Gist for When arguments are mutable

arguments_relationship-strict_mode.js

// using strict mode
var fn3 = function (a) {
  "use strict";
  a = "foo"
  console.log(arguments[0])
}

// arguments, and the arguments object no longer track each other
fn3(2) // 2

// containing rest, defaults or destructured parameters
var fn4 = function (a = 0) {
  arguments[0] = "bar"
  console.log(a)
}

fn4(2) // 2

var fn5 = function (...args) {
  arguments[0] = "bar"
  console.log(args[0])
}

fn5(2) // 2

var fn6 = function () {
  const [a] = arguments
  arguments[0] = "bar"
  console.log(a)
}

fn6(2) // 2

arguments_relationship.js

// non-strict function, not containing rest,
// defaults or destructured parameters
var fn1 = function (a) {
  a = "foo"
  console.log(arguments[0])
}

// argument `a` tracks arguments[0]
fn1(2) // "foo"

var fn2 = function (a) {
  arguments[0] = "bar"
  console.log(a)
}

// arguments[0] tracks argument `a`
fn2(2) // "bar"

param_assignment.js

var bar = function (a) {
  var aa = arguments[0]
  a = 'foo'
  console.log(aa)
}

//this works as expected
bar(2) // 2

param_reassign.js

var bar = function (a) {
  a = 'foo'
  console.log(arguments[0])
}

bar(2)