The getReader()
method of the
ReadableStream
interface creates a reader and locks the stream to it.
While the stream is locked, no other reader can be acquired until this one is released.
Syntax
var reader = readableStream.getReader({mode});
Parameters
- {mode} Optional
- An object containing a property
mode
, specifying the type of reader to create. Values can be:"byob"
, which results in aReadableStreamBYOBReader
being created that can read readable byte streams (i.e. can handle "bring your own buffer" reading).undefined
(or not specified at all — this is the default), which results in aReadableStreamDefaultReader
being created that can read individual chunks from a stream.
Return value
A ReadableStreamDefaultReader
or ReadableStreamBYOBReader
object instance, depending on the mode
value.
Exceptions
- RangeError
- The provided mode value is not
"byob"
orundefined
. - TypeError
- The stream you are trying to create a reader for is not a
ReadableStream
.
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 = value;
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 'getReader()' in that specification. |
Living Standard | Initial definition. |
Browser compatibility
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