nikjohn nikjohn - 1 month ago 3
Javascript Question

How to access results of a promise at a later part of a chain

I have a promise chain like so

functionOne()
.catch(errorHandlerOne)
.then(functionTwo) // calls functionTwo(responseOne)
.catch(errorHandlerTwo)
.then(functionThree) // calls functionThree(responseTwo)
.catch(errorHandlerThree)
.finally(finalHandler)


This might seem to be an obvious answer, but my question is this:

I can access the
responseOne
in
functionTwo()
fine, but how can I access the
responseOne
in
functionThree()
or
finalHandler()
?


Edit: I thought about assigning it to a variable and accessing it later, but it seems to be quite hacky and against the flow of the promise chain. I'm looking for a better way

Answer

how can I access the responseOne in functionThree() or finalHandler()?

By passing them forward in some way, as the return value of the then callback (or the resolution value of a promise it returns, which is effectively the same thing).

Example:

function asyncOp(name) {
  return new Promise(resolve => {
    resolve(name);
  });
}
asyncOp("one")
  .then(result1 => {
    return asyncOp("two")
      .then(result2 => [result1, result2]);
  })
  .then(results => {
    console.log("results", results);
  });

An array like that is just one option; you could use an object, you could store the interim results in variables the handler close over, ...

Same example in ES5 (just in case someone needs it):

function asyncOp(name) {
  return new Promise(function(resolve) {
    resolve(name);
  });
}
asyncOp("one")
  .then(function(result1) {
    return asyncOp("two")
      .then(function(result2) {
        return [result1, result2];
      });
  })
  .then(function(results) {
    console.log("results", results);
  });

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