Question

For instance, in Lua and Python:

num = 3;

On the other hand, other languages like JavaScript use ‘var’:

var num = 3;

I presume that the use of ‘var’ word should be fast to parse “var num = 3” as a variable declaration than “num = 3” (or am I wrong?).

So, why same script languages doesn’t use a “var” word to the variable declaration?
Is it irrelevant?

Answer 1

The are several possible reasons:

1. The semantics might be different (e.g. in JavaScript foo = 1 and var foo = 1 do different things)
2. It might be required for the grammar of the language (to help the parser disambiguate between multiple parse trees)
3. It might be there because the designers of the language decided that it reads more consistently this way (IIRC I read somewhere that from the parser’s perspective C# could have dropped the var in implicitly type variables, but keeping it makes the language consistent)