indentable.js

function indentable(textarea, len = 2) {
  const tab = Array(len + 1).join(' ')
  textarea.addEventListener('keypress', e => {
    const pos = textarea.selectionStart
    const val = textarea.value
    if (e.keyCode === 9) { /* tab */
      e.preventDefault()
      if (e.shiftKey) {
        const firstNL = val.lastIndexOf('\n', pos - 1) + 1
        const start = val.slice(0, firstNL)
        let rmvLen
        const end = val
          .slice(firstNL)
          .replace(new RegExp(`^ {0,${len}}`), a => ((rmvLen = a.length), ''))
        textarea.value = start + end
        textarea.selectionEnd = (textarea.selectionStart = pos - rmvLen)
      } else {
        textarea.value = val.slice(0, pos) + tab + val.slice(pos)
        textarea.selectionEnd = (textarea.selectionStart = pos + len)
      }
    } else if (e.keyCode === 13) { /* enter */
      const pos = textarea.selectionStart - 1
      const val = textarea.value
      const firstNL = val.lastIndexOf('\n', pos) + 1
      const tab = /^ */.exec(val.slice(firstNL))[0]
      setTimeout(() => {
        const pos = textarea.selectionStart
        const val = textarea.value
        textarea.value = val.slice(0, pos) + tab + val.slice(pos)
        textarea.selectionEnd = (textarea.selectionStart = pos + tab.length)
      });
    }
  });
}