How do I make an array of another array's duplicate values only in Javascript?

Question

I have an array with registrants from multiple companies (some from the same companies, some not) and I need to count how many people from the same company have registered. So I need a number that tells me how many extra people (after the first) from unique companies have registered.

Say I have an array:

var company_names = ['acme', 'acme', 'bobo', 'comanche', 'acme', 'comanche'];

and a variable:

var companies_eligible_for_discount = 0;

How would I count that 3 discounts are to be assigned? (2 for "acme" and 1 for "comanche")

Answer 1

var dupes = {};

company_names.forEach(function(v,i) {
    if( v in dupes ) {
        dupes[v]++;
        companies_eligible_for_discount++;
    }
    else dupes[v] = 0;
});

---

var dupes = {}, v, i;

for( i = 0; i < company_names.length; ++i ) {
    v = company_names[i];
    if( v in dupes ) {
        dupes[v]++;
        companies_eligible_for_discount++;
    }
    else dupes[v] = 0;
}

Answer 2

I've provided a snippet that you can run in your console to test the functionality, and created a demo function that you could use forthwith (removing console.log statements). It returns an array of company names.

Effectively what I'm doing is using the fact that Javascript has native associative arrays for objects, so I am assigning the toLowerCase fieldname (in your case the company) as the field for the associative array lookup point. If the fieldname is not already a property, then this is the first time we've added it. On the first time we add one (consider "bobo") we set it to zero. On the subsequent times, we increment it by one.

function getCompaniesOver(companyArray, discountMinimum){
  var tallyObject = {},
      retArray = [],
      has = Object.prototype.hasOwnProperty; //I'm making sure that we have a clean reference to the hasOwnProperty
  for(var k in companyArray){
    var s = companyArray[k]+''; s = s.toLowerCase();
    if (has.call(tallyObject,s)){
      tallyObject[s]++;
    } else {
      tallyObject[s] = 0;
    }
  }

  console.log(tallyObject); // for debugging insepection.

  console.log('companies with ' +companies_eligible_for_discount+ ' number of employees above 1 attending')
  console.log('--------')
  for (var k in tallyObject){
    if (tallyObject[k] >= companies_eligible_for_discount){
       console.log(k);
       retArray.push(k);
    }
  }
  console.log('--------')

  return retArray;
}

var company_names_long = ['acme', 'acme', 'bobo', 'comanche', 'acme', 'comanche', 'comanche', 'acme', 'sanford & sons', 'Sanford & Sons', 'Johnson&Johnson', 'johnson&johnson'];

var company_names = ['acme', 'acme', 'bobo', 'comanche', 'acme', 'comanche'],
    companies_eligible_for_discount = 2; //this is the range you can supply

 getCompaniesOver(company_names, companies_eligible_for_discount );

 companies_eligible_for_discount = 1;
 getCompaniesOver(company_names_long, companies_eligible_for_discount );