How to bind the resolved value of Javascript promise in ClojureScript

Question

How do I bind the resolved return value (in this case a json object) of a Javascript promise in a let block? all I get is a #object[Promise [object Promise]]

For instance:

(.then (fn-that-returns-a-js-promise) #(print (.-prop %)))

prints the value of prop to the the console, while:

(let [prop (.then (fn-that-returns-a-js-promise) #(.-prop %))] (print prop))

prints #object[Promise [object Promise]]

Dusan Miloradovic · Accepted Answer · 2016-09-28 09:02:25Z

1

You can't bind it.

(.then (fn-that-returns-a-js-promise) #(.-prop %))

returns javascript Promise. Every operation on a promise returns the new promise, and your program logic executes within the context of that promise, you can't escape it.

answered Sep 28, 2016 at 9:02

Dusan Miloradovic

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6 Comments

mac Over a year ago

But calling print apparently forces it?

Madara's Ghost Over a year ago

@mac .then(fn)'s return value is always a Promise, regardless of what function you've passed to it. That's a "limitation" of JavaScript, not ClojureScript.

Dusan Miloradovic Over a year ago

In your first expression , print is called inside the promise, and it will be executed (printed) once the original promise is released. Your second expression just prints the promise, not its value. The value can be accessed just ffrom the promise itself

mac Over a year ago

Thanks for the comments. Why is it that the property access inside the second callback does not return the value instead of the promise? it is afterall executed "inside" the promise.

Dusan Miloradovic Over a year ago

It is returning the value, but "wrapped" inside the promise. If you want to read the value, you need to access it from the promise: (.then (.then (fn-that-returns-a-js-promise) #(.-prop %)) (fn[x](println x))) .There is no way for a value inside the promise to escape outside it, you have to chain the promises to operate on them(every call to then returns the new promise)

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