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Fiber/coroutine support for v8 and node.

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NOTE OF OBSOLESCENCE -- The author of this project recommends you avoid its use if possible. The original version of this module targeted nodejs v0.1.x in early 2011 when JavaScript on the server looked a lot different. Since then async/await, Promises, and Generators were standardized and the ecosystem as a whole has moved in that direction.

I'll continue to support newer versions of nodejs as long as possible but v8 and nodejs are extraordinarily complex and dynamic platforms. It is inevitable that one day this library will abruptly stop working and no one will be able to do anything about it.

I'd like to say thank you to all the users of fibers, your support over the years has meant a lot to me.

Update [April 13th, 2021] -- Fibers is not compatible with nodejs v16.0.0 or later. Unfortunately, v8 commit dacc2fee0f is a breaking change and workarounds are non-trivial.

fibers(1) -- Fiber support for v8 and Node

npm version isc license travis build npm downloads

Fibers, sometimes called coroutines, are a powerful tool which expose an API to jump between multiple call stacks from within a single thread. This can be useful to make code written for a synchronous library play nicely in an asynchronous environment.



via npm

  • npm install fibers
  • You're done! (see "supported platforms" below if you run into errors)

from source

  • git clone git://
  • cd node-fibers
  • npm install

Note: node-fibers uses node-gyp for building. To manually invoke the build process, you can use

node-gyp rebuild
. This will put the compiled extension in
. However, when you do
, it will expect the module to be in, for example,
. You can manually put the module here every time you build, or you can use the included build script. Either
npm install
node build -f
will do this for you. If you are going to be hacking on node-fibers, it may be worthwhile to first do
node-gyp configure
and then for subsequent rebuilds you can just do
node-gyp build
which will be faster than a full
npm install
node-gyp rebuild

meteor users please read this

If you're trying to get meteor running and you ended up at this page you're probably doing something wrong. Please uninstall all versions of NodeJS and Meteor, then start over. See meteor#5124 for more information.

supported platforms

If you are running 64-bit nodejs version 12.x or 14.x on Linux, OS X, or Windows (7 or later) then you should be able to install fibers from npm just fine. If you are running nodejs v10.x then you will need to use

npm install [email protected]
. Older versions of nodejs will require older and older version of fibers.

(special thanks to Jeroen Janssen for his work on fibers in Windows)

If you do end up needing to compile fibers first make sure you have node-gyp installed as a global dependency (

npm install -g node-gyp
), and that you have setup your build environment by following the instructions at node-gyp. Ubuntu-flavored Linux users may need to run
sudo apt-get install g++
as well.


The examples below describe basic use of

, but note that it is not recommended to use
without an abstraction in between your code and fibers. See "FUTURES" below for additional information.


This is a quick example of how you can write sleep() with fibers. Note that while the sleep() call is blocking inside the fiber, node is able to handle other events.

$ cat sleep.js
var Fiber = require('fibers');

function sleep(ms) { var fiber = Fiber.current; setTimeout(function() {; }, ms); Fiber.yield(); }

Fiber(function() { console.log('wait... ' + new Date); sleep(1000); console.log('ok... ' + new Date); }).run(); console.log('back in main');

$ node sleep.js
wait... Fri Jan 21 2011 22:42:04 GMT+0900 (JST)
back in main
ok... Fri Jan 21 2011 22:42:05 GMT+0900 (JST)

Incremental Generator

Yielding execution will resume back in the fiber right where you left off. You can also pass values back and forth through yield() and run(). Again, the node event loop is never blocked while this script is running.

$ cat generator.js
var Fiber = require('fibers');

var inc = Fiber(function(start) { var total = start; while (true) { total += Fiber.yield(total); } });

for (var ii =; ii <= 10; ii = { console.log(ii); }

$ node generator.js

Fibonacci Generator

Expanding on the incremental generator above, we can create a generator which returns a new Fibonacci number with each invocation. You can compare this with the ECMAScript Harmony Generator Fibonacci example.

$ cat fibonacci.js
var Fiber = require('fibers');

// Generator function. Returns a function which returns incrementing // Fibonacci numbers with each call. function Fibonacci() { // Create a new fiber which yields sequential Fibonacci numbers var fiber = Fiber(function() { Fiber.yield(0); // F(0) -> 0 var prev = 0, curr = 1; while (true) { Fiber.yield(curr); var tmp = prev + curr; prev = curr; curr = tmp; } }); // Return a bound handle to run on this fiber return; }

// Initialize a new Fibonacci sequence and iterate up to 1597 var seq = Fibonacci(); for (var ii = seq(); ii <= 1597; ii = seq()) { console.log(ii); }

$ node fibonacci.js

Basic Exceptions

Fibers are exception-safe; exceptions will continue travelling through fiber boundaries:

