index.js

var toString = Object.prototype.toString

var isModern = (
  typeof Buffer.alloc === 'function' &&
  typeof Buffer.allocUnsafe === 'function' &&
  typeof Buffer.from === 'function'
)

function isArrayBuffer (input) {
  return toString.call(input).slice(8, -1) === 'ArrayBuffer'
}

function fromArrayBuffer (obj, byteOffset, length) {
  byteOffset >>>= 0

  var maxLength = obj.byteLength - byteOffset

  if (maxLength < 0) {
    throw new RangeError("'offset' is out of bounds")
  }

  if (length === undefined) {
    length = maxLength
  } else {
    length >>>= 0

    if (length > maxLength) {
      throw new RangeError("'length' is out of bounds")
    }
  }

  return isModern
    ? Buffer.from(obj.slice(byteOffset, byteOffset + length))
    : new Buffer(new Uint8Array(obj.slice(byteOffset, byteOffset + length)))
}

function fromString (string, encoding) {
  if (typeof encoding !== 'string' || encoding === '') {
    encoding = 'utf8'
  }

  if (!Buffer.isEncoding(encoding)) {
    throw new TypeError('"encoding" must be a valid string encoding')
  }

  return isModern
    ? Buffer.from(string, encoding)
    : new Buffer(string, encoding)
}

function bufferFrom (value, encodingOrOffset, length) {
  if (typeof value === 'number') {
    throw new TypeError('"value" argument must not be a number')
  }

  if (isArrayBuffer(value)) {
    return fromArrayBuffer(value, encodingOrOffset, length)
  }

  if (typeof value === 'string') {
    return fromString(value, encodingOrOffset)
  }

  return isModern
    ? Buffer.from(value)
    : new Buffer(value)
}

module.exports = bufferFrom