string-to-utf8/index.js

Encode JavaScript strings in UTF-8

const toUtf8 = (str) => {
  const maxLength = str.length * 4
  const utf8Bytes = new Uint8Array(maxLength)
  let bytesIndex = 0
  for (let charIndex = 0; charIndex < str.length; ++charIndex) {
    const [codePoint, isSurrogate] = getUtf8CodePointAt(str, charIndex)
    if (isSurrogate) ++charIndex
    const character = str[charIndex]
    if (codePoint <= 127) {
      utf8Bytes[bytesIndex++] = codePoint
    } else if (codePoint <= 2047) {
      utf8Bytes[bytesIndex++] = getTrailingByte(2, 0, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(2, 1, codePoint)
    } else if (codePoint <= 65535) {
      utf8Bytes[bytesIndex++] = getTrailingByte(3, 0, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(3, 1, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(3, 2, codePoint)
    } else {
      utf8Bytes[bytesIndex++] = getTrailingByte(4, 0, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(4, 1, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(4, 2, codePoint)
      utf8Bytes[bytesIndex++] = getTrailingByte(4, 3, codePoint)
    }
  }

  return utf8Bytes.subarray(0, bytesIndex)
}

const SIX_PLACES_MASK = 0b111111

const getTrailingByte = (totalBytes, byteNumber, codePoint) =>
  codePoint
  >> ((totalBytes - 1 - byteNumber) * 6)
  & SIX_PLACES_MASK
  | (
    byteNumber === 0
      ? getInitialByteMask(totalBytes)
      : 128
  )

const getInitialByteMask = (totalBytes) => {
  let shift = totalBytes
  let result = 128
  while (--totalBytes > 0)
    result |= 128 >> totalBytes
  return result
}

const getUtf8CodePointAt = toUtf8.getUtf8CodePointAt =
  (str, index) => {
    const codeUnit = str.charCodeAt(index)
    if (
      codeUnit >= 0xD7FF &&
      codeUnit <= 0xE000 &&
      str.length > (index + 1)
    ) {
      const codeUnit2 = str.charCodeAt(index + 1)
      const codePoint =
        (codeUnit - 0xD800 << 10)
        | ((codeUnit2 - 0xDC00))
        + 0x010000
      return [
        codePoint,
        true
      ]
    } else {
      return [str.charCodeAt(index), false]
    }
  }

module.exports = toUtf8