Convergence helpers for testing big
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@bigtest/convergence CircleCI

Recognize a desired state and synchronize on when that state has been achieved.

Why Convergence?

Let's say you want to write an assertion to verify a simple cause and effect: when a certain button is clicked, a dialog appears containing some text that gets loaded from the network.

In order to do this, you have to make sure that your assertion runs after the effect you're testing has been realized.

Image of assertion after an effect

If not, then you could end up with a false negative, or "flaky test" because you ran the assertion too early. If you'd only waited a little bit longer, then your test would have passed. So sad!

Image of false negative test

In fact, test flakiness is the reason most people shy away from writing big tests in JavaScript in the first place. It seems almost impossible to write robust tests without having visibility into the internals of your runtime so that you can manually synchronize on things like rendering and data loading. Unfortunately, those can be a moving target, and worse, they couple you to your framework.

But what if instead of trying to run our assertions at just the right time, we ran them many times until they either pass or we decide to give up?

Image of convergent assertion

This is the essence of what @bigtest/convergence provides: repeatedly testing for a condition and then allowing code to run when that condition has been met.

And it isn't just for assertions either. Because it is a general mechanism for synchronizing on any observed state, It can be used to properly time test setup and teardown as well.

Creating a Convergence

A convergent function is a function that runs repeatedly until it no longer returns false or throws an error. When the function is finally successfully executed, it is considered to be passing and a converging promise will resolve. However, if the converging function does not pass within the provided timeout period the promise will reject with the last error thrown from the function.

Using Convergence

The Convergence class allows you to create a convergence, add assertions to converge on, and execute the convergent assertions all within a single timeout period.

import Convergence from '@bigtest/convergence'

// starts a new queue
new Convergence()
  // adds a convergent function to the queue
  .when(() => expect($el).to.exist)
  // called when the previous function converges
  .do(() => $el.get(0).click())
  // adds a convergent function that resolves when it is true
  // for the remaining timeout period
  .always(() => expect($el).to.have.prop('disabled', true))
  // starts converging and returns a promise that resolves
  // when all convergences have been met


Convergence has an immutable interface, so it's methods will return new instances.

// will converge when the total equals 5
let convergeWhen = new Convergence()
  .when(() => total === 5);

// will log the total after the first convergence
let convergeAndLog = convergeWhen
  .do(() => console.log(total));

// after logging the total, will converge when it remains 5 for the
// duration of the remaining timeout
let convergeAlways = convergeAndLog
  .always(() => total === 5);

// all three convergences can be ran in parallel;;;

Note: .run() does not return a new instance, instead it returns a promise.


new Convergence([timeout = 2000]);



You can define the convergences total timeout during initialization of a Convergence instance. This timeout will be used for the set of convergent functions unless the .timeout() method is used to change the total timeout period.

let converge = new Convergence(1000);



Given a timeout (in ms), the .timeout() method will return a new Convergence instance with the new timeout period replacing the old one. When no argument is given, this method will return the current timeout period of the convergence.

let convergeLong = converge.timeout(5000);

converge.timeout();  // => 1000
convergeLong.timeout(); // => 5000


Returns a new Convergence instance and adds the provided assertion to its queue. When this instance is ran, the assert function will be looped over repeatedly until it passes, or until the convergence's timeout has been exceeded.

// this convergence will resolve when `total` equals `5`
converge.when(() => total === 5)

.always(assert[, timeout])

Another common pattern is asserting that something has not changed. With a typical convergence, the state may change after an assertion converges immediately. So for these scenarios, you want to converge when the assertion passes for the duration of a timeout. .always() is just like .when() above, except that the assert function is looped over repeatedly until it fails for the first time or never fails for the duration of the timeout.

// this convergence will resolve when total is always 5
// for the duration of the remaining timeout
converge.always(() => total === 5)

When a convergence added with .always() is last in the queue, it will default to the remaining timeout to converge on its assertion always passing. When it is not last in the queue, timeout defaults to one-tenth of the total timeout or 20ms (whichever is greater).

  // will be given any remaining timeout
  .always(() => total === 5)

  // waits 500ms
  .always(() => total === 5, 500)

  // defaults to 200ms (one-tenth 2000ms)
  .always(() => total === 5)
  .when(() => total === 10)


This method is useful when you need to execute something after a convergence in the queue, but before other convergences are ran. This can help with debugging in between convergences and also allows you to run side effects between convergences.

