Why is there a different output for counting each element in an array?

December 28, 2023

In the code below if count[item] is truthy increment it else set it’s initial value to 1
however I don’t understand why when I give the property a string value other than 0 (of course) the final result is a:2

const letters = ["a", "b", "a", "b", "c", "d", "a"];

let count = {"a": 0}; // 0 or "0"

letters.forEach(function (item){
   if (count[item]){
    count[item]++
   }else {
    count[item] = 1
   }
})

console.log(count)
// result
a: 3
b: 2
c: 1
d: 1

now the result here is different

let count = {"a": "asdasd"}; // or any other string

letters.forEach(function (item){
   if (count[item]){
    count[item]++
   }else {
    count[item] = 1
   }
})

// result

a: 2
b: 2
c: 1
d: 1

Can please someone explain?

>Solution :

When you insert an arbitrary string in there, you end up doing this:

count["asdasd"]++

This will result in:

{a: NaN}

Because you can’t increment a string. The next time the letter a is encountered in letters though, the if checks the following:

if (count['a']) {  // if (NaN) {

Since NaN is falsey, that check is false, and the NaN is overwritten by 1 due to count['a'] = 1. That means the first increment was lost, so the final count is one less than it should be.