Synchronous JavaScript executor

synchronous, javascript, promise, async, await
npm install nsynjs@0.1.3


Build Status


Nsynjs is JavaScript execution engine + state machine that allows to write javascript code without callbacks, and execute it in synchronous manner.

Nsynjs has following unique features:

  • Pseudo-threads that are executed synchronously and that can be gracefully stopped,
  • Asynchronous new operator,
  • No compilation or transpilation required,
  • No need to mark functions with '*', async or await keywords,
  • Not dependant on promises.
  • Compatible with nodejs and all browsers, including Internet Explorer.

Nsynjs is written in ES2015 and does not require any dependencies.

It supports most of the features of ES2015, but with some limitations (see below).

How it works

It accepts function pointer as an input parameter, and performs following:

  • It parses code of input function, and builds a few internal structures, such as:

    • an internal tree structure of operators and expressions, that represents code of input function,
    • hash array with local variables names, that were defined in input function using var statement,
  • It "compiles" each operator and expression by

    • modifying source of operator by changing references to local variables
    • creating internal function that contain modified code
  • It creates execution context that contains local variables, execution stack with program counters, and other information, that is necessary in order to represent the latest state, and to stop and resume execution.

  • It executes structure of operators (code) against execution context (data).

With nsynjs you can write code like this:

    var i=0;
    while(i<5) {
        wait(1000); // <<-- long-running function with callback
        console.log(i, new Date());

Or like this:

    function getStats(userId) {
        var res;
        try {
            res = { // <<-- expression with few long-running functions, evaluated one after another
                friends: dbQuery("select * from firends where user_id = "+userId).data,
                comments: dbQuery("select * from comments where user_id = "+userId).data,
                likes: dbQuery("select * from likes where user_id = "+userId).data,
        catch(e) {
            res = {
                error: e
        return res;

How to start

Step 1. Get nsynjs

In Node.JS:

npm install nsynjs

var nsynjs = require('nsynjs');

Note: latest version of examples or documantation files are available on this GitHub repository. NPM package is updated only when changes are made to core library files.

In browser:

<script src="nsynjs.js"></script>

Step 2. Wrap all functions with callbacks into nsynjs-aware wrappers or promises

If all your asynchronous functions return promises, you can skip this step.

If some functions are returning results via callbacks, you can either promisify them, or wrap into nsynjs-aware wrapper.

Here is an example of wrapper to setTimeout function:

    var wait = function (ctx, ms) {
        setTimeout(function () {
            ctx.resume(); // <<-- resume execution of nsynjs pseudo-thread, referred by ctx
        }, ms);
    wait.nsynjsHasCallback = true; // <<-- indicates that nsynjs should stop and wait when calling this function

Another example - wrapper to jQuery's getJSON(), that can return data or throw an exception back to nsynjs-executed code:

    var ajaxGetJson = function (ctx,url) {
        var res = {}; // <<-- results will be posted back to nsynjs via method to this object
        var ex; // <<-- possible exception
        $.getJSON(url, function (data) {
   = data; // <<-- capture data from callback, or
        .fail(function(e) {
            ex = e; // <<-- capture exception
        .always(function() {
            ctx.resume(ex); // <<-- resume pseudo-thread
        return res;
    ajaxGetJson.nsynjsHasCallback = true; // <<-- indicates that nsynjs should stop and wait on evaluating this function
Q: Promises or nsynjs-aware wrappers?

Many async functions already have promisified versions, so you can just use them in your synchronous code.

However, if you want to be able to stop pseudo-threads gracefully and with proper cleaning, you should use nsynjs-aware wrappers, because they provide mechanism to terminate active underlying functions (e.g. terminate setTimeout timer with clearTimeout).

More on wrappers

Wrappers included with nsynjs

Step 3. Write your synchronous code

Put your synchronous code into function:

    function myTestFunction1() {
        var i=0;
        while(i<5) {
            wait(nsynjsCtx,1000); // <<-- reserved variable nsynjsCtx is a reference to current pseudo-thread
            console.log(res, new Date());
        return "myTestFunction1 finished";

If you use functions that return promises, you can just use them like this:

    var req = window.fetch(url).data;

In this example window.fetch() will return pending promise. Nsynjs will check if returned object is a promise, and if yes, it will wait until promise is resolved or rejected. Once promise resolves, it's value is assigned to 'data' propery of an object, that is returned to the caller. If promise rejects, it will trigger exception in the caller.

