JavaScript allows us to redeclare variables with the same name using var, but redeclaration should not be used since it can make variables hard to trace.

In addition, this lint rule disallows redeclaration using let or const as well, although ESLint allows. This is useful because we can notice a syntax error before actually running the code.

As for functions and parameters, JavaScript just treats these as runtime errors, throwing SyntaxError when being run. It's also beneficial to detect this sort of errors statically.

Invalid:

var a = 3;
var a = 10;

let b = 3;
let b = 10;

const c = 3;
const c = 10;

function d() {}
function d() {}

function e(arg: number) {
  var arg: number;
}

function f(arg: number, arg: string) {}

Valid:

var a = 3;
function f() {
  var a = 10;
}

if (foo) {
  let b = 2;
} else {
  let b = 3;
}