The WritableStreamDefaultWriter
interface of the Streams API is the object returned by WritableStream.getWriter()
and once created locks the writer to the WritableStream
ensuring that no other streams can write to the underlying sink.
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(() => {
return 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) => {
var buffer = new ArrayBuffer(1);
var view = new Uint8Array(buffer);
view[0] = chunk;
var decoded = decoder.decode(view, { stream: true });
var listItem = document.createElement('li');
listItem.textContent = "Chunk decoded: " + decoded;
list.appendChild(listItem);
result += decoded;
resolve();
});
},
close() {
var 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.