Javascript Promise Unsettled

Introduction

New promises are created using the Promise constructor.

This constructor accepts a single argument: a function called the executor.

It contains the code to initialize the promise.

The executor is passed two functions named resolve() and reject() as arguments.

The resolve() function is called when the executor has finished successfully and the promise is ready to be resolved.

The reject() function indicates that the executor has failed. 

let fs = require("fs");

function readFile(filename) {
    return new Promise(function(resolve, reject) {

        // trigger the asynchronous operation
        fs.readFile(filename, { encoding: "utf8" }, function(err, contents) {

            // check for errors
            if (err) {
                reject(err);//w w w.ja  va  2  s .  c o m
                return;
            }

            // the read succeeded
            resolve(contents);

        });
    });
}

let promise = readFile("main.js");

// listen for both fulfillment and rejection
promise.then(function(contents) {
    // fulfillment
    console.log(contents);
}, function(err) {
    // rejection
    console.error(err.message);
});

In this example, the fs.readFile() asynchronous call is wrapped in a promise.

The executor either passes the error object to the reject() function or passes the file contents to the resolve() function.




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