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; 
                    }); 
          });

Answer 1

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> 

Answer 2

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 });