Filter an array in underscorejs by using another array

Question

var list1 =[{user: "A", id: 'a'},
            {user: "B", id: 'b'},
            {user: "C", id: 'c'},
            {user: "D", id: 'd'},
            {user: "E", id: 'e'}];
var list2 = ["A","B","C"];

I have above two arrays and i want to filter list1 by using list2. My output should be [{id: 'a'},{id: 'b'},{id: 'c'}] or only ['a','b','c'].

I am doing following way to filter but not getting any result. What is wrong here?

var ids = _.filter(list1, function(id) { 
                    _.each(list2, function(name){ 
                            return id.user === name; 
                    }); 
          });

Nina Scholz · Accepted Answer · 2017-04-04 06:33:24Z

4

In plain Javascript, you could just filter the array and then map id to the result set.

var list1 =[{ user: "A", id: 'a' }, { user: "B", id: 'b' }, { user: "C", id: 'c' }, { user: "D", id: 'd' }, { user: "E", id: 'e' }],
    list2 = ["A","B","C"],
    result = list1.filter(a => list2.includes(a.user)).map(a => a.id);
    
console.log(result);

With underscore's

_.pluck for getting the value of a property,
_.contains for a check if the values is in an array and
_.filter for returning a sub set.

var list1 =[{ user: "A", id: 'a' }, { user: "B", id: 'b' }, { user: "C", id: 'c' }, { user: "D", id: 'd' }, { user: "E", id: 'e' }],
    list2 = ["A","B","C"],
    result = _.pluck(_.filter(list1, a => _.contains(list2, a.user)), 'id');
    
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>

edited Apr 4, 2017 at 6:33

answered Apr 4, 2017 at 6:28

Nina Scholz

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2 Comments

emkarachchi Over a year ago

_.pluck and _.contains are no longer available in newer versions. so use _.map instead of _.pluck and _.map instead of _.contains

Nina Scholz Over a year ago

@emkarachchi, great to know. you could edit this answer, if you like.

Derlin · Accepted Answer · 2017-04-04 06:28:41Z

3

The function inside filter should return a boolean meaning "keep it" or "don't keep it". each performs an operation on each element of an array, but doesn't return anything.

A simply fix is to use find instead of each:

var ids = _.filter(list1, function(id) {      
  return _.find(list2, function(name) { return id.user === name; });
});

To extract only the ids, use map:

ids = ids.map(function(user){ return user.id });

answered Apr 4, 2017 at 6:28

Derlin

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