Master the JavaScript Interview: What is a Promise?

Eric Elliott
Jan 23, 2017 · 11 min read
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Photo by Kabun (CC BY NC SA 2.0)

What is a Promise?

An Incomplete History of Promises

How Promises Work

wait — promise example on CodePen

Important Promise Rules

promise.then(
onFulfilled?: Function,
onRejected?: Function
) => Promise

Promise Chaining

fetch(url)
.then(process)
.then(save)
.catch(handleErrors)
;
Promise chaining behavior example on CodePen

Error Handling

save().then(
handleSuccess,
handleError
);
save()
.then(handleSuccess)
.catch(handleError)
;
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Without .catch(), an error in the success handler is uncaught.
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With .catch(), both error sources are handled. (diagram source)
save()
.then(
handleSuccess,
handleNetworkError
)
.catch(handleProgrammerError)
;

I recommend ending all promise chains with a .catch().

How Do I Cancel a Promise?

Adding .cancel() to the promise

Forgetting to clean up

Forgetting to handle a rejected cancel promise

Overly complex

Rethinking Promise Cancellation

Cancellable wait — try it on CodePen

Abstracting Promise Cancellation

speculation(fn: SpecFunction, shouldCancel: Promise) => Promise
SpecFunction(resolve: Function, reject: Function, onCancel: Function) => Void
npm install --save speculation

Extras of the Native JS Promise

Conclusion

Explore the Series



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