The constructor accepts a function called executor. This executor function accepts two parameters resolve and reject which are in turn functions. Promises are generally used for easier handling of asynchronous operations or blocking code, examples for which being file operations, API calls, DB calls, IO calls etc.The initiation of these asynchronous operations happens within the executorfunction. If the asynchronous operations are successful then the expected result is returned by calling the resolvefunction by the creator of the promise. Similarly if there was some unexpected error the reasons is passed on by calling the rejectfunction.

Understanding promises in JavaScript

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