Need private packages and team management tools?Check out npm Orgs. »


1.6.0 • Public • Published

Javascript Code Runner npm version

exec.js (922 bytes) is a low latency javascript code runner that enables to isolate and abort javascript code execution, including setTimeout/setInterval, promises and Fetch requests. It supports all browsers.

The code is executed in an isolated container with fine grained restriction capabilities, access to DOM and the ability to return functions and objects without serialization or cloning.

For some workloads, the javascript performance appears to be better than a WebWorker in modern browsers, without blocking UI (see tests). The startup latency can be reduced to <1ms compared to ~100ms for a WebWorker (2018).

In future versions of Chrome exec.js may be executed in a separate thread (multi-threading), offering an advantage over Web Workers. For more information, see Out-of-Process iframes (OOPIFs).

Table of contents


with npm:

npm install exec.js

with bower:

bower install exec.js


Include exec.js in the HTML document.

<script src="exec.min.js"></script>

Use var runner = new exec(code[, onmessage][, sandbox][, csp]); to execute javascript code in an isolated container. You can provide the code as a string or as a function.

// start code runner with onmessage callback
var runner = new exec('setInterval(function() {console.log("startup code")},200);onmessage=function(data){console.log("UI sent",data);};', 
    function onmessage(data) {'response from container:', data);
// start code runner with security isolation
// var runner = new exec('console.log("secured code");',null,['allow-pointer-lock'],{"default-src": "","script-src":"'sha256-076c8f1ca6979ef156b510a121b69b6265011597557ca2971db5ad5a2743545f'"});
// execute code in container
runner.exec('console.log("some code");');
// redefine onmessage callback
runner.on(function message(data) {'response from container (redefined)',data);
// post data to container'some data');
// execute code in container
runner.exec(function(postMessage) {
    // reconfigure message handler
    onmessage = function(data) {
        postMessage("received " + data + " in container");
// execute code in container
runner.exec(function(postMessage) {
    // reconfigure message handler to process functions
    onmessage = function(fn) {
        fn(); // function passed from UI
        // pass a function back to UI
        postMessage(function(x,y,z) { /* ... */ });
// post function to container { console.log('test OK'); });
// stop code execution
runner.stop(); // this will abruptly stop any code execution including unfinished promises
// chain
new exec('onmessage=function(data){console.log(data);}',null,['allow-pointer-lock'])
    .post('test 1')
    .post('test 2')
    .on(function message(data) {'response from container:',data);
    .exec('console.log("test 3");')
    .exec(function(postMessage){postMessage("post to UI");})

You can return data from your code using the postMessage(data) function. You can receive data in the container by defining onmessage=function(e) { // } in the container.

Simple Fetch request

var runner = new exec(function(postMessage) {
        .catch(function(err) {
}, function(response) {
    // process fetch response    
    response.text().then(function(text) {
        console.log('fetch result', text.length);
// timeout in 5 seconds
setTimeout(function() {
    runner.stop(); // cancel Fetch request

Abortable Fetch

Include exec-fetch.js (221 bytes) in the HTML document.

<script src="exec.min.js"></script>
<script src="exec-fetch.min.js"></script>

The native fetch API is now enhanced with a .abort() method. This is not a Polyfill.

// normal fetch request
var request = fetch('')
    .then(function(response) {
        response.text().then(function(text) {
            console.log('response', text.length);
    }).catch(function(err) {
        console.log('err', err.message);
// abort request after 10ms
setTimeout(function() {
}, 10);

Fine tune the timeout to test Fetch request and/or response cancellation.

Cancelled Fetch API Request and Response

Abortable fetch requires a dedicated exec.js container per fetch request. Use a container pool to improve performance.


Enhance the performance of exec.js when making subsequent/simultaneous requests by creating a container pool. Code isolation can be applied as a third parameter.

// create container pool for performance
// exec(5,null,[]); // pool with code isolation enabled
console.time('abortable fetch with pool');
var url = '';
console.timeEnd('abortable fetch with pool');

Abort / cancel code execution

To cancel code execution, use runner.stop().

var runner = new exec('setInterval(function() {console.log(123);},100);');
setTimeout(function() {

Access to DOM

To access the DOM, use parent.document (info). DOM access is available in all browsers.

var runner = new exec('setInterval(function() {var h = parent.document.createElement(\'h1\');h.innerHTML = \'test\';parent.document.body.insertBefore(h, parent.document.body.firstChild);},100);');
setTimeout(function() {

Security / code isolation

It is possible to isolate the code to be executed and block access to DOM, navigation, popups, form submission and other specific privileges by passing a third parameter with an array of sandbox parameters and/or a fourth parameter with Content-Security-Policy (CSP) configuration.

// enable code isolation
new exec(code,onmessage,[])
// enable code isolation with forms and API's enabled
new exec(code,null,['allow-forms','allow-pointer-lock']);
// enable code isolation and block loading of images, objects and media
new exec(code,null,[],{"img-src":"'none'","media-src":"'none'","object-src":"'none'"});
// restrict resource access to domain without code isolation
new exec(function(){
    // test CSP using Fetch
    fetch('').catch(function(err) {});

Code isolation is disabled by default. When enabled, the allow-scripts and allow-same-origin sandbox parameters are enabled by default.

Content Security Policy (CSP) Whitelist

To whitelist exec.js add script-src 'nonce-execjs' to the Content-Security-Policy. To use the runner.exec() method, script-src: 'unsafe-eval' is required.


npm i exec.js

Downloadsweekly downloads








last publish


  • avatar
Report a vulnerability