An implementation of continuations in JavaScript
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Latest commit 086322b Sep 27, 2016 @jlongster committed on GitHub Update


An implementation of continuations in JavaScript. Includes built-in support for breakpoints (implemented with continuations) and setting breakpoints on running scripts.

See this post for a deeper explanation and interactive tutorials.

This implements the paper "Exceptional Continuations in JavaScript". It started as a fork of regenerator from January 2014, so the code is outdated. However, it is useful for demos and exploring interesting patterns.

Do not build actual software with this. Not only is it an old regenerator fork, but my work on top of it is hacky. There are no tests, as I was figuring out what was even possible. You will likely hit bugs when trying to write non-trivial code against this.

With that said, fixing those bugs is usually straight-forward. Each expression needs to mark itself correcly in the state machine. Usually this is a matter of changing 1 or 2 lines of code.

There is little ES6 support, but that could be fixed by first transforming code with Babel.

Getting Started

The simplest way is to write some code in a file called program.js and compile it with ./bin/compile program.js. A file called a.out will be generated, or you can specify an output file as the second argument.

$ ./bin/compile program.js <output-file>
$ node <output-file>

There is also a browser editor included in the browser directory. Open browser/index.html to run it, and you will be able to interactively write code and set breakpoints.

Continuation API

Use callCC to capture the current continuation. It will be given to you as a function that never returns.

function foo() {
  var cont = callCC(cont => cont);
  if(typeof cont === "function") {
  return cont;

console.log(foo()); // -> 5

See this post for more interesting examples, including resumable exceptions.

Machine API

At the bottom of the generated file, you will see where the program is run by the virtual machine. This virtual machine does not interpret the code; the code is real native JavaScript. All the virtual machine does is check the behavior of the code and handle runtime information of continuations (such as frames).

Some useful methods of the VM:

  • toggleBreakpoint(line) - set/remove a breakpoint
  • continue() - resume execution
  • step() - step to the next expression
  • getTopFrame() - if paused, get the top frame
  • abort() - stop executing and clear out all state

Events (subscribe to events with vm.on):

  • paused - fired when the code stops (breakpoint, stepped, etc)
  • error - fired when an uncaught error occurs
  • resumed - fired when the code resumed from being paused
  • finish - fired when the code completes
  • cont-invoked - fired when a continuation is invoked


I have turned off issues because I know there are many bugs in here and I do not have time to triage them. However, I welcome PRs that have a clear bugfix or purpose.

Some things I would like to see:

  • Minor bugfixes and general stability improvements

  • Clean up lib/visit.js and break up the large functions

  • Remove so much manual AST construction. It would be great to give something a string of code and generate the AST I need automatically, but without having to parse the same code each time.

  • Introduce two compiler modes: debugger and continuations. If we don't support the "debugger" mode which allows live breakpoints, we can do further optimizations and don't need to convert every single expression into the state machine. But I want this project to continue to support breakpoints, so it would be nice if we could have different compiler modes (or maybe optimization levels?)

  • Tests. Oh god help me, there are no tests.

Some things I am going to reject:

  • Major refactorings without any discussion beforehand. I don't have time to go through it.