Why do I need to test for the existence of a property on an object before incrementing it, when counting the number of letters in an object? - node.js

Question

I came across a youtube video showing how to count each letters' occurrence using Javascript, eg when the input is "hello", the function will return
{h:1 e:1 l:2 o:1}
Like this:
const obj = {};
for (let i = 0; i < wordInput.length; i++) {
const char = wordInput[i];
if (!obj[char]) {
obj[char] = 0;
}
obj[char]++;
}
Why do we need the if statement? He said something like "some of these are undefined", but I'm not really sure what that means, can you explain why we need the if (!obj[char])?

Answer 1

In the first run of the loop
i = 0
-----------
char = 'h'
obj = {}
obj[char] ==> obj['h'] ==> undefined
So if you try to do
obj[char]++ ==> obj['h']++ , it will throw error as the value is undefined. Instead initialize it to 0 first, using the if statement, then increment it.

Answer 2

The obj object starts out empty. Trying to access any property of it will return undefined, and if you try to use ++ on undefined, you'll get NaN (Not a Number), which is not what you want:
const obj = {};
obj.foo++;
console.log(obj.foo);
To fix that, before incrementing, check to see if the property exists first, and if it doesn't, set it to 0. 0 is incrementable; undefined is not.
const obj = {};
if (!obj.foo) {
obj.foo = 0;
}
obj.foo++;
console.log(obj.foo);
The code in your question does the same sort of thing, except it iterates over chars (characters of the wordInput string). If the character hasn't been put as a property of the object yet, it must be set before incrementing, otherwise the resulting object will contain useless NaN values:
const wordInput = 'foo bar';
const obj = {};
for (let i = 0; i < wordInput.length; i++) {
const char = wordInput[i];
obj[char]++;
}
console.log(obj);

Answer 3

The if statement is for the first entry of a character. Before 'a' comes, there's no property in obj called 'a'. So obj[char] defines a property and sets its value to 0. Before that, there's no property, so it's undefined. You can't apply operator to undefined.
This is how obj looks like when the first letter comes in 'hello'
before the if statement:
obj = {};
so obj['h'] = undefined;
It enters the if statement, then:
obj = { 'h' : 0 }