index.js

/**
 * Exercise in consolidating common response handling using promises
 * Duplicated code is on lines 34 and 68.  How can we consolidate it?
 */

var http = require("http"),
    express = require("express"),
    errorFactory = require("error-factory"),
    app = express(),

    // Custom errors for common response problems.  These errors are emitted
    // by various requests when bad things happen
    EmptyResponseError = errorFactory("EmptyResponseError", {status: 404}),
    BadRequestError = errorFactory("BadRequestError", {status: 400}),
    ServerError = errorFactory("ServerError", {status: 500});

app.use("/foo", function (req, res, next) {
    new Promise(function (resolve, reject) {
        if (req.query.hasOwnProperty("empty")) {
            // You may throw an error -- this counts as a rejection
            throw new EmptyResponseError;
        }
        else if (req.query.hasOwnProperty("client")) {
            throw new BadRequestError;
        }
        else if (req.query.hasOwnProperty("server")) {
            throw new ServerError;
        }
        resolve();
    }).then(
        function () {
            res.status(200).end("foo");
        },
        next
    );
});

app.use("/bar", function (req, res, next) {
    new Promise(function (resolve, reject) {
        if (req.query.hasOwnProperty("empty")) {
            // Calling `reject` explicitly is also acceptable.  This is done
            // so that the implementation of the /bar route is different from
            // the /foo route which would ordinarily be the case.  This
            // duplication is not at issue right now
            reject(new EmptyResponseError);
        }
        else if (req.query.hasOwnProperty("client")) {
            reject(new BadRequestError);
        }
        else if (req.query.hasOwnProperty("server")) {
            reject(new ServerError);
        }
        resolve();
    }).then(
        function () {
            res.status(200).end("bar");
        },
        next
    );
});

app.use(function (err, req, res, next) {
    res.status(err.status || 500).json(err);
});

if (require.main === module) {
    http.createServer(app).listen(process.env.PORT || 3001, function () {
        console.log("Listening on port", process.env.PORT || 3001);
    });
}

module.exports = app;

package.json

{
  "name": "promise-problem",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "bluebird": "^2.9.24",
    "error-factory": "^0.1.5",
    "express": "^4.12.3"
  },
  "devDependencies": {
    "supertest": "^0.15.0"
  }
}

test-index.js

var app = require("supertest")(require("./")),
    assert = require("assert");

describe("Routes", function () {
    describe("/foo", function () {
        it("emit foo", function (done) {
            app
                .get("/foo")
                .expect(200)
                .expect(function (res) {
                    assert.equal("foo", res.text);
                })
                .end(done);
        });

        it("emit 404 for empty", function (done) {
            app
                .get("/foo?empty")
                .expect(404)
                .end(done);
        });

        it("emit 400 for client", function (done) {
            app
                .get("/foo?client")
                .expect(400)
                .end(done);
        });

        it("emit 500 for client", function (done) {
            app
                .get("/foo?server")
                .expect(500)
                .end(done);
        });
    });

    describe("/bar", function () {
        it("emit bar", function (done) {
            app
                .get("/bar")
                .expect(200)
                .expect(function (res) {
                    assert.equal("bar", res.text);
                })
                .end(done);
        });

        it("emit 404 for empty", function (done) {
            app
                .get("/bar?empty")
                .expect(404)
                .end(done);
        });

        it("emit 400 for client", function (done) {
            app
                .get("/bar?client")
                .expect(400)
                .end(done);
        });

        it("emit 500 for client", function (done) {
            app
                .get("/bar?server")
                .expect(500)
                .end(done);
        });
    });
});