files/en-us/web/api/writablestreamdefaultwriter/write/index.md
{{APIRef("Streams")}}{{AvailableInWorkers}}
The write() method of the
{{domxref("WritableStreamDefaultWriter")}} interface writes a passed chunk of data to a
{{domxref("WritableStream")}} and its underlying sink, then returns a
{{jsxref("Promise")}} that resolves to indicate the success or failure of the write
operation.
Note that what "success" means is up to the underlying sink; it might indicate that the chunk has been accepted, and not necessarily that it is safely saved to its ultimate destination.
write(chunk)
chunk
WritableStream.A {{jsxref("Promise")}}, which fulfills with the undefined upon a
successful write, or rejects if the write fails or stream becomes errored before the
writing process is initiated.
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 {{domxref("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) {
// defaultWriter is of type WritableStreamDefaultWriter
const defaultWriter = writableStream.getWriter();
const encoder = new TextEncoder();
const encoded = encoder.encode(message);
encoded.forEach((chunk) => {
defaultWriter.ready
.then(() => defaultWriter.write(chunk))
.then(() => {
console.log("Chunk written to sink.");
})
.catch((err) => {
console.log("Chunk error:", err);
});
});
// Call ready again to ensure that all chunks are written
// before closing the writer.
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(
{
// Implement the sink
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.
{{Specifications}}
{{Compat}}