ReadableStreamDefaultReader

The ReadableStreamDefaultReader interface provides a way to read data from a ReadableStream in a pull-based manner. It is based on the Web APIs standard ReadableStreamDefaultReader interface.

Note:

You cannot directly construct a ReadableStreamDefaultReader object. Use the ReadableStream.getReader method to obtain one：

const { readable, writable } = new TransformStream();
const reader = readable.getReader();

Properties

closed

• Type: Promise<void> (read-only)
• Description: A promise that resolves when the stream is closed and rejects if there is an error in the stream or the reader’s lock is released.

Methods

read()

reader.read(): Promise<{ value: any, done: boolean }>

Return Value: Returns a Promise containing the read data (chunk) and the reading status.

Description: Reads data from the stream.

Note: Calling the read() method to initiate another read operation before the previous read operation completes is not allowed.

The reader.read method returns a Promise that resolves with an object in the format of { value: theChunk, done: boolean }, where:

• If a chunk is available, the Promise will be fulfilled with done: false.
• If the stream is closed, the Promise will be fulfilled with value: undefined and done: true.
• If the stream encounters an error, the Promise will be rejected and contain the relevant error information.

cancel()

reader.cancel(reason?: string): Promise<void>

Parameters:

• reason (optional): A human-readable string indicating the reason for cancellation.

Description: Cancels the stream. The reason is passed to the underlying source’s cancel algorithm. If this readable stream is one side of a TransformStream, then its cancellation algorithm causes the transform’s writable side to become errored with the reason.

Warning: Any data not yet read will be lost.

releaseLock()

reader.releaseLock(): void;

Description: Releases the lock on the readable stream.

Note: A lock cannot be released if the reader has pending read operations. If this occurs, a TypeError is thrown, and the reader remains locked.

References

• Streams
• Readable streams in the WHATWG Streams API specification
• MDN ReadableStreamDefaultReader Documentation