WritableStream: getWriter() method
The getWriter()
method of the WritableStream
interface returns a new instance of WritableStreamDefaultWriter
and locks the stream to that instance.
While the stream is locked, no other writer can be acquired until this one is released.
Syntax
js
getWriter()
Parameters
None.
Return value
A WritableStreamDefaultWriter
object instance.
Exceptions
TypeError
-
The stream you are trying to create a writer for is not a
WritableStream
.
Examples
The following example illustrates several features of this interface. It shows the
creation of the 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.
js
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, { 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);
});
});
// 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
Specification |
---|
Streams Standard # ref-for-ws-get-writer① |
Browser compatibility
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