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How Can Promise Chaining Simplify Nested Promises in Asynchronous Operations?
How Can Promise Chaining Simplify Nested Promises in Asynchronous Operations?
Dec 10, 2024 pm 03:10 PM
Unraveling Nested Promises
Introduction
Nested promises can arise when working with asynchronous operations that build upon each other. In the provided code, the viewFile function makes multiple network requests using boxContentRequest and boxViewerRequest. These requests are currently structured within nested promises, making the code difficult to follow.
Chaining Promises
To eliminate the nested promises, we can employ promise chaining. This technique allows us to connect multiple asynchronous operations in a sequential manner, where the output of one promise becomes the input of the next.
Refactored Code
The refactored code below achieves the desired chaining:
exports.viewFile = function (req, res) {
var fileId = req.params.id;
boxContentRequest('files/' + fileId + '/content', req.user.box.accessToken)
.then(function (response) {
// Return the next promise directly
return boxViewerRequest('documents', { url: response.request.href }, 'POST');
})
.then(function (response) {
// Return the final promise directly
return boxViewerRequest('sessions', { document_id: response.body.id }, 'POST');
})
.then(function (response) {
console.log(response);
});
};
Explanation
In the refactored code, we return the next promise directly from each then callback. This allows us to chain the promises seamlessly, ensuring that the execution flow follows the desired sequence.
Generic Promise Chaining Pattern
The generic pattern for promise chaining is as follows:
// Option 1: Single-line syntax
somePromise
.then((r1) => nextPromise)
.then((r2) => anyValue);
// Option 2: Multi-line syntax
somePromise
.then(function (r1) {
return nextPromise;
})
.then(function (r2) {
return anyValue;
});
Both options will resolve the final promise with the value of anyValue. This pattern provides a concise and elegant way to connect asynchronous operations, simplifying the code structure and improving readability.
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