How to create a Promise from the nested kotlin.js.Promise?

Question

kotlin.js.Promise has function then with this declaration:

open fun <S> then(
    onFulfilled: (T) -> S, 
    onRejected: (Throwable) -> S = definedExternally
): Promise<S>

I have two functions a() and b(). They both return a Promise<Int>. (They represent some requests to the server.) I need to combine them and create a new function like:

fun c(): Promise<Int> {
    a().then({
        b()
    })
}

But it is not possible, because return type is Promise<Promise<Int>> and not Promise<Int>.

I think this is possible in Javascript. How can I chain promises in Kotlin?

holi-java · Accepted Answer · 2017-06-14T13:38:35.247

2

you need an additional Promise for that, for example:

fun c(): Promise<Int> {
    return Promise({ resolve, reject ->
        a().then({
            b().then(resolve, reject);
        });
    })
}

the code above also can simplified by using single-expression function as below:

fun c() = Promise({ resolve, reject ->
    a().then({
        b().then(resolve, reject);
    });
});

edited Jun 14 '17 at 13:38

answered Jun 14 '17 at 12:59

holi-java

29,655
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score 1 · Answer 2 · answered Jun 15 '17 at 20:39

1

fun c(): Promise<Int> {
    return a().then ({
        b().unsafeCast<Int>() 
        //Result of b is Promise<Int>, not Int, but then has bad type declaration
    })
}

answered Jun 15 '17 at 20:39

user3706629

209
2
9

1

While this code may answer the question, providing additional context regarding why and/or how this code answers the question improves its long-term value. – Donald Duck Jun 17 '17 at 10:22
This is actually useful in certain situations. Basically in JS you can return a promise as a result of a promise, but in Kotlin the typings are screwed up since forever. – LppEdd Jun 24 '23 at 12:21

How to create a Promise from the nested kotlin.js.Promise?

2 Answers2