Testing functions containing async (without setTimeout)

Question

I want to test myFunction(), which is expected to turn var appState from "old state" to "new state". However, myFunction() invokes n functions fetcher() which contain the async function fetch - similar to the psudocode below.

// Application code

myFunction() = function {
    for (var i = 0, len = myarray.length; i < len; i++) {
          fetcher()
     }
}

fetcher = function(){
    fetch(url) // Is asynchronous
    .then(response => response.json())
    .then(data => {
        if(data == 'abc'){ appState = "new state" }
    }
}

// Test

myFunction()

setTimeout(() => {  
    assert.equal(appState, "New State")
}, 10);

This timeout works fine and the test passes, but it feels like a code smell. Is there a more correct way to wait for the async functions generated by myFunction() to complete before testing the new state?

Well there is nothing you can attach to to know the call is complete. And there is no async/await. You would have to change the code to use Promise.all to know when all the calls are done. So the answer is you have to change your code. — epascarello
– epascarello, Commented Jan 27, 2022 at 22:28

Felipe André · Accepted Answer · 2022-01-27 23:03:02Z

2

async / await is the way to go!

// Application code

myFunction() = async function {
    for (var i = 0, len = myarray.length; i < len; i++) {
          await fetcher()
     }
}

fetcher = async function(){
    const respons = await fetch(url) // Is asynchronous
    const data = await response.json())
    if(data == 'abc'){ appState = "new state" }
}

// Your test now should look like this:

async function test() {
    await myFunc()
    assert.equal(appState, "New State")
}

test()

async / await is awesome!

answered Jan 27, 2022 at 23:03

Felipe André

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1 Comment

Will Taylor Over a year ago

Thanks! I didn't realise you could put async on a fundamentally not-asynchronous function.

epascarello · Accepted Answer · 2022-01-27 23:08:46Z

You would need to utilize promises or use Async/Await

const fakeFetch = (url) => new Promise((resolve) => {
  setTimeout(() => {
    resolve({
      json: () => new Promise((resolveJSON) => resolveJSON({
        myKey: url === 3 ? 'abc' : 'foo'
      })),
    });
  }, Math.ceil(1000 * Math.random()));
});

let appState = 'x';
const myarray = [0, 1, 2, 3, 4];


const myFunction = function() {
  var myPromises = [];
  for (var i = 0, len = myarray.length; i < len; i++) {
    myPromises.push(fetcher(myarray[i]))
  }
  return Promise.all(myPromises);
  // return Promise.allSettled(myPromises);
}

// const myFunction = function() {
//   return Promise.allSettled(myarray.map(x => fetcher(x)));
// }


const fetcher = function(url) {
  return fakeFetch(url)
    .then(response => response.json())
    .then(data => {
      if (data.myKey == 'abc') {
        appState = "new state"
      }
    });
}


myFunction().then( function () {
  console.assert(appState === "New State", "appState", appState);
});

const fakeFetch = (url) => new Promise((resolve) => {
  setTimeout(() => {
    resolve({
      json: () => new Promise((resolveJSON) => resolveJSON({
        myKey: url === 3 ? 'abc' : 'foo'
      })),
    });
  }, Math.ceil(1000 * Math.random()));
});

let appState = 'x';
const myarray = [0, 1, 2, 3, 4];


const myFunction = async function() {
  for (var i = 0, len = myarray.length; i < len; i++) {
    await fetcher(myarray[i])
  }
}

const fetcher = async (url) => {
  const response = await fakeFetch(url);
  const data = await response.json();
  if (data.myKey == 'abc') {
    appState = "new state"
  }
}

const test = async () => {
  await myFunction();
  console.assert(appState === "New State", "appState", appState);
};

test();

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