WritableStream: WritableStream() constructor
Note: This feature is available in Web Workers.
The WritableStream()
constructor creates a new WritableStream
object instance.
Syntax
new WritableStream(underlyingSink)
new WritableStream(underlyingSink, queuingStrategy)
Parameters
underlyingSink
Optional-
An object containing methods and properties that define how the constructed stream instance will behave. The
controller
parameter passed to this object's methods is aWritableStreamDefaultController
that provides abort and error signaling.underlyingSink
can contain the following:start(controller)
Optional-
This is a method, called immediately when the object is constructed. The contents of this method are defined by the developer, and should aim to get access to the underlying sink. If this process is to be done asynchronously, it can return a promise to signal success or failure.
write(chunk, controller)
Optional-
This method, also defined by the developer, will be called when a new chunk of data (specified in the
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. This method will be called only after previous writes have succeeded, and never after the stream is closed or aborted (see below). close(controller)
Optional-
This method, also defined by the developer, will be called if the app signals that it has finished writing chunks to the stream. The contents should do whatever is necessary to finalize writes to the underlying sink, and release access to it. If this process is asynchronous, it can return a promise to signal success or failure. This method will be called only after all queued-up writes have succeeded.
abort(reason)
Optional-
This method, also defined by the developer, will be called if the app signals that it wishes to abruptly close the stream and put it in an errored state. It can clean up any held resources, much like
close()
, butabort()
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. Thereason
parameter contains a string describing why the stream was aborted.
queuingStrategy
Optional-
An object that optionally defines a queuing strategy for the stream. This takes two parameters:
highWaterMark
-
A non-negative integer — this defines the total number of chunks that can be contained in the internal queue before backpressure is applied.
size(chunk)
-
A method containing a parameter
chunk
— this indicates the size to use for each chunk, in bytes.
Note: You could define your own custom
queuingStrategy
, or use an instance ofByteLengthQueuingStrategy
orCountQueuingStrategy
for this object value. If noqueuingStrategy
is supplied, the default used is the same as aCountQueuingStrategy
with a high water mark of 1.
Return value
An instance of the WritableStream
object.
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.
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.
Backpressure
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.
-
The
highWaterMark
property, which is set when creating the counting strategy usingnew CountQueuingStrategy
, sets the maximum amount of data that theWritableStream
instance will handle in a singlewrite()
operation. In this example, it's the maximum amount of data that can be sent todefaultWriter.write()
, in thesendMessage
function. -
The
defaultWriter.ready
property returns a promise that resolves when the sink (the first property of theWritableStream
constructor) is done writing data. The data source can either write more data usingdefaultWriter.write()
or calldefaultWriter.close()
, as demonstrated in the example above. Callingclose()
too early can prevent data from being written. This is why the example callsdefaultWriter.ready
twice. - The
Promise
returned by the sink'swrite()
method tells theWritableStream
and its writer when to resolvedefaultWriter.ready
.
Specifications
Specification |
---|
Streams Standard # ref-for-ws-constructor④ |
Browser compatibility
BCD tables only load in the browser