str_segments('heyyaa', inp_markers))
console.log("IN: 'heyyaaaaaa'")
console.log(mark_str_segments('heyyaaaaaa', inp_markers))
console.log("IN: 'heyyaaaaaaa'")
console.log(mark_str_segments('heyyaaaaaaa', inp_markers))
console.log("IN: 'heyyaaaaaaaa'")
console.log(mark_str_segments('heyyaaaaaaaa', inp_markers))
console.log("IN: 'heyyaaaaaaaaaa'")
console.log(mark_str_segments('heyyaaaaaaaaaa', inp_markers))