Compute changed range from ProseMirror steps

changedRanges.js

function changedRanges(history, group) {
  let ranges = []
  history.forEach((step, i) => {
    let map = step.posMap()
    ranges = ranges.map(range => {
      let from = map.map(range.from, 1), to = Math.max(from, map.map(range.to, -1))
      return {from, to}
    })
    if (group.indexOf(i) > -1) {
      let newRanges = []
      map.forEach((_f, _t, from, to) => newRanges.push({from, to}))
      ranges = joinRanges(ranges, newRanges)
    }
  })
  return ranges
}

function joinRanges(a, b) {
  let ranges = a.slice(), i = 0
  b.forEach(({from, to}) => {
    while (i < ranges.length && ranges[i].to < from) i++
    let end = i
    while (end < ranges.length && ranges[end].from <= to) {
      from = Math.min(ranges[end].from, from)
      to = Math.max(ranges[end].to, to)
      end++
    }
    ranges.splice(i, end - i, {from, to})
  })
  return ranges
}