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async.js

Ryan Roemer (@ryan_roemer)

Node.DC - Feb. 20, 2012

Does this look familiar?

var fs = require('fs'),
  path = require('path'),
  file1 = "file1.txt",
  file2 = "file2.txt";

// Check if file1 exists, write to file2,
// then display new file2 contents.
path.exists(file1, function (exists) {
  if (!exists) throw new Error("No file!");

  fs.readFile(file1, function (err, data) {
    if (err) throw err;

    fs.writeFile(file2, data, function (err) {
      if (err) throw err;

      fs.readFile(file2, function (err, data) {
        if (err) throw err;
        console.log(data.toString());
      });
    });
  });
});

fun1(params, function (err, results) {
  fun2(params, function (err, results) {
    fun3(params, function (err, results) {
      fun4(params, function (err, results) {
        fun5(params, function (err, results) {
          fun6(params, function (err, results) {
            fun7(params, function (err, results) {
              fun8(params, function (err, results) {
                fun9(params, function (err, results) {
                  fun10(params, function (err, results) {
                    // Do something interesting here.
                  });
                });
              });
            });
          });
        });
      });
    });
  });
});

Isn't this a bit frustrating and complicated?

async.js

A control-flow library for
Node.js and the browser.

github.com/caolan/async

Serial Control Flow

series
iterator
waterfall

var readData; // Chained data object.

// Straight series.
async.series([
  function (cb) {
    path.exists(file1, function (exists) {
      cb(exists ? null : new Error("No file!"));
    });
  },
  function (cb) {
    fs.readFile(file1, function (err, data) {
      readData = data;
      cb(err);
    });
  },
  function (cb) {
    fs.writeFile(file2, readData, cb);
  },
  function (cb) {
    fs.readFile(file2, function (err, data) {
      if (!err) console.log(data.toString());
      cb(err);
    });
  }
], function (err) {
  if (err) throw err;
});

Pitfall: Out-of-order execution
and chained objects.

// Now with waterfall...
async.waterfall([
  function (cb) {
    path.exists(file1, function (exists) {
      cb(exists ? null : new Error("No file!"));
    });
  },
  function (cb) {
    fs.readFile(file1, cb); // Pass cb directly.
  },
  function (data, cb) { // "data" now first arg.
    fs.writeFile(file2, data, cb);
  },
  function (cb) {
    fs.readFile(file2, function (err, data) {
      if (!err) console.log(data.toString());
      cb(err);
    });
  }
], function (err) {
  if (err) throw err;
});

Parallel Control Flow

parallel
queue

// Parallel.
async.parallel({
  read1: function (cb) {
    fs.readFile(file1, cb);
  },

  read2: function (cb) {
    fs.readFile(file2, cb);
  }
}, function (err, results) {
  if (err) throw err;

  // Our results object with named members.
  console.log(results.read1.toString() +
              results.read2.toString());
});

Collections

forEach | forEachSeries
map | mapSeries
filter | filterSeries
reduce | reduceRight
concat | concatSeries

function readPrint(file, callback) {
  fs.readFile(file, function (err, data) {
    console.log(data.toString());
    callback(err, file);
  });
}

// Collection "forEach" iteration.
async.forEach([file1, file2], readPrint, function (err) {
  if (err) throw err;
});

// Mapped collection (also aggregates returned result).
async.map([file1, file2], readPrint, function (err, results) {
  if (err) throw err;
  console.log("File names: " + results);
});

Everything Together

auto: dependency graphs!

// Read two files and write to third.
async.auto({
  read1:  function (cb) {
    fs.readFile(file1, cb);
  },

  read2:  function (cb) {
    fs.readFile(file2, cb);
  },

  write3: ['read1', 'read2', function (cb, results) {
    var data = results.read1.toString() +
               results.read2.toString();
    fs.writeFile(file3, data, cb);
  }],

  read3:  ['write3', function (cb) {
    fs.readFile(file3, cb);
  }]
}, function (err, results) {
  if (err) throw err;
  console.log(results.read3.toString());
});

Thanks!

slides - bit.ly/nodedc-async
async.js - github.com/caolan/async

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