$ cat error.js
var Fiber = require('fibers');

var fn = Fiber(function() { console.log('async work here...'); Fiber.yield(); console.log('still working...'); Fiber.yield(); console.log('just a little bit more...'); Fiber.yield(); throw new Error('oh crap!'); });

try { while (true) {; } } catch(e) { console.log('safely caught that error!'); console.log(e.stack); } console.log('done!');

$ node error.js
async work here...
still working...
just a little bit more...
safely caught that error!
Error: oh crap!
        at error.js:11:9


Using the

class without an abstraction in between your code and the raw API is not recommended.
is meant to implement the smallest amount of functionality in order make possible many different programming patterns. This makes the
class relatively lousy to work with directly, but extremely powerful when coupled with a decent abstraction. There is no right answer for which abstraction is right for you and your project. Included with
is an implementation of "futures" which is fiber-aware. Usage of this library is documented below. There are several other externally-maintained options which can be found on the wiki. You should feel encouraged to be creative with fibers and build a solution which works well with your project. For instance,
is not a good abstraction to use if you want to build a generator function (see Fibonacci example above).


to wrap existing node functions. At no point is the node event loop blocked:
$ cat ls.js
var Future = require('fibers/future');
var fs = Future.wrap(require('fs'));

Future.task(function() { // Get a list of files in the directory var fileNames = fs.readdirFuture('.').wait(); console.log('Found '+ fileNames.length+ ' files');

// Stat each file
var stats = [];
for (var ii = 0; ii &lt; fileNames.length; ++ii) {
} {

// Print file size
for (var ii = 0; ii &lt; fileNames.length; ++ii) {
    console.log(fileNames[ii]+ ': '+ stats[ii].get().size);


$ node ls.js 
Found 11 files
bin: 4096
fibers.js: 1708
.gitignore: 37 8664
future.js: 5833
.git: 4096
src: 4096
ls.js: 860
Makefile: 436
package.json: 684

The future API is designed to make it easy to move between classic callback-style code and fiber-aware waiting code:

$ cat sleep.js
var Future = require('fibers/future'), wait = Future.wait;

// This function returns a future which resolves after a timeout. This // demonstrates manually resolving futures. function sleep(ms) { var future = new Future; setTimeout(function() { future.return(); }, ms); return future; }

// You can create functions which automatically run in their own fiber and // return futures that resolve when the fiber returns (this probably sounds // confusing.. just play with it to understand). var calcTimerDelta = function(ms) { var start = new Date; sleep(ms).wait(); return new Date - start; }.future(); //

$ node sleep.js
Set timer for 2000ms, waited 2009ms


Fiber's definition looks something like this:

 * Instantiate a new Fiber. You may invoke this either as a function or as
 * a constructor; the behavior is the same.
 * When run() is called on this fiber for the first time, `fn` will be
 * invoked as the first frame on a new stack. Execution will continue on
 * this new stack until `fn` returns, or Fiber.yield() is called.
 * After the function returns the fiber is reset to original state and
 * may be restarted with another call to run().
function Fiber(fn) {
    [native code]


  • Fiber.current will contain the currently-running Fiber. It will be
  • undefined if there is no fiber (i.e. the main stack of execution).
  • See "Garbage Collection" for more information on responsible use of
  • Fiber.current.
  • / Fiber.current = undefined;


  • Fiber.yield() will halt execution of the current fiber and return control
  • back to original caller of run(). If an argument is supplied to yield(),
  • run() will return that value.
  • When run() is called again, yield() will return.
  • Note that this function is a global to allow for correct garbage
  • collection. This results in no loss of functionality because it is only
  • valid to yield from the currently running fiber anyway.
  • Note also that yield is a reserved word in Javascript. This is normally
  • not an issue, however some code linters may complain. Rest assured that it
  • will run fine now and in future versions of Javascript.
  • / Fiber.yield = function(param) { [native code] }


  • run() will start execution of this Fiber, or if it is currently yielding,
  • it will resume execution. If an argument is supplied, this argument will
  • be passed to the fiber, either as the first parameter to the main
  • function [if the fiber has not been started] or as the return value of
  • yield() [if the fiber is currently yielding].
  • This function will return either the parameter passed to yield(), or the
  • returned value from the fiber's main function.
  • / = function(param) { [native code] }


  • reset() will terminate a running Fiber and restore it to its original
  • state, as if it had returned execution.
  • This is accomplished by causing yield() to throw an exception, and any
  • futher calls to yield() will also throw an exception. This continues
  • until the fiber has completely unwound and returns.
  • If the fiber returns a value it will be returned by reset().
  • If the fiber is not running, reset() will have no effect.
  • / Fiber.prototype.reset = function() { [native code] }


  • throwInto() will cause a currently yielding fiber's yield() call to
  • throw instead of return gracefully. This can be useful for notifying a
  • fiber that you are no longer interested in its task, and that it should
  • give up.
  • Note that if the fiber does not handle the exception it will continue to
  • bubble up and throwInto() will throw the exception right back at you.
  • / Fiber.prototype.throwInto = function(exception) { [native code] }

