How do I build an object counting occurrences in an Array in JavaScript?

Question

I want to count how often a number in an Array occurs. For example, in Python I can use Collections.Counter to create a dictionary of how frequently an item occurs in a list.

This is as far as I've gotten in JavaScript:

var array = [1,4,4,5,5,7];
var obj = {};
for (var i=0; i < array.length; i++) {
 /* obj[array[i]] = +=1 */ <= pseudo code
}

How can I create this frequency counter object?

Answer 1

Close but you can't increment undefined so you need to set initial value if it doesn't exist

var array = [1,4,4,5,5,7];
var obj = {};
for (var i=0; i < array.length; i++) {
  obj[array[i]] = (obj[array[i]] || 0) +1 ;
}