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Javascript Varsnap Client. For use in both browser and node.js


Install from NPM - npm install --save varsnap


The varsnap decorator needs to be configured with a config object with these variables:

  • varsnap - Should be either true or false. Varsnap will be disabled if the variable is anything other than true.
  • env - If set to development, the client will receive events from production. If set to production, the client will emit events.
  • branch - Used for labeling test runs by (git) branch. Test runs on the master branch will update your varsnap badge
  • producerToken - Only clients with this token may emit production snapshots. Copied from
  • consumerToken - Only clients with this token may emit development snapshots. Copied from

If you’re deploying varsnap to a browser, note that Varsnap needs unmangled code in production to work correctly.


Wrap your functions with the varsnap decorator for any function you’d like to make better:

// Node.js
// var varsnap = require('varsnap');
import varsnap from 'varsnap';

// Browser
var varsnap = window.varsnap;

// Configuration
  varsnap: true,
  env: 'production',
  branch: 'master',
  producerToken: 'producer-abcd'

// Integration
function example(x, y, z) {
  // ...
example = varsnap(example);

example(1, 2, 3);

See example/example.js for example usage.


If you are using typescript, make sure your tsconfig.json has a "target" to "es2015", "es6", or later. This is needed to persist function names after compiling to javascript. See for details.


Use the varsnap.runTests() function.

function test() {
  const status = varsnap.runTests().then(function(status) => {

If you are using mocha and chai, you can include this test script:

import { expect } from 'chai';
import varsnap from 'varsnap';

// Load the modules where varsnap is being used
import './example.js';

context('Varsnap', function() {
  this.timeout(30 * 1000);
  it('runs with production', function() {
    return varsnap.runTests().then(status => {

See example/test.js for example usage.


This client currently does not work with minification/obfuscation libraries that change class, function, and argument names. Disable any code processors that change these names. If you are using the terser package, use these configuration options to allow the varsnap client to match production and development Snaps together:

import { minify } from 'terser';

const options = {
  mangle: false,
  keep_fnames: true,
  keep_classnames: true,
minify(code, options);

Git Branch

Providing a git branch to the client allows Varsnap to categorize test runs. Tests on the master branch will also be reflected in your Varsnap badge image.

Client Development


  1. Update Changelog with new version
  2. Update const version in varsnap/core.ts
  3. Update webpack.config.js version and run npm run package
  4. Update package.json version and run npm install
  5. Git commit, tag, and push
  6. Upload dist/* to