Future's definition looks something like this:

  • Returns a future-function which, when run, starts running the target
  • function and returns a future for the result.
  • Example usage:
  • var funcy = function(arg) {
  • return arg+1;
  • }.future();
  • funcy(1).wait(); // returns 2
  • / Function.prototype.future = function() { ... }
  • /**

    • Future object, instantiated with the new operator.
    • / function Future() {}


    • Wrap a node-style async function to return a future in place of using a callback.
    • fn - the function or object to wrap
    • array - indicates that this callback will return more than 1 argument after err. For example,
    •     `child_process.exec()` returns [err, stdout, stderr]
    • suffix - appends a string to every method that was overridden, if you passed an object
    • Example usage: Future.wrap(asyncFunction)(arg1).wait()
    • / Future.wrap = function(fn, multi, suffix) { ... }


    • Invoke a function that will be run in its own fiber context and return a future to its return
    • value.
    • Example:
    • Future.task(function() {
    • // You can safely wait on stuff here
    • }).detach();
    • / Future.task = function(fn) { ... }


    • Wait on a series of futures and then return. If the futures throw an exception this function
    • /won't/ throw it back. You can get the value of the future by calling get() on it directly. If
    • you want to wait on a single future you're better off calling future.wait() on the instance.
    • Example usage: Future.wait(aFuture, anotherFuture)
    • / Future.wait = function(/* ... */) { ... }


    • Return the value of this future. If the future hasn't resolved yet this will throw an error.
    • / Future.prototype.get = function() { ... }


    • Mark this future as returned. All pending callbacks will be invoked immediately.
    • value - the value to return when get() or wait() is called.
    • Example usage: aFuture.return(value)
    • / Future.prototype.return = function(value) { ... }


    • Throw from this future as returned. All pending callbacks will be invoked immediately.
    • Note that execution will continue normally after running this method,
    • so make sure you exit appropriately after running throw()
    • error - the error to throw when get() or wait() is called.
    • Example usage: aFuture.throw(new Error("Something borked"))
    • / Future.prototype.throw = function(error) { ... }


    • "detach" this future. Basically this is useful if you want to run a task in a future, you
    • aren't interested in its return value, but if it throws you don't want the exception to be
    • lost. If this fiber throws, an exception will be thrown to the event loop and node will
    • probably fall down.
    • / Future.prototype.detach = function() { ... }


    • Returns whether or not this future has resolved yet.
    • / Future.prototype.isResolved = function() { ... }


    • Returns a node-style function which will mark this future as resolved when called.
    • Example usage:
    • var errback = aFuture.resolver();
    • asyncFunction(arg1, arg2, etc, errback)
    • var result = aFuture.wait();
    • / Future.prototype.resolver = function() { ... }


    • Waits for this future to resolve and then invokes a callback.
    • If only one argument is passed it is a standard function(err, val){} errback.
    • If two arguments are passed, the first argument is a future which will be thrown to in the case
    • of error, and the second is a function(val){} callback.
    • / Future.prototype.resolve = function(/* errback or future, callback */) { ... }


    • Propogate results to another future.
    • Example usage: future1.proxy(future2) // future2 gets automatically resolved with however future1 resolves
    • / Future.prototype.proxy = function(future) { ... }


    • Differs from its functional counterpart in that it actually resolves the future. Thus if the
    • future threw, future.wait() will throw.
    • / Future.prototype.wait = function() { ... }


    • Support for converting a Future to and from ES6 Promises.
    • / Future.fromPromise = function(promise) { ... } Future.prototype.promise = function() { ... }


    If you intend to build generators, iterators, or "lazy lists", you should be aware that all fibers must eventually unwind. This is implemented by causing yield() to throw unconditionally when the library is trying to unwind your fiber-- either because reset() was called, or all handles to the fiber were lost and v8 wants to delete it.

    Something like this will, at some point, cause an infinite loop in your application:

    var fiber = Fiber(function() {
        while (true) {
            try {
            } catch(e) {}

    If you either call reset() on this fiber, or the v8 garbage collector decides it is no longer in use, the fiber library will attempt to unwind the fiber by causing all calls to yield() to throw. However, if you catch these exceptions and continue anyway, an infinite loop will occur.

    There are other garbage collection issues that occur with misuse of fiber handles. If you grab a handle to a fiber from within itself, you should make sure that the fiber eventually unwinds. This application will leak memory:

    var fiber = Fiber(function() {
        var that = Fiber.current;
    fiber = undefined;

    There is no way to get back into the fiber that was started, however it's impossible for v8's garbage collector to detect this. With a handle to the fiber still outstanding, v8 will never garbage collect it and the stack will remain in memory until the application exits.

    Thus, you should take care when grabbing references to


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