script.js

var editor = document.getElementById("editor")

function columnWidth(rows, columnIndex) {
  return Math.max.apply(null, rows.map(function(row) {
    return ('' + row[columnIndex]).length
  }))
}

function looksLikeTable(data) {
  return true
}

editor.addEventListener("paste", function(event) {
  var clipboard = event.clipboardData
  var data = clipboard.getData('text/plain').trim()

  if(looksLikeTable(data)) {
    event.preventDefault()
  }else{
    return
  }

  var rows = data.split((/[\u0085\u2028\u2029]|\r\n?/g)).map(function(row) {
    row = row.replace('\n', ' ')
    return row.split("\t")
  })

  var colAlignments = []

  var columnWidths = rows[0].map(function(column, columnIndex) {
    var alignment = "l"
    var re = /^(\^[lcr])/i
    var m = column.match(re)
    if (m) {
        var align = m[1][1].toLowerCase()
        if (align === "c") {
          alignment = "c"
        } else if (align === "r") {
          alignment = "r"
        }
      }
    colAlignments.push(alignment)
    column = column.replace(re, "")
    rows[0][columnIndex] = column
    return columnWidth(rows, columnIndex)
  })
  var markdownRows = rows.map(function(row, rowIndex) {
    // | Name         | Title | Email Address  |
    // |--------------|-------|----------------|
    // | Jane Atler   | CEO   | jane@acme.com  |
    // | John Doherty | CTO   | john@acme.com  |
    // | Sally Smith  | CFO   | sally@acme.com |
    return "| " + row.map(function(column, index) {
      return column + Array(columnWidths[index] - column.length + 1).join(" ")
    }).join(" | ") + " |"
    row.map

  })
  markdownRows.splice(1, 0, "|" + columnWidths.map(function(width, index) {
    var prefix = ""
    var postfix = ""
    var adjust = 0
    var alignment = colAlignments[index]
    if (alignment === "r") {
      postfix = ":"
      adjust = 1
    } else if (alignment == "c") {
      prefix = ":"
      postfix = ":"
      adjust = 2
    }
    return prefix + Array(columnWidths[index] + 3 - adjust).join("-") + postfix
  }).join("|") + "|")

  // https://www.w3.org/TR/clipboard-apis/#the-paste-action
  // When pasting, the drag data store mode flag is read-only, hence calling
  // setData() from a paste event handler will not modify the data that is
  // inserted, and not modify the data on the clipboard.

  event.target.value = markdownRows.join("\n")
  return false
})