Immutability in javascript with numbers

test.js

// That’s a slightly different thing. Number in JS are always immutable already
let a = 4;
function getA() {
  return a++
}

let v1 = getA() // v1 = 4
let v2 = getA() // v1 = 4, v2 = 5
let v3 = getA() // v1 = 4, v2 = 5, v3 = 6
// Because the value of the number doesn’t change in place.

// By contrast:
let ary = []
return getAry() {
  ary.push(ary.length)
  return ary
}

let v1 = getAry() // v1 = [0]
let v2 = getAry() // v1 = [0,1], v2 = [0,1]
let v3 = getAry() // v1 = [0,1,2], v2 = [0,1,2], v3 = [0,1,2]

//but the fact that numbers might be immutable doesn’t mean the variable reference is. would be.
// a is actually set to 5 after the first call but the operation returns the existing value before inc’ing it.

// i’d say the lesson there is to rely on named functions 
// for transformation that are referentially transparent and idempotent
let inc = (x)=> x + 1

// there’s a difference between a mutable variable and a mutable value.
// Generally you want to avoid both, but mutable values are far worse than mutable variables.

// A mutable variable can only damage its own lexical scope,
// while a mutable value can poison a whole application.

// mutable values are array situation
// mutable variables are things without the const variable