How Can Promise Chaining Simplify Nested Promises in Asynchronous Operations?

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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