Function Transform

lib/internal/streams/transform.js:82–124

(options)

const kCallback = Symbol('kCallback');

function Transform(options) {
  if (!(this instanceof Transform))
    return new Transform(options);

  // TODO (ronag): This should preferably always be
  // applied but would be semver-major. Or even better;
  // make Transform a Readable with the Writable interface.
  const readableHighWaterMark = options ? getHighWaterMark(this, options, 'readableHighWaterMark', true) : null;
  if (readableHighWaterMark === 0) {
    // A Duplex will buffer both on the writable and readable side while
    // a Transform just wants to buffer hwm number of elements. To avoid
    // buffering twice we disable buffering on the writable side.
    options = {
      ...options,
      highWaterMark: null,
      readableHighWaterMark,
      writableHighWaterMark: options.writableHighWaterMark || 0,
    };
  }

  Duplex.call(this, options);

  // We have implemented the _read method, and done the other things
  // that Readable wants before the first _read call, so unset the
  // sync guard flag.
  this._readableState.sync = false;

  this[kCallback] = null;

  if (options) {
    if (typeof options.transform === 'function')
      this._transform = options.transform;

    if (typeof options.flush === 'function')
      this._flush = options.flush;
  }

  // When the writable side finishes, then flush out anything remaining.
  // Backwards compat. Some Transform streams incorrectly implement _final
  // instead of or in addition to _flush. By using 'prefinish' instead of
  // implementing _final we continue supporting this unfortunate use case.
  this.on('prefinish', prefinish);
}

function final(cb) {
  if (typeof this._flush === 'function' && !this.destroyed) {