The close()
method of the WritableStreamDefaultWriter
interface closes the associated writable stream.
The underlying sink will finish processing any previously-written chunks, before invoking the close behavior. During this time any further attempts to write will fail (without erroring the stream).
A Promise
, which fulfills with the undefined
if all remaining chunks were successfully written before the close, or rejects with an error if a problem was encountered during the process.
The following example shows the creation of a WritableStream
with a custom sink and an API-supplied queuing strategy. It then calls a function called sendMessage()
, passing the newly created stream and a string. Inside this function it calls the stream's getWriter()
method, which returns an instance of WritableStreamDefaultWriter
. A forEach()
call is used to write each chunk of the string to the stream. Finally, write()
and close()
return promises that are processed to deal with success or failure of chunks and streams.
const list = document.querySelector("ul");
function sendMessage(message, writableStream) {
const defaultWriter = writableStream.getWriter();
const encoder = new TextEncoder();
const encoded = encoder.encode(message, { stream: true });
encoded.forEach((chunk) => {
defaultWriter.ready
.then(() => {
defaultWriter.write(chunk);
})
.then(() => {
console.log("Chunk written to sink.");
})
.catch((err) => {
console.log("Chunk error:", err);
});
});
defaultWriter.ready
.then(() => {
defaultWriter.close();
})
.then(() => {
console.log("All chunks written");
})
.catch((err) => {
console.log("Stream error:", err);
});
}
const decoder = new TextDecoder("utf-8");
const queuingStrategy = new CountQueuingStrategy({ highWaterMark: 1 });
let result = "";
const writableStream = new WritableStream(
{
write(chunk) {
return new Promise((resolve, reject) => {
const buffer = new ArrayBuffer(1);
const view = new Uint8Array(buffer);
view[0] = chunk;
const decoded = decoder.decode(view, { stream: true });
const listItem = document.createElement("li");
listItem.textContent = `Chunk decoded: ${decoded}`;
list.appendChild(listItem);
result += decoded;
resolve();
});
},
close() {
const listItem = document.createElement("li");
listItem.textContent = `[MESSAGE RECEIVED] ${result}`;
list.appendChild(listItem);
},
abort(err) {
console.log("Sink error:", err);
},
},
queuingStrategy,
);
sendMessage("Hello, world.", writableStream);
You can find the full code in our Simple writer example.