WritableStreamDefaultController: signal property

The read-only signal property of the WritableStreamDefaultController interface returns the AbortSignal associated with the controller.

Value

An AbortSignal object.

Examples

Aborting a long write operation

In this example, we simulate a slow operation using a local sink: We do nothing when some data is written but to wait for a second. This gives us enough time to call the writer.abort() method and to immediately reject the promise.

const writingStream = new WritableStream({
  // Define the slow local sink to simulate a long operation
  write(controller) {
    return new Promise((resolve, reject) => {
      controller.signal.addEventListener("abort", () =>
        reject(controller.signal.reason)
      );

      // Do nothing but wait with the data: it is a local sink
      setTimeout(resolve, 1000); // Timeout to simulate a slow operation
    });
  },
});

// Perform the write
const writer = writingStream.getWriter();
writer.write("Lorem ipsum test data");

// Abort the write manually
await writer.abort("Manual abort!");

Transferring the AbortSignal to the underlying layer

In this example, we use the Fetch API to actually send the message to a server. The Fetch API also support AbortSignal: It is possible to use the same object for both the fetch method and the WritableStreamDefaultController.

const endpoint = "https://www.example.com/api"; // Fake URL for example purpose
const writingStream = new WritableStream({
  async write(controller, chunk) {
    // Write to the server using the Fetch API
    const response = await fetch(endpoint, {
      signal: controller.signal, // We use the same object for both fetch and controller
      method: "POST",
      body: chunk,
    });
    await response.text();
  },
});

// Perform the write
const writer = writingStream.getWriter();
writer.write("Lorem ipsum test data");

// Abort the write manually
await writer.abort("Manual abort!");

Specifications

Specification
Streams Standard
# ref-for-ws-default-controller-signal①

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