Top tip for mastering JavaScript promises

return makeFirstPromise()
    .then(function(res1) {
       (...)
    })
    .then(function(res2) {
       (...)
    })
    .then(function(res3) {
        // **here I need to access res1**
    });

Is there a recommended approach for accessing a previous promise result in a subsequent function of the promise chain?

I have identified two potential solutions:

var r1;
return makeFirstPromise()
    .then(function(res1) {
       r1 = res1;
       (...)
    })
    .then(function(res2) {
       (...)
    })
    .then(function(res3) {
        console.log(r1);
    });

Alternatively, nesting the promises after the first one, but this method disrupts the visual flow of the chain:

return makeFirstPromise()
    .then(function(res1) {
       (...)
       return secondPromise(res2)
           .then(function(res3) {
               console.log(res1);
           });
    });

Any thoughts or suggestions on this matter?

Answer №1

It is recommended to utilize promise syntax in the initial way, as described here. The second syntax can become perplexing rather quickly.
Remember to pass the result onto the subsequent promise.

var r1;
return makeFirstPromise()
    .then(function(res1) {
       r1 = res1;
       (...)
       return r1;
    })
    .then(function(r1) {
       (...)
    });

Answer №2

When working with promises proxy values, the most efficient method is to treat them as proxies for values. This is the main concept they encapsulate:

const result1 = createFirstPromise();
const result2 = result1.then(createSecondPromise);
Promise.all([result1, result2]).spread(function(firstOutput, secondOutput){
    // Access both values easily without the need for nested functions or closure tricks.
});

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