Const in JavaScript when to use it and is it necessary

Question

In JavaScript, I recently came across the const keyword. It appears to be used to create immutable variables, and I checked it to guarantee that it cannot be redefined (in Node.js):

const x = 'const';
const x = 'not-const';

// Will give an error: 'constant 'x' has already been defined'

I understand that it isn't yet standardized across all browsers - but I'm primarily interested in the context of Node.js V8, and I've seen that some developers/projects appear to favor it excessively when the var keyword may achieve the same result.

1. When should const be used in place of var?
2. Should it be used whenever a variable that will not be re-assigned is declared?
3. Does it make a difference if var is used instead of const or vice versa?

Answer 1

Your questions have two parts: what are the technical implications of using const instead of var, and what are the human

 implications.

The technical distinction is crucial.

In compiled languages, a constant is substituted at build time, allowing for various optimizations such as dead code elimination to further improve the code's runtime performance.

Recent (loosely defined) JavaScript engines actually compile JS code to improve efficiency, thus utilising the const keyword would alert them that the above-mentioned optimizations are available and should be performed. This leads to improved performance.

The semantics of the term are the human-related aspects.

A variable is a data structure that stores information that is likely to change.

A constant data structure stores information that will never change. Var should always be used when there is space for mistakes.

However, not all information that never changes over a program's lifetime must be declared with const.

If the information should change under various situations, use var to indicate that, even if the actual change does not occur in your

code.