Web-based Compiler

To run the compiler from a web page, include the script stopify-full.bundle.js. Use the following command to get the path to this script on your local machine:

stopify-path stopify-full.bundle.js

This bundle exposes the following function:

stopify.stopifyLocally(url: string, copts?: CompileOpts, ropts?: RuntimeOpts): AsyncRun

Command-line Compiler

The Stopify CLI compiler requires the name of the input file and the name of the output file. In addition, the compiler has several optional flags:

stopify [compile-opts] input.js output.js

To load a compiled file in the browser, the Stopify runtime provides the following function:

stopify.stopify(url: string, opts?: RuntimeOpts): AsyncRun

Compiler as a Node Library

To use the Stopify compiler a Node library, first import the Stopify library:

const stopify = require('stopify');

This library exposes the following function:

stopify.stopify(url: string, copts?: CompileOpts): string

To load a compiled file in the browser, the Stopify runtime provides the following function:

stopify.stopify(url: string, opts?: RuntimeOpts): AsyncRun

Compiler Options

The Stopify compiler accepts the following options:

interface CompilerOpts {
  captureMethod?: "lazy" | "catch" | "retval" | "eager" | "original", // --transform from the CLI
  newMethod?: "wrapper" | "direct",                                   // --new from the CLI
  getters?: boolean,                                                  // --getters from the CLI
  debug?: boolean,                                                    // --debug from the CLI
  eval?: boolean,                                                     // --eval from the CLI
  eval2?: boolean,                                                     // --eval2 from the CLI
  es?: "sane" | "es5",                                                // --es from the CLI
  jsArgs?: "simple" | "faithful" | "full",                            // --js-args from the CLI

If an option is not set, Stopify picks a default value that is documented below. By default, Stopify is not completely faithful to the semantics of JavaScript (certain JavaScript features are difficult to support and incur a high runtime cost). Instead, Stopify’s default values work with a number of compilers that we’ve tested. By default, Stopify does not support getters, setters, eval, builtin higher-order functions, implicit operations, arguments-object aliasing, and single-stepping. If you think you may need these features, you will need to set their corresponding flags.

Transformation (.captureMethod)

Stopify uses first-class continuations as a primitive to implement its execution control features. Stopify can represent continuations in several ways; the fastest approach depends on the application and the browser. The valid options are "lazy", "catch", "retval", "eager", and "original". For most cases, we recommend using "lazy".

Constructor Encoding (.newMethod)

Stopify implements two mechanisms to support suspending execution within the dynamic extent of a constructor call.

  • "wrapper" desugars all new expressions to ordinary function calls, using Object.create.
  • "direct" preserves new expressions, but instruments all functions to check if they are invoked as constructors, using

The fastest approach depends on the browser. We recommend using wrapper.

Eval Support (.eval)

How should Stopify handle JavaScript’s eval function? By default, this flag is false and Stopify leaves eval unchanged. Since Stopify typically does not rename variables, using a stopfied program can use eval, but the evaluated code may lock-up the browser if it has an infinite loop.

If set to true, Stopify rewrites calls to JavaScript’s eval function to invoke the Stopify compiler. (Note: Stopify does not rewrite new Function and dynamically generated <script> tags.) This allows Stopify to control execution in dynamically generated code. Naturally, this requires the online compiler. However, the feature incurs considerable overhead.

Alternative Eval Support (.eval2)

The eval2 flag implements an alternative approach to supporting JavaScript’s eval function from within Stopified code. This flag is mutually exclusive with the eval compiler flag; only one of the two can be specified at compile-time.

If set to true, Stopify supports evaluating new code in the same global environment as the main program. This means that code executed by the eval function can refer to global variables and declare global variables that escape the scope of eval.

Implicit Operations (.es)

Stopify can suspend execution within user-written valueOf() and toString() methods that JavaScript invokes implicitly.

For example, the following program is an infinite loop in JavaScript:

var x = { toString: function() { while(true) { } } };
x + 1;

With the implicit operations flag is set to "es5", Stopify will be able to gracefully suspend the program above. With the flag set to "sane", Stopify will not be able to detect the the infinite loop. We have found that most source language compilers do not rely on implicit operations, thus it is usually safe to use "sane".

Fidelity of arguments (.jsArgs)

The arguments object makes it difficult for Stopify to resume execution after suspension. Stopify supports arguments in full, but it also supports two simple special cases that improve performance.

  • Use "simple" if the program (1) does not use arguments to access declared formal arguments and (2) only reads additional arguments using the arguments object.
  • Use "faithful" if the program (1) does not use arguments to access declared formal arguments and (2) may read or write additional arguments using the arguments object.
  • Use "full" for full support of JavaScript’s arguments object.

Getters and Setters (.getters)

Programs that suspend execution within getters/setters incur a lot of overhead with Stopify. The .getters flag has two possible values:

  • Use true to have Stopify instrument the program to support suspension within getters and setters.
  • Use false if the program does not use getters and setters.

Single-stepping and Breakpointing (.debug)

Set .debug to true to enable support for single-stepping and breakpointing. However, note that this requires more instrumentation and slows the program down further.