countBy Method into a Function using Javascript

I'm trying to write a countBy function, identical to the countBy method in Javascript. Here is what I have so far:

var array = [1,2,4,4,5];
function countBy(collection, func) { var object = {} ; for (var i=0;i<=0; i++) { for (var key in object) { if (func(collection[i]) === object.key) { object.key ++; } else { object.key = 1; } } } return object
}
alert(countBy(array, function(n) {return Math.floor(n);}));

What the code intends to do is search through the collection array to see if a value there matches a key in object. If it has found a match in the collection array, increment that key value by one. If it has not found one, create a new key value. Therefore, the result that should be alerted is: {4:2, 1:1, 2:1, 5:1}. But it seems like my output is [object Object]. What am I doing wrong??

2 Answers

for (var i=0;i<=0; i++) {

This will run exactly once instead of iterating through the collection.

for (var key in object) { if (func(collection[i]) === object.key) {

This does the wrong thing; you should call func only once in each iteration and then look up the result of the function call in your object.

object.key ++;

This doesn't do what you want; it's better to use the array dereference operator instead.

function countBy(collection, func)
{ var object = Object.create(null); collection.forEach(function(item) { var key = func(item); if (key in object) { ++object[key]; } else { object[key] = 1; } }); return object;
}

function countby(arr, selector) { selector = selector ?? (x => x) // default is identity const f = (acc, x) => { let k = selector(x); acc[k] = (acc[k] ?? 0) + 1; return acc } return arr.reduce(f, {})
};
let r
r = countby([2, 3, 1, 3, 4, 4])
console.log(r)
r = countby([{a:2}, {a:3, b:"bunny"}, {a:2}], (x => x.a))
console.log(r)