Draft
This page is not complete.
This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The read()
method of the ReadableStreamDefaultReader
interface returns a promise providing access to the next chunk in the stream's internal queue.
Syntax
Promise<result> readableStreamDefaultReaderInstance.read();
Parameters
None.
Return value
A Promise
, which fulfills/rejects with a result depending on the state of the stream. The different possibilities are as follows:
- If a chunk is available, the promise will be fulfilled with an object of the form
{ value: theChunk, done: false }
. - If the stream becomes closed, the promise will be fulfilled with an object of the form
{ value: undefined, done: true }
. - If the stream becomes errored, the promise will be rejected with the relevant error.
Exceptions
- TypeError
- The source object is not a
ReadableStreamDefaultReader
, or the stream has no owner.
Examples
In the following simple example, a previously-created custom ReadableStream
is read using a ReadableStreamDefaultReader
created using getReader()
. (see our Simple random stream example for the full code). Each chunk is read sequentially and output to the UI, until the stream has finished being read, at which point we return out of the recursive function and print the entire stream to another part of the UI.
function fetchStream() { const reader = stream.getReader(); let charsReceived = 0; // read() returns a promise that resolves // when a value has been received reader.read().then(function processText({ done, value }) { // Result objects contain two properties: // done - true if the stream has already given you all its data. // value - some data. Always undefined when done is true. if (done) { console.log("Stream complete"); para.textContent = result; return; } // value for fetch streams is a Uint8Array charsReceived += value.length; const chunk = value; let listItem = document.createElement('li'); listItem.textContent = 'Received ' + charsReceived + ' characters so far. Current chunk = ' + chunk; list2.appendChild(listItem); result += chunk; // Read some more, and call this function again return reader.read().then(processText); }); }
Specifications
Specification | Status | Comment |
---|---|---|
Streams The definition of 'read()' in that specification. |
Living Standard | Initial definition. |
Browser compatibility
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
No compatibility data found. Please contribute data for "path.to.feature.NameOfTheProperty" (depth: 1) to the MDN compatibility data repository.