Regarding your first question: there is no difference between your 1st and 2nd example.
For your second question, let's look at this small example:
charCount = { a: 2, b:3 }
char = 'a'
print(charCount[char]) // 2
print(charCount.char) // error
As pointed out in the comment, when you call charCount[char], it will dynamically access object property using variable. What it does is just simply replace the variable char with its current value (which is a in this case). So, the call becomes charCount['a'], and you got the result.
When you call charCount.char, it literally tries to access the property named char of the object charCount. In this case, that property does not exist, which is why you got an error.
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