  .when(() => total === 5)
  // executes after the total is equal to 5
  .do(() => total *= 100)
  // starts converging after the total has been multiplied
  .always(() => total === 500)

Functions in a Convergence queue curry their return value between other functions in the queue. This means that what you return from one function in the queue will be available as the argument given to the next function in the queue.

  // returns the element when it exists in the DOM
  .when(() => {
    let $el = $('[data-test-element]');
    return $el;
  // clicks the element and returns it for chaining
  .do(($el) => {
    return $el;
  // converges when the element is disabled for the remaining timeout
  .always(($el) => {
    expect($el).to.have.prop('disabled', true)

You can even return promises, or other convergences from this method.

  .do(() => Promise.resolve(5))
  .do((total) => new Convergence()
    // the final always will still get the remaining time
    .always(() => total === 5 && total))
  // ... unless it is followed by additional methods
  .do((total) => total *= 100)


Combines convergences to allow composing them together to create brand new convergence instances.

let converge1 = converge.when(() => total === 1)
let converge5 = converge.when(() => total === 5)

// converges when the total first equals `1` and then equals `5`


In order to actually run a Convergence instance, you must call the .run() method. This method does not return another instance, instead it returns a promise that resolves when all assertions in the queue have converged. The returned promise will keep track of the current timeout and ensure that all convergences in the queue converge within that period.

Because .run() returns a promise, and Convergence is immutable, you can call .run() multiple times on the same convergence instance.
  .catch(() => {
    console.log('failed. trying again...');
  .then(() => {

The promise returned from .run() resolves with a stats object. This stats object holds various information about how the convergences in the queue ran. The queue property is an array of stats objects specific to each function in the queue. => {
  stats.start    // start time of the convergences
  stats.end      // end time of the convergences
  stats.elapsed  // time taken to converge on the entire queue
  stats.runs     // number of times functions in the queue ran
  stats.timeout  // this Convergence instance's timeout
  stats.value    // value returned from the last function in the queue
  stats.queue    // an array of stats objects from the queue

  // each function in the queue produces similar stats objects
  stats.queue[0].start    // start time of this convergence
  stats.queue[0].end      // end time of this convergence
  stats.queue[0].elapsed  // time taken for this convergence
  stats.queue[0].runs     // number of times this convergence ran
  stats.queue[0].timeout  // the timeout this convergence was given
  stats.queue[0].always   // whether this convergence used .always()
  stats.queue[0].value    // value returned from this convergence

Note: functions added using .do() produce a small stats object as well, but they omit runs, timeout, and always since they do not apply to these functions.


In addition to the Convergence class, this package also exports a few convergence helpers.


Returns true if the given object implements a Convergence interface. A Convergence interface consists of a queue, a #timeout() method, and a #run() method.

class CustomConvergence extends Convergence() {

isConvergence(new Convergence()) // => true
isConvergence(new CustomConvergence()) // => true
isConvergence(new Array()) // => false

when(assertion[, timeout=2000])

Starts converging on the given assertion, resolving when it passes within the timeout period. The assertion will run once every 10ms and is considered to be passing when it does not error or return false. If the assertion never passes within the timeout period, the promise will reject as soon as it can with the last error it recieved.

// simple boolean test
await when(() => total === 100)

// with chai assertions
await when(() => {
  expect(add(total, 1)).to.equal(101)

Returns a thennable function that can both be used as a callback and awaited on directly. For example, testing frameworks that support async tests work well with convergent assertions.

test('will eventually become bar within 1s', when(() => {
}, 1000));

Note: when will throw an error after it fails to converge within the timeout. When using with testing frameworks, be aware of any test timeouts that may occur before the convergence fails.

always(assertion[, timeout=200])

Starts converging on the given assertion, resolving when it passes throughout the timeout period. Like when(), The assertion will run once every 10ms and is considered to be passing when it does not error or return false. However, if the assertion does not pass consistently throughout the entire timeout period, it will reject the very first time it encounters a failure.

// simple boolean test
await always(() => total !== 100)

// with chai assertions
await always(() => {
  expect(add(total, 1)).to.equal(101)

Also returns a thennable function that can both be used as a callback and awaited on directly.

test('remains foo for at least 1s', always(() => {
}, 1000));

Note: always will resolve after it converges throughout the entire timeout. When using with testing frameworks, be aware of any test timeouts that may occur before the convergence resolves.