Find number of occurences of string elements in an Array using lodash or underscore js

Question

I have an array in the following format:

var array = [ 
  { id: '555weibo' },
  { id: '578weibo' },
  { id: '111facebook' },
  { id: '123facebook' },
  { id: '145facebookpage' },
  { id: '145facebookpage' },
  { id: '766facebook' },
  { id: '242facebook' },
  { id: '432twitter' },
  { id: '432exing' }
 ];

I need to find the number of occurrences of facebook, twitter, xing, and weibo inside that array. eg:

{
  weibo: 2,
  facebook: 6,
  twitter: 1,
  xing: 1
}

I've searched for a solution, but nothing seems to work.

The following code doesn't give the expected result. Your help is much appreciated. Thank you.

var filtered = _.map(diff, function(value, key) {
   return { id: key, count:value };
});

Answer 1

Here's how you could do it using lodash:

_.countBy(array, _.flow(
    _.method('id.replace', /^\d+/, ''),
    _.method('replace', /page$/, '')
));

• countBy() returns an object with counts as values. The keys are generated by the passed in callback, so this is where you can parse and manipulate the keys you want to count by.
• flow() generates a function, usually for callbacks to map() and so on. It takes an arbitrary number of functions as arguments. The input is filtered through each of these functions and the result is returned. This is a powerful way to compose callbacks because it's easy to rearrange/add/remove it's parts.
• method() returns a function that's invoked on the given object. You want to replace the beginning numbers with an empty string here.
• method() is doing the same thing here, except it's removing page from the end of the string. Notice how there's no id prefix since it's just being passed the result of the function in front of it.

Answer 2

You must specify the possible keys as there's no way to guess them in the initial array.

Here's a solution, you don't need any library:

var filtered = array.reduce(function(m,v){
    for (var k in m) if (~v.id.indexOf(k)) m[k]++;
    return m;
},{facebook:0, weibo:0, twitter:0, xing:0});

Demonstration