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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The WritableStream()
constructor creates a new WritableStream
object instance.
var writableStream = new WritableStream(underlyingSink[, queuingStrategy]);
underlyingSink
can contain the following: controller
parameter passed to this method is a WritableStreamDefaultController
. This can be used by the developer to control the stream during set up.chunk
parameter) is ready to be written to the underlying sink. It can return a promise to signal success or failure of the write operation. The controller
parameter passed to this method is a WritableStreamDefaultController
that can be used by the developer to control the stream as more chunks are submitted for writing. This method will be called only after previous writes have succeeded, and never after the stream is closed or aborted (see below).controller
parameter passed to this method is a WritableStreamDefaultController
, which can be used to control the stream at the end of writing.close()
, but abort()
will be called even if writes are queued up — those chunks will be thrown away. If this process is asynchronous, it can return a promise to signal success or failure. The reason parameter contains a DOMString
describing why the stream was aborted.chunk
— this indicates the size to use for each chunk, in bytes.Note: You could define your own custom queuingStrategy
, or use an instance of ByteLengthQueuingStrategy
or CountQueuingStrategy
for this object value. If no queuingStrategy
is supplied, the default used is the same as a CountQueuingStrategy
with a high water mark of 1.
An instance of the WritableStream
object.
The following example illustrates several features of this interface. It shows the creation of the WritableStream
with a custom sink and an API-supplied queueing 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) { // 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(() => { return 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) => { var buffer = new ArrayBuffer(2); var view = new Uint16Array(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.
Because of how backpressure is supported in the API, its implementation in code may be less than obvious. To see how backpressure is implemented look for three things.
highWaterMark
property, which is set when creating the counting strategy (line 33), sets the maximum amount of data that the WritableStream
instance will handle in a single write()
operation. In this example, it's the maximum amount of data that can be sent to defaultWriter.write()
(line 9).writer.ready
property returns a promise that resolves when the sink (the first property of the WritableStream
constructor) is done writing data. The data source can wither write more data (line 9) or call close()
(line 21). Calling close() too early can prevent data from being written. This is why the example calls writer.ready
twice (lines 7 and 19). Promise
returned by the sink's write()
method (line 38) tells the WritableStream
and its writer when to resolve writer.ready
.Specification | Status | Comment |
---|---|---|
Streams The definition of 'WritableStream()' in that specification. | Living Standard | Initial definition. |
No compatibility data found. Please contribute data for "path.to.feature.NameOfTheConstructor" (depth: 1) to the MDN compatibility data repository.
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/WritableStream/WritableStream