JS logical issues in condition

Question

I am building a chat application with Kinvey mbaas as database. I have a collection storing chats with the following data columns: _id, firstUser, otherUsers, history. So far the idea is: when a message is posted GET is requested to find if there is chat between the two users. GET takes the whole collection and iterates over entries and checks if firstUser and otherUsers match. This is where the problem comes: they don't match ever. Code bellow:

for (let entity = 0; entity < response.length; entity++) {
console.log('DEV_firstUser: ' 
                 + response[entity]['firstUser'] + '|' + firstUser);
    console.log('DEV_otherUsers: |' 
                 + response[entity]['otherUsers'] + '|' + otherUsers + "|");

    console.log(response[entity]['firstUser'] === firstUser);
    console.log(response[entity]['otherUsers'] === otherUsers);
    // The problematic condition - the logs above are demonstrations.
    if (response[entity]['firstUser'] === firstUser 
            && response[entity]['otherUsers'] === otherUsers) {
                id = response[entity]['_id'];
                console.log('DEV_id: ' + id);
                index = entity;
                console.log(response[index][id]);
            }
    }

'response' is the collection - array of objects from what I can see. 'entity' is straightforward - each entity inside the collection. 'otherUsers' is array.

This is what I get on the console:

DEV_firstUser: alex|alex
DEV_otherUsers:|Ganka|Ganka|
true
false

Please boil this down for a couple dozen lines of code. – Aug 20 '16 at 16:08 — , Aug 20 '16 at 16:08

score 2 · Accepted Answer · edited May 23 '17 at 11:59

There are two issues with your code (disclaimer: answer is based on the original version of the question).

firstUser comparison

console.log('DEV_firstUserComparison: ' 
    + response[entity]['firstUser'] == firstUser);

yields false, because + has a higher precedence than == or ===, so you're actually comparing a string to a user object (note how you also don't see your "DEV_firstUserComparison:" in the output).

Instead, put the comparison in brackets:

console.log('DEV_firstUserComparison: ' 
        + (response[entity]['firstUser'] == firstUser));

Short snippet to demonstrate the issue:

console.log('foo' === 'foo');      //true
console.log('f' + 'oo' === 'foo'); //true

otherUsers comparison

Even after fixing the first issue

console.log('DEV_otherUsersComparison: ' 
    + (response[entity]['otherUsers'] == otherUsers));

still returns false. This is due to the fact that otherUsers is an array and you cannot simply compare those with ==.

Instead, you have a few other options. See the following questions on Stack Overflow for details:

Basically: either write your own comparison method or convert both arrays to string before comparing (which will not work when the elements are ordered differently).

The easiest way for your use case would probably be:

response[entity]['otherUsers'].length == otherUsers.length
  && response[entity]['otherUsers'].every(function(v, i) { return v == otherUsers[i]})

Again, a short snippet to demonstrate:

var array1 = ['foo', 'bar'];
var array2 = ['foo', 'bar'];

function eq(a1, a2) {
  return a1.length == a2.length && a1.every(function(v, i) {
    return v === a2[i]
  });
}

console.log(array1 == array2);   //false
console.log(eq(array1, array2)); //true

Well the data on the server is created by previous execution of the exact same function, therefore it should be equal. I know it doesn't work, because I look at the console prints for each iteration. If you would look at GitHub: https://github.com/AwesomeChat-devs/Awesome-chat/tree/Alex — Alex, Aug 20 '16 at 15:09
Meanwhile I will try and add more information on the post for easier clarification. — Alex, Aug 20 '16 at 15:18
Yes it is an array, since I want to have the functionality of adding multiple users in one chat. But I can't see the problem, as long as it is saved in the BD as array. — Alex, Aug 20 '16 at 15:43
See my updated answer. You cannot simply compare arrays with `==`. — Marvin, Aug 20 '16 at 16:03
I'll apply that solution right away. I boiled down my question as was suggested by @torazaburo and thus reverted it to it original form. Thanks for the tip :) — Alex, Aug 20 '16 at 19:08

JS logical issues in condition

1 Answers1

firstUser comparison

otherUsers comparison