SerialPort: close() method

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

Note: This feature is available in Dedicated Web Workers.

The SerialPort.close() method of the SerialPort interface returns a Promise that resolves when the port closes.

Description

close() closes the serial port if previously-locked SerialPort.readable and SerialPort.writable members are unlocked, meaning the releaseLock() methods have been called for their respective reader and writer.

However, when continuously reading data from a serial device using a loop, the associated readable stream will always be locked until the reader encounters an error. In this case, calling reader.cancel() will force reader.read() to resolve immediately with { value: undefined, done: true } allowing the loop to call reader.releaseLock().

// Without transform streams.

let keepReading = true;
let reader;

async function readUntilClosed() {
  while (port.readable && keepReading) {
    reader = port.readable.getReader();
    try {
      while (true) {
        const { value, done } = await reader.read();
        if (done) {
          // reader.cancel() has been called.
          break;
        }
        // value is a Uint8Array.
        console.log(value);
      }
    } catch (error) {
      // Handle error...
    } finally {
      // Allow the serial port to be closed later.
      reader.releaseLock();
    }
  }

  await port.close();
}

const closedPromise = readUntilClosed();

document.querySelector("button").addEventListener("click", async () => {
  // User clicked a button to close the serial port.
  keepReading = false;
  // Force reader.read() to resolve immediately and subsequently
  // call reader.releaseLock() in the loop example above.
  reader.cancel();
  await closedPromise;
});

Closing a serial port is more complicated when using transform streams. See Close a serial port for guidance.