Using lodash, why is the map method within flow not working?

Question

I have a users object am trying to use the lodash map() method on it to have it return only the userIds, while filtering out any users with the currentUserId. I wanted to avoid using chain() since it pulls in the entire library, so it seemed that the flow() method is perfect, yet it's not mapping to an array of Id's.

import {
  map, filter, flow,
} from 'lodash';

const users = {
    123: {
      uid: 123
    },
    456: {
      uid: 456
    }
};

const currentUserId = 123;

const userIds = _.flow(
  _.map(user => user.uid),
  _.filter(userId => userId !== currentUserId),
)(users);

Unfortunately, this is returning the same object as was passed into it. How can I get an array with the ids of all the users that are not the current user?

user_3980115 · Answer

The answer applies for the standard version of lodash. Please see @Vlaz's answer for a look at the functional programming version of lodash.

When you write _.map(user => user.uid) it is actually invoking the function at that time. What you're really attempting to do is to create function that is similar to _.map, but has one of its arguments already set.

Fortunately, Lodash has a built-in to handle this situation - _.partialRight

const userIds = _.flow(
  _.partialRight(_.map, user => user.uid)
  _.partialRight(_.filter, userId => userId !== currentUserId),
)(users);

Documentation

Alternatively if you wish to use plain JS rather than importing a new function, you can simply wrap it in an anonymous function to pass the arguments properly.

const userIds = _.flow(
  (users) => _.map(users, user => user.uid),
  (users) => _.filter(users, userId => userId !== currentUserId),
)(users);

user_3689450 · Answer

It appears that you are not using the functional version of lodash.

var users = {
    123: {
      uid: 123
    },
    456: {
      uid: 456
    }
};
var currentUserId = 123;

const userIds = _.flow(
  _.map(user => user.uid),
  _.filter(userId => userId !== currentUserId)
)(users);

console.log("result with normal lodash", userIds);

var users = {
    123: {
      uid: 123
    },
    456: {
      uid: 456
    }
};
var currentUserId = 123;

const userIds = _.flow(
  _.map(user => user.uid),
  _.filter(userId => userId !== currentUserId)
)(users);

console.log("result with lodash FP", userIds);

As per the Lodash FP guide, you should be importing that into your files

// Load the fp build.
var fp = require('lodash/fp');

In your case, you can just import just map, filter, and flow but you should get their lodash FP variants.

user_1175966 · Answer

Can do this with a simple native JS reduce of array created by Object#values() with no more code than using lodash

const users = {123: {uid: 123},456: {uid: 456}};

const currId = 123;


const userIds = Object.values(users)
            .reduce((a, {uid}) => uid !== currId ? a.concat(uid): a, [])
    

console.log(userIds)

Using lodash, why is the map method within flow not working?

3 Answers