Filtering two MongoDB arrays not working (ES6)

Question

I am asking MongoDB for two arrays of objectIDs, and then trying to compare the two arrays to find the difference.

I'm also passing the arrays through express middleware using res.locals.

Here is the code:

let questions = res.locals.questions;
let answers = res.locals.answers;
let outstanding = questions.filter(oid => !answers.includes(oid));

console.log("The outstanding oids: "+outstanding)

The result is always blank.

Maybe something to do with MongoDB's format, or the res.locals format? Appreciate any suggestions.

Here is what the two arrays look like:

questions: ["5f522bc55dd8993e58283526","5f522ab45dd8993e58283521","5f522ba65dd8993e58283525","5f522a5e5dd8993e5828351f","5f47a9a0b1764c3e285d4666"]

answers: ["5f522ab45dd8993e58283521","5f522bc55dd8993e58283526"]

When I perform typeof() test it returns "object". They are objectIDs in mongoose, so I assume they are objectIDs? — Izzi
– Izzi, Commented Sep 9, 2020 at 7:57

Ilijanovic · Accepted Answer · 2020-09-09 08:05:34Z

1

I guess your values inside of the array are ObjectIDs and not strings so you cannot just compare them with like == operator.

Thankfully mongoose provides .equals() to compare object IDs.

let outstanding = questions.filter(oid => !answers.some(answer => oid.equals(answer)));

http://mongodb.github.io/node-mongodb-native/api-bson-generated/objectid.html#equals

edited Sep 9, 2020 at 8:05

answered Sep 9, 2020 at 7:59

Ilijanovic

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2 Comments

Izzi Over a year ago

Oh wow, that makes sense. Except, I want to return the ones that do not find a match, so needs to have !answers.some...etc

Ilijanovic Over a year ago

@IsraelPeck answers.some returns either true or false, then just invert it !

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