ReadableStream: locked property

The locked read-only property of the ReadableStream interface returns whether or not the readable stream is locked to a reader.

A readable stream can have at most one active reader at a time, and is locked to that reader until it is released. A reader might be obtained using ReadableStream.getReader() and released using the reader's releaseLock() method.

const stream = new ReadableStream({
  // ...
});

const reader = stream.getReader();

stream.locked;
// should return true, as the stream has been locked to a reader