FAQ: JavaScript Promises - Using Promise.all()

Where do the onFulfill and onReject come from and how can we use them if they weren’t defined in any of our programs

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Having difficulty understanding whats going on with Promises.
Would help to see an explanation using foreground/background, interrupt handlers and pointers.

How I can look at the code in library.js in the terminal? Thanks.

Super late but maybe someone else finds this useful:
I think it is because checkPants etc. aren’t functions, they’re variables. So you aren’t invoking a function directly - you’re passing the variables as arguments to Promise.all. The value of the variables is the result of invoking the checkInventory() function. So you are indirectly invoking a function, but what you’re passing to Promise.all is ‘just’ a value.


To add on to what you said, the function has the console.log commands in it already, so if the function is called it will print to the console without needing the console.log(whileCallingTheFunction). If the function returned strings, then you would need to do the console.log while calling the function, otherwise nothing will be printed to the console.

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I agree with what you say, except this part…

I don’t think we can call the variables checkSunglasses , checkPants and checkBags themselves arguments. They are promises returned from calling checkAvailability() three times, and which are then assigned (by reference to their variable names) as individual elements to an array. It’s this array which is passed to the Promise.all() function as its single argument/parameter.

@p.ryan.dorangmail.co, @tochmy, @toskap3

This post answers your question…

You may already know the answer by now, but you have at least two options:

  1. At the top of the the ‘Code Editor’ pane you should see a folder icon and two tabs (one for app.js and one for library.js). If you click library.js, the code should now be visible.
  2. In the bash terminal, there are a variety of commands you can use. The simplest one is probably
    more library.js

They were already defined as functions in the Lesson 10 version of app.js, in the section above the following comment
// Write your code below:
They both just write console.log entries

To call a function, we use functionName(parameters). That’s left empty if there are no parameters.
Now check what checkSunglasses, checkPants, and checkBags are. Are they functions? No. If this confusing to you, I suggest you review functions. checkSunglasses, checkPants, and checkBags store the result of calling checkAvailability(itemName, distributorName) on a certain item and distributor.

If you look at the code for checkAvailability(itemName, distributorName), you’ll see that it returns a promise. Then we call Promise.all(promises) on an array of promises! Since checkSunglasses, checkPants, and checkBags are the results of calling checkAvailabity(itemName, distributorName), they are promises, so we can make them items of that array.

Edit: Whoops, I didn’t see the indirect comment chains that already answered the question. Hope someone finds this useful anyways.

Regarding itemsArray array on app.js file:

  1. Where in the code I see it is declared as an array?
  2. How do it gets its variables from? Is it from the resolve(itemName) on checkAvailability function on library.js file ?

Thank you, I just love to understand it all.


Could anyone help me with this exercise. I can’t seem to past step 3.
This is my code:

const promise checkSunglasses = checkAvailability(‘sunglasses’, ‘Favorite Supply Co.’);

const promise checkPants = checkAvailability(‘pants’, ‘Favorite Supply Co.’);

const promise checkBags = checkAvailability(‘bags’, ‘Favorite Supply Co.’);

Promise.all([checkSunglasses, checkPants, checkBags]).then(onFulfill).catch(onReject);

Code academy is popping up with a syntax error and when I execute the code, it produces also a syntax error directing to ‘sunglasses’. I have been staring at this for far too long and I feel like I must be missing something obvious here.

Many thanks in advance.

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So technically, this below shouldn’t work? We were using an arrow function in .then and .catch all long, and now it is not working?

Promise.all([checkSunglasses, checkPants, checkBags])
.then((onFulfill) => {console.log(onFulfill)})
.catch((onReject) => {console.log(onReject)})


Promise.all([checkSunglasses, checkPants, checkBags]).then(onFulfill));

it should work. :crossed_fingers:

I copied and pasted what you said and it still gives me a red ‘X’ on step 4?

Would need to see your code, and the error message if one was provided in order to hazard a guess as to why.

I see what I did! I put a ; after the .then statement. Evidently you cannot continue to the .catch statement if you do that!

Thank you for the quick reply!

  1. As I understand (and that’s not a very hopeful premise), it’s because the resolution of the promises from the functions checking availability is the item name if they are all successful. Meaning that if Promise.all resolves, the argument is passes is the array of all the resolutions.

  2. Yes as I alluded to in answer (1), we have this bit of code in ‘library.js’

if (restockSuccess()) {
     console.log(`${itemName} are in stock at ${distributorName}`)

For each of the variable which calls ‘checkAvailability’ function returns promise (as given in library.js module), either resolved with ‘ItemName’ or rejected with reason.

Case 1: When all promises resolved, it would return ‘ItemName’ for each ‘checkAvailability’ function

const checkSunglasses = checkAvailability('sunglasses', 'Favorite Supply Co.');
const checkPants = checkAvailability('pants', 'Favorite Supply Co.'); 
const  checkBags = checkAvailability('bags', 'Favorite Supply Co.');

Now, the values returned from these are stored in Array inside Promise.all()

let myPromise = Promise.all([checkSunglasses, checkPants, checkBags]);

so for all promises resolved, we would get the output like below

let myPromise = Promise.all(['sunglasses', 'pants', 'bags']);

which when used with .then() and .catch(), would give the expected result.

Case 2: When any of the promises fail, this would trigger catch() and would call onReject

Hope this makes sense !!

My version of the code is below:

  • I used the similar way they have explained in example to call ‘.then()’ and ‘.catch()’. Initially I have to make it work the way the program wanted to make pass step 4 to 6. Later, I changed as below, it gives me the same result. This way, I could understand in better way what variable is passed and into which function.

Please let me know, if there is any other ways of writing this… :slight_smile:

My question is actually a small maybe unrelated question regarding the random 80% true/false generator at the end… don’t exactly nderstand the syntax of the ‘Math.random() > .2’ how does the > signb work in this case?
Thanks in advance :slight_smile:

I am getting the following output on the console:
bags are in stock at Favorite Supply Co.
pants are in stock at Favorite Supply Co.
Error: sunglasses is unavailable from Favorite Supply Co. at this time.

So I think the promise.all works in this fashion that it executes the success handler for each promise of the array first, and in case a promise was rejected it executes the failure handler at last? The code is as follows:

Promise.all([checkSunglasses, checkPants, checkBags])

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