got one newbie question here. I need to declare the const in the top level of my Typescript file, so it´s accesible by all functions below. The problem is that the value I need to assign to this constant gets returned by the asynchronous function and this is where I got stuck. Asynchronous function cannot be called from the top-level - I can do something like this, but in this case, the constant is no more on the top level and cannot be accessed by other functions.

(async () => {
    const myConst = await asyncFunction(params);
})(); 

The other option is to use let instead of const, like this, but I rather had const up there

let myConst;
(async () => {
    myConst = await asyncFunction(params);
})();

Could you advise me? Is there some way out of it so I can declare the const for the global scope and assign it a value based on the async function?

Thanks a lot :)

got one newbie question here. I need to declare the const in the top level of my Typescript file, so it´s accesible by all functions below. The problem is that the value I need to assign to this constant gets returned by the asynchronous function and this is where I got stuck. Asynchronous function cannot be called from the top-level - I can do something like this, but in this case, the constant is no more on the top level and cannot be accessed by other functions.

(async () => {
    const myConst = await asyncFunction(params);
})(); 

The other option is to use let instead of const, like this, but I rather had const up there

let myConst;
(async () => {
    myConst = await asyncFunction(params);
})();

Could you advise me? Is there some way out of it so I can declare the const for the global scope and assign it a value based on the async function?

Thanks a lot :)

• javascript
• typescript
• asynchronous
• scope
• constants

Share Improve this question Follow asked Oct 15, 2018 at 15:18 JozefJozef 49311 gold badge1212 silver badges4444 bronze badges 4

• 3 Generally what you want to do is not really useful, because the global variable will only be available after the asynchronous operation is plete. – Pointy Commented Oct 15, 2018 at 15:22
• Closely related: Javascript set const variable inside of a try block – apsillers Commented Oct 15, 2018 at 15:23
• This is by design. You aren't meant to mix sync and async code in this fashion. If you need the variable in the future, you use a Promise to describe that, and the way you normally do that, is by using async functions. This way you can await on the value to ensure that it's there (as opposed to your current approach, in which you can only guess, or poll) – Madara's Ghost Commented Oct 15, 2018 at 15:23
• Either use a promise, either wrap the code that needs that constant specifically inside the async block. This is intended by design, as far as I know. – briosheje Commented Oct 15, 2018 at 15:23

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3 Answers 3

Sorted by: Reset to default 5

Using an async result in a top-level variable is problematic because there is no static guarantee that the result will be ready when the functions in your module attempt to read the value. In my opinion the proper & safe way to do this would be to assign a promise to the top-level variable, and make any function that reads the value await the result:

const myConst = asyncFunction(params);

async function example() {
  doStuff(await myConst);
}

But if it is important to have the async result in a top-level variable, and you have some way to guarantee that the value will be ready before it is read then I would go with your let assignment. I don't think there is any good way to make that variable a const in that case.

The problem is that since the operation providing the value is asynchronous, the other functions in your module could try to use the variable/constant before its value was set. So you're best off avoiding doing this, perhaps by doing what Jesse Hallett suggests and forcing the other functions in the module to use the promise.

If you can't avoid it, and you know for sure those functions won't be called before the asynchronous operation getting the value returns, use the let option initializing the variable with null or some other flag value, and check for that flag value in the functions that supposedly aren't called until it's ready and have them throw. E.g.:

let myConst = null; // null or some other value that isn't valid for this
(async () => {
    try {
        myConst = await asyncFunction(params);
    } catch (e) {
        // Handle the fact the async function failed -- don't skip this!
    }
})();

function assumeMyConst() {
    if (myConst === null) {
        throw new Error("State error: myConst not available yet");
    }
}

function foo() {
    assumeMyConst();
    // ...
}

But again, avoid it if you possibly can.

---

Side note: Notice the error checking in that async IIFE. Don't skip error checking. :-)

Since myConst gets its value after async code pletes, it is of Promise type. Hence your declaration should be pretty simple:

declare const myConst: Promise<any> // or type of the value instead of any

Use it anywhere like this:

myConst.then((v) => console.log(v))

P.S. My personal advice is to use modules (import/export) whenever possible. Global variables is a code smell.