function args not visible inside return new Promise(function(resolve, reject)

Question

arguments et and instance, passed to the function which is promisified are not visible (seen as null) in the return new promise block. Note:

rp = require('request-promise');
Promise = require('bluebird');

.. and some of my own modules like urlLib are 'required' at the top. rp library is promisified request library... It works if I explicitly set et and instance variables inside the return statement. (see commented et and instance declarations).

module.exports.getEntityInstance = function (et, instance) {

return new Promise(function(resolve, reject) {

    // explicit setting, to test, works fine.
    // var et = "device", instance = "33";

    if ((!et) || (!instance)) {
        // ****** FAILING HERE *******
        reject(new Error(utilLib.errorAsJSON ("Invalid et/instance arg")));
    }
    OPTS.url = base_url + et + "/" + instance;
    rp(OPTS)
        .then(function (data) {
            resolve(data)
        })
    .catch(function (err) {
            reject(err);
        });
});
}

Here's the unadulterated code:

module.exports.em7GetEntityInstance = function (et, instance) {

log.info("----- et before promise:(" + et + ")");
log.info("----- instance before promise:(" + instance + ")");

return new Promise(function(resolve, reject) {
    // var et = "device";
    // var instance = "33";

    // if null or undefined...
    if (!et || !instance) {
        log.error("---- et and/or instance args null.  rejecting");
        reject(new Error(utilLib.errorAsJSON ("getEntityInstace():Invalid entity type argument")));
    }
    var et = et.toLowerCase().trim();
    var url = baseurl + et + "/" + instance
    log.info("url:(" + url + ")");
    OPTS.url = url;
    rp(OPTS)
        .then(function (data) {
            log.debug ("success. invoking resolve()");
            resolve(data)
        })
    .catch(function (err) {
            log.error("+++ rp failed:(" + err  + ")");
            reject(err);
        });
});

}

And the log statements (its a server process (no console.log). Using winston log module.

2016-05-05T15:44:06.866Z - info: ----- et before promise:(device)
2016-05-05T15:44:06.867Z - info: ----- instance before promise:(33)
2016-05-05T15:44:06.867Z - error: ---- et and/or instance args null.  rejecting

They *are* visible unless you did shadow them by re-declaring `var et, instance`. Please show us your call, it's more likely that you passed in `null`. — Bergi, May 05 '16 at 15:28
They'll be visible. What else happens in that `.getEntityInstance()` function? Have you used `console.log()` or the browser debugger to see what values the parameters have *outside* the Promise? — Pointy, May 05 '16 at 15:28
Avoid the [`Promise` constructor antipattern](http://stackoverflow.com/q/23803743/1048572)! Rather [use `Promise.reject`](http://stackoverflow.com/q/35856041/1048572). — Bergi, May 05 '16 at 15:29
Yes I used logging just after entering getEntityInstance and JUST after entering return - I see null. I have commented out the settings because i wanted to test to make sure the rest of the functionality works as expected IF values are available - and it does. So the now commented shadowing was for testing. @Bergi - can you please suggest what's the correct pattern here to avoid my antipattern please? => I see the link and that should work!! - as there is no explicit wrapper return(function()){} there. While i do that though, any suggestions why above's failing? — user3213604, May 05 '16 at 15:30
@user3213604: Please post the code that includes the `console.log` statements and the exact console output you are getting. — Bergi, May 05 '16 at 15:32
I have sanitized the posted codefor code clarity... I am not shadowing the variables. I added them there to see IF they are available inside return, it works correctly. Here's from my log: `2016-05-05T15:00:06.948Z - info: ----- et before promise:(device) 2016-05-05T15:00:06.948Z - info: ----- instance before promise:(33) 2016-05-05T15:00:06.948Z - error: +++++ et null. rejecting` — user3213604, May 05 '16 at 15:35
@Bergi: added entire code. followed by logs (fresh execution). — user3213604, May 05 '16 at 15:47

score 1 · Accepted Answer · edited May 23 '17 at 11:52

1

The var et in your inner function does shadow the variable with the same name from the outer scope. It's undefined when the test is evaluted.

Avoid the var, use a different variable name, or just do

var url = baseurl + et.toLowerCase().trim() + "/" + instance;

without the intermediate variable at all.

But you shouldn't use an inner function at all, rather do

module.exports.em7GetEntityInstance = function(et, instance) {
    log.info("----- et before promise:(" + et + ")");
    log.info("----- instance before promise:(" + instance + ")");
    // if null or undefined...
    if (et==null || instance==null) {
        log.error("---- et and/or instance args null.  rejecting");
        return Promise.reject(new Error(utilLib.errorAsJSON ("getEntityInstace():Invalid entity type argument")));
    }
    var url = baseurl + et.toLowerCase().trim() + "/" + instance;
    log.info("url:(" + url + ")");
    OPTS.url = url;
    return rp(OPTS).then(function(data) {
        log.debug ("success. invoking resolve()");
        return data;
    }, function(err) {
        log.error("+++ rp failed:(" + err  + ")");
        throw err;
    });
};

edited May 23 '17 at 11:52

Community

1
1

answered May 05 '16 at 15:53

Bergi

630,263
148
957
1,375

First of all the commented out var et and var instance just inside the 'return' function were to test (purposefully shadow, so I have values I expect to test if the rest of the functionailty works. But those stayed commented as shown in the code Furthermore, even if I use et and instance directly in the URL - I don't reach there. Lastly, I do need to promisify it all, since I am invoking this function from another module - which uses the promise pattern over this function. That 'rp' module supports promises is incidental. I need entire function promisified for use outside this module. – user3213604 May 05 '16 at 15:57
I did not mean the commented out `var et`, I meant the one in the line `var et = et.toLowerCase().trim();`. – Bergi May 05 '16 at 16:00
I see! but I am just not reaching that point - since the failure occurs before that. However - I noticed one change you did in your suggested snippet => and that is 'RETURN" rp(opts)..... which my earlier code didn't have. all the code before RP is invoked is synchronous - so no issues and 'rp' IS a promise anyway and it should work. Let me try that:) – user3213604 May 05 '16 at 16:02
I don't understand what your problem with promisification is? My version works just like yours, always returning promises. – Bergi May 05 '16 at 16:02
@user3213604: I doesn't matter whether evaluation reaches that point or not, `var et` is a variable declaration that is hoisted to the function scope and shadows the `et` variable from the outer scope. – Bergi May 05 '16 at 16:03
AHHH of course! forgot hoisting!! anyway, let me change the code to return rp()... and see what happens. Thanks. – user3213604 May 05 '16 at 16:05
yay. The problem WAS hoisting!! Thanks a bunch @Bergi! Its often easy to get sucked into believing something complex is going wrong - when often its the simplest things. thanks again. – user3213604 May 05 '16 at 16:07

function args not visible inside return new Promise(function(resolve, reject)

1 Answers1