node-awaitify-stream

Read or write to a stream using while and await, not event handlers.

• Read and write streams using familiar constructs, without resorting to synchronous methods for I/O.
• Read and write long streams without runaway memory usage.
• Process streams one chunk or line at a time. Await asynchronous operations inbetween.

Requirements

• Node v8+ recommended. async/await support was added to Node v7. Node v6 will throw "SyntaxError: Unexpected token function" for the examples below. See secondExample_without_await.js for an example of using awaitify-stream without await.

Install

npm install awaitify-stream

Reader functions

• readAsync([size]): Promise wrapper around readable.read. Returns a promise for the next chunk of data. Resolves to null at the end of the stream.

Writer functions

• writeAsync(chunk[, encoding]): Promise wrapper around writable.write. Returns a promise that resolves following a drain event (if necessary) and a call to write. Doesn't wait for the chunk to be flushed.

• endAsync([chunk][, encoding]): Promise wrapper around writable.end. Returns a promise that resolves when the stream is finished.

Reader/Writer API

Use createReader, createWriter or createDuplexer to create a wrapper around the stream. The stream property can be used to later access the stream.

const fs = require('fs');
const aw = require('awaitify-stream');

async function run() {
    let readStream = fs.createReadStream('firstExample.js');
    let reader = aw.createReader(readStream);
    let writer = aw.createWriter(process.stdout);

    // Read the file and write it to stdout.
    let chunk, count = 0;
    while (null !== (chunk = await reader.readAsync())) {
        // Perform any synchronous or asynchronous operation here.
        await writer.writeAsync(chunk);
        count++;
    }

    console.log(`\n\nDone. Read the file in ${count} chunk(s).`);
}

run();

Augment Stream API

Use addAsyncFunctions to add the reader and/or writer functions to a stream object. The reader functions are added if stream.readable is true. The writer functions are added if stream.writable is true.

const fs = require('fs');
const aw = require('awaitify-stream');
const lineLength = 6; // 5 digits in a zip code, plus the newline character.

function delay(ms) {
    return new Promise((resolve) => {
        setTimeout(resolve, ms);
    });
}

async function run() {
    let stream = aw.addAsyncFunctions(fs.createReadStream('zipCodes.lftxt'));
    stream.setEncoding('utf8');

    // Read and print zip codes, slowly.
    let zipCode;
    while (null !== (zipCode = await stream.readAsync(lineLength))) {
        // Remove the newline character. If you didn't set the encoding
        // above, use zipCode.toString().trim()
        zipCode = zipCode.trim();

        console.log(zipCode);
        await delay(200);
    }
}

run();

Line Reading

You can use awaitify-stream in combination with a package like byline to read a line at a time.

const fs = require('fs');
const aw = require('awaitify-stream');
const byline = require('byline');
const readline = require('readline'); // Used for prompting the user.

function checkGuess(rl, guess) {
    return new Promise((resolve) => {
        rl.question(`Is the ${guess} your card? [y/n] `, (answer) => {
            resolve(answer.startsWith('y'));
        });
    });
}

async function run() {
    let stream = fs.createReadStream('millionsOfGuesses.txt');
    stream.setEncoding('utf8');

    let lineStream = byline.createStream(stream, { keepEmptyLines: false });
    let reader = aw.createReader(lineStream);

    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout
    });

    try {
        let line;
        while (null !== (line = await reader.readAsync())) {
            let guessedCard = await checkGuess(rl, line);

            if (guessedCard) {
                console.log('Huzzah!');
                return;
            }
        }

        console.log('Drat!');
    }
    finally {
        rl.close();
    }
}

run();

Related Packages

• byline: Useful for reading streams line-by-line. I recommend using byline over node's builtin readline because you can pause the stream or await asynchronous operations inbetween each line.

• stream-consume-promise: Similar to this package, but returns an iterator-style response with value and done properties. Also see stream-produce-promise.

Notes

• The library has no dependencies. mocha and byline are required only for testing.

Credits

byline served as an example package as I was writing awaitify-stream, my first package.

davedoesdev contributed fixes.

mknj identified an issue with error handling.