async-barrier

Helper function to increase your expresiveness when async/await testing

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async-barrier
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AsyncBarrier
Helper function to increase your expresiveness when async/await testing

Quick Use

Install with npm:
npm install async-barrier

Just require or import the make function and create a barrier. This barrier will ensure that all async functions reaches the desired line at the same time.
const makeAsyncBarrier = require("async-barrier");

test("setTimeout executes in next tick", async () => {
  const log = [];
  const barrier = makeAsyncBarrier(2);

  setTimeout(async () => {
    log.push("timeout");
    await barrier();
  });

  log.push("begin");
  await barrier();
  log.push("end");

  expect(log).toEqual(["begin", "timeout", "end"]);
});

By omitting the await keyword it can be used to define checkpoints and force async events to happen in the desidered order.
const makeAsyncBarrier = require("async-barrier");

test("setTimeout executes in next tick", async () => {
  const log = [];
  const endTimeout = makeAsyncBarrier(2);

  setTimeout(async () => {
    log.push("timeout");
    endTimeout();
  });

  log.push("begin");
  await endTimeout();
  log.push("end");

  expect(log).toEqual(["begin", "timeout", "end"]);

Motivation

Async/await are hard to test specially when we need a fine grain specification. If you try to use promises to create checkpoints and control async execution order code can become difficult to read. For example, the following code is equivalent to the previous test:
test("setTimeout executes in next tick", async () => {
  const log = [];
  let endTimeoutResolve;
  let endTimeoutPromise = new Promise(resolve => {
    endTimeoutResolve = resolve;
  });

  setTimeout(async () => {
    log.push("timeout");
    endTimeoutResolve();
  });

  log.push("begin");
  await endTimeoutPromise;
  log.push("end");

  expect(log).toEqual(["begin", "timeout", "end"]);
});

makeAsyncBarrier

const barrier = makeAsyncBarrier(2);

It creates a barrier function and returns it. The argument that receives is an integer and represents the number of times that the barrier function will be executed. The created barrier function receives no arguments and returns a promise. The promise returned will be satisfied when the barrier function is executed as many times as indicated in makeAsyncBarrier argument.

Multiple barriers

Each barrier function created with makeAsyncBarrier is independent and represents a different barrier. Each function will only wait for its own calls.
You can use multiple barriers to synchronize steps:
const makeAsyncBarrier = require("async-barrier");

test("synchronize multiple steps in the correct order", async () => {
  const log = [];
  const beginStep1 = makeAsyncBarrier(2);
  const beginStep2 = makeAsyncBarrier(2);
  const endStep2 = makeAsyncBarrier(2);

  (async () => {
    await beginStep2();
    log.push("step 2");
    endStep2();
  })();
  (async () => {
    await beginStep1();
    log.push("step 1");
    beginStep2();
  })();

  log.push("begin");
  beginStep1();
  await endStep2();
  log.push("end");

  expect(log).toEqual(["begin", "step 1", "step 2", "end"]);
});

Excessive calls

Once it is called as many times as indicated in makeAsyncBarrier function argument, next calls have no effect and return a resolved promise.
const makeAsyncBarrier = require("async-barrier");

test("barrier can be called more times that specified", async () => {
  const barrier = makeAsyncBarrier(2);

  barrier();
  barrier();
  await barrier();
});

Don't

Do not wait twice for the same barrier in the same event. It will wait forever:
// don't do this
const barrier = makeAsyncBarrier(2);
await barrier();
await barrier();