SerialPort: close() method

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

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

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().

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

Examples

Closing a port after reading once the stream is done

The following example shows how to close a port after continuously reading data from it, once the stream is done. A keepReading flag controls when to stop reading. A button click sets keepReading to false and cancels the reader, which causes reader.read() to resolve immediately so the loop can release the lock and close() can be called.

// 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;
});