ReadableStreamDefaultReader.read()
The read()
method of the
ReadableStreamDefaultReader
interface returns a Promise
providing access
to the next chunk in the stream's internal queue.
Syntax
var promise = readableStreamDefaultReader.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
Example 1 - simple example
This example shows the basic API usage, but doesn't try to deal with complications like stream chunks not ending on line boundaries for example.
In this example stream
is a previously-created custom
ReadableStream
. It 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 as an array of UTF-8 bytes, 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);
});
}
Example 2 - handling text line by line
This example shows how you might fetch a text file and handle it as a stream of text lines. It deals with stream chunks not ending on line boundaries and converting from Uint8Array to strings.
async function* makeTextFileLineIterator(fileURL) {
const utf8Decoder = new TextDecoder("utf-8");
let response = await fetch(fileURL);
let reader = response.body.getReader();
let {value: chunk, done: readerDone} = await reader.read();
chunk = chunk ? utf8Decoder.decode(chunk, {stream: true}) : "";
let re = /\r\n|\n|\r/gm;
let startIndex = 0;
for (;;) {
let result = re.exec(chunk);
if (!result) {
if (readerDone) {
break;
}
let remainder = chunk.substr(startIndex);
({value: chunk, done: readerDone} = await reader.read());
chunk = remainder + (chunk ? utf8Decoder.decode(chunk, {stream: true}) : "");
startIndex = re.lastIndex = 0;
continue;
}
yield chunk.substring(startIndex, result.index);
startIndex = re.lastIndex;
}
if (startIndex < chunk.length) {
// last line didn't end in a newline char
yield chunk.substr(startIndex);
}
}
for await (let line of makeTextFileLineIterator(urlOfFile)) {
processLine(line);
}
Specifications
Specification |
---|
Streams Standard # ref-for-default-reader-read① |
Browser compatibility
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