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ReadableStream: from() static method - Web APIs

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The ReadableStream.from() static method returns a ReadableStream from a provided iterable or async iterable object.

The method can be used to wrap iterable and async iterable objects as readable streams, including arrays, sets, arrays of promises, async generators, ReadableStreams, Node.js readable streams, and so on.

ReadableStream.from(anyIterable)

A ReadableStream.

TypeError

Thrown if the passed parameter is not an iterable or async iterable (does not define the [Symbol.iterator]() or [Symbol.asyncIterator]() method). Also thrown if, during iteration, the result of the next step is not an object or is a promise that does not resolve to an object.

This live example demonstrates how you can convert an async iterable to a ReadableStream, and then how this stream might be consumed.

The HTML is consists of single <pre> element, which is used for logging.

The example code creates a log() function to write to the log HTML element.

const logElement = document.getElementById("log");
function log(text) {
  logElement.innerText += `${text}\n`;
}

It then checks if the static method is supported, and if not, logs the result.

if (!ReadableStream.from) {
  log("ReadableStream.from() is not supported");
}

The async iterable is an anonymous generator function that yields the values of 1, 2 and 3 when it is called three times. This is passed to ReadableStream.from() to create the ReadableStream.

// Define an asynchronous iterator
const asyncIterator = (async function* () {
  yield 1;
  yield 2;
  yield 3;
})();

// Create ReadableStream from iterator
const myReadableStream = ReadableStream.from(asyncIterator);

Using readable streams demonstrates several ways to consume a stream. The code below uses a for ...await loop, as this method is the simplest. Each iteration of the loop logs the current chunk from the stream.

consumeStream(myReadableStream);

// Iterate a ReadableStream asynchronously
async function consumeStream(readableStream) {
  for await (const chunk of myReadableStream) {
    // Do something with each chunk
    // Here we just log the values
    log(`chunk: ${chunk}`);
  }
}

The output of consuming the stream is shown below (if ReadableStream.from() is supported).

This example demonstrates how you can convert an Array to a ReadableStream.

const logElement = document.getElementById("log");
function log(text) {
  logElement.innerText += `${text}\n`;
}

if (!ReadableStream.from) {
  log("ReadableStream.from() is not supported");
}

The iterable is just an array of strings that is passed to ReadableStream.from() to create the ReadableStream.

// An Array of vegetable names
const vegetables = ["Carrot", "Broccoli", "Tomato", "Spinach"];

// Create ReadableStream from the Array
const myReadableStream = ReadableStream.from(vegetables);

consumeStream(myReadableStream);

// Iterate a ReadableStream asynchronously
async function consumeStream(readableStream) {
  for await (const chunk of myReadableStream) {
    // Do something with each chunk
    // Here we just log the values
    log(`chunk: ${chunk}`);
  }
}

We use the same approach as in the previous example log and to consume the stream, so that is not shown here.

The output is shown below.

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