Answer 1

First look at Tibor Santa's correction note, <b> should be an object if we want to access its member by member-access operator (dot)

Output can only be 151 when <c> was declared as string, rather than an array.

[ "199" ] [ 0 ] yields "199", the first array element

"199" [ 0 ] yields "1" the first character

(Edit)

[ "199" ] [ 0 ] [ 0 ] also yields the first character from the first array element

You might be questioning the correctness of the code now that you see this ...

Answer 2

Syntax error. Your variable b is invalid.

If you fix it to be a javascript object (change to curly braces):

let b = {x: 4, y: 10}.y;

Then the result is:
5 + 10 + "199" = 15199

Answer 3

Sony Hossain You can always see the output of any code by running it. That's what Code Playground is for.

Answer 4

Sony Hossain I see.
Get used to be specific in the questions. When you need to understand how the code works, ask that. Don't leave that for the readers guesses.
Ah, always include a tag with the language name.