dopromise

A Simple Promise Library for JavaScript

Install

$ npm install dopromise

Usage

Nodejs

var doPromise = require("dopromise").doPromise;

Browser

Just include "promise.js" script file. The global variable dopromise is made available to you.

Example

The function doPromise takes a set of functions as its arguments. Each function is called with a continuation function, except the last function.

The called function promises to call the continuation from within it. It may call the continuation synchronously or asynchronously (from inside a callback).

If at any instance one of the functions wants to halt the operations, it can simply not call the continuation.

All the called functions are called with the same this. So to pass values from one function to the other, just attach it to this. To access this inside a callback, save it as that, before making the async call.

The example below is a nodejs program to asynchronously copy an input file to an output file. It makes three asynchronous calls. fs.exists, fs.readFile, fs.writeFile

var fs = require("fs");
var doPromise = require("dopromise").doPromise;

doPromise(
    function(continuation) {
        var that = this;
        this.infile = process.argv[2] || "";
        fs.exists(this.infile, function (exists) {
            if (exists) {
                continuation();
            } else {
                console.log("File does not exist: " + that.infile);
            }
        });
    },
    function(continuation) {
        this.outfile = process.argv[3];
        if (this.outfile) {
            continuation();
        } else {
            console.log("Output File Name is Required");
        }
    },
    function(continuation) {
        var that = this;
        fs.readFile(this.infile, function (err, data) {
            if (err) {
                console.log("Error reading File: " + that.infile);
            } else {
                that.contents = data;
                continuation();
            }
        });
    },
    function() {
        var that = this;
        fs.writeFile(this.outfile, this.contents, function (err) {
            if (err) {
                console.log("Error writing File: " + that.outfile);
            }
        });
    }
);