Promises can also be chained:

    var text = window.fetch(url).data.text().data;

Step 4. Execute it

Execute your function via nsynjs engine:,null, function (ret) {
    console.log('done all:', ret);

The result will look like this:

i=0 Sun Dec 25 2016 12:25:41 GMT-0700 (Mountain Standard Time)
i=1 Sun Dec 25 2016 12:25:42 GMT-0700 (Mountain Standard Time)
i=2 Sun Dec 25 2016 12:25:43 GMT-0700 (Mountain Standard Time)
i=3 Sun Dec 25 2016 12:25:44 GMT-0700 (Mountain Standard Time)
i=4 Sun Dec 25 2016 12:25:45 GMT-0700 (Mountain Standard Time)
done all: myTestFunction1 finished

nsynjs Reference

var ctx =,obj, param1, param2 [, param3 etc], callback) (function, to be called to execute function synchronously)


  • myTestFunction1: pointer to a function that needs to be executed synchronously
  • obj: some object that will be accessed via "this" in myTestFunction1 (could be null)
  • param1, param2, etc - any number of parameters
  • callback: some function to call once myTestFunction1 is finished.


  • pseudo-thread execution context

nsynjs.compile(myTestFunction1) (function, compiles myTestFunction1 and puts it into "nsynjsBin" propery of myTestFunction1)


  • myTestFunction1: pointer to a function that needs to be compiled


  • none

Pseudo-thread execution context reference

Pseudo-thread execution context is available inside nsynjs-executed code via predefined variable nsynjsCtx.

For use inside nsynjs-aware wrapper functions

ctx.resume ([exception])

Wrapper function should always call this to indicate that all callbacks are done, and that pseudo-thread may continue.

  • exception: optional exception to be thrown back to nsynjs-executed code

ctx.setDestructor (func)

Set destructor function, that will be called if pseudo-therad is terminated.

  • func: function that will do the cleanup (e.g. abort pending XHR request, or call to cleanTimeout)


This notifies nsynjs engine, that wrapper is not going to call slow function with callback, and that execution of the caller may be continued.

Supported JS features

  • var
  • if ... then ... [else...]
  • while
  • do ... while
  • for(;;)
  • for(var ;;)
  • for(.. in ..)
  • for(var .. in ..)
  • switch
  • break [label]
  • continue [label]
  • return
  • expr1 ? expr2 : expr3
  • try ... catch
  • throw
  • typeof
  • closures

Not supported

  • const
  • let
  • for ... of
  • arrow functions

Other limitations

  1. Operators that are executed via nsynjs should all be separated with semicolon.

  2. Nsynjs is not able to execute native functions with callbacks, such as, or Array.forEach(). But in many cases this can be done by running polyfills via nsynjs. Please see 'browser-array-map-polyfill.html' for an example.

Under the hood

When some function is executed via,...), nsynjs will check if someFunc.nsynjsBin property exists. This property holds tree-like structure that represents the code of someFunc, an is required for nsynjs to run. This parsing/compiling is done only once per function pointer.

Whe nsynjs parses code of function, it also parses all nested function definitions. These nested functions would have stub body but valid someFunc.nsynjsBin property, as they intended to fail if called directly. Instead, they should only be called from nsynjs-executed code.

When nsynjs executes code and encounters some function call, it checks what type of function is called. There could be 3 types:

  • function with someOtherFunc.nsynjsBin property defined: these functions executed in synchronous manner by nsynjs.
  • function without someOtherFunc.nsynjsBin property defined
    • With someOtherFunc.nsynjsHasCallback property defined: this means that someOtherFunc is nsynjs-aware wrapper, so nsynjs should stop and wait untill ctx.resume() is called by wrapper.
    • Without someOtherFunc.nsynjsHasCallback property defined: these functions are executed immediately.

Performance considerations

Nsynjs tries to optimize internal structure by packing as many elements as possible into each internal function. For example, consider following code:

    for(i=0; i<arr.length; i++) {
        res += arr[i];

Since it does not have any function calls in it, it will be packed into one internal function:

    this.execute = function(state) {
        for(state.localVars.i=0; state.localVars.i<arr.length; state.localVars.i++) {
            state.localVars.res += state.localVars.arr[state.localVars.i];

This function will be executed almost as fast as native code.

However, if some function is called inside this code, there is generally no way to find out type of that function at compile time, therefore nsynjs will evaluate such expressions one piece at a time.

For example, following code will not be optimized:

var n = Math.random()

It will be split into following internal functions:

this.execute = function(state) {
    return Math
this.execute = function(state,prev) {
    return prev.random
this.execute = function(state,prev) {
    return prev()
this.execute = function(state,prev, v) {
    return state.localVars.n = v