JavaScript Group By Array

You can loop through each index and save it in a dictionary and increment it when every that key is found.

count = {};
for(a in array){
  if(count[array[a]])count[array[a]]++;
  else count[array[a]]=1;
}

Output will be:

Boat: 1
Car: 2
Truck: 2

var arr = [ 'Car', 'Car', 'Truck', 'Boat', 'Truck' ];
var hist = {};
arr.map( function (a) { if (a in hist) hist[a] ++; else hist[a] = 1; } );
console.log(hist);

results in

{ Car: 2, Truck: 2, Boat: 1 }

This works, too:

hist = arr.reduce( function (prev, item) { 
  if ( item in prev ) prev[item] ++; 
  else prev[item] = 1; 
  return prev; 
}, {} );