JSFiddle - React, Tailwind, and code Playground

JavaScript

var n = 10000000;
// The number of times the variable will initialized.
// This number is relatively low.
// For more accurate results, n = 100000000 or even n=1000000000, but this will hang the browser for a moment;

console.time("number no init");
    for (var i = n; i >= 0; i--) { var x; x = 42; };
console.timeEnd("number no init");

console.time("number init to null");
    for (var i = n; i >= 0; i--) { var x = null; x = 42; };
console.timeEnd("number init to null");

console.time("number init to number");
    for (var i = n; i >= 0; i--) { var x = 42; x = 42; };
console.timeEnd("number init to number");

console.time("string no init");
    for (var i = n; i >= 0; i--) { var x; x = 'test'; };
console.timeEnd("string no init");

console.time("string init to null");
    for (var i = n; i >= 0; i--) { var x = null; x = 'test'; };
console.timeEnd("string init to null");

console.time("string init to string");
    for (var i = n; i >= 0; i--) { var x = ''; x = 'test'; };
console.timeEnd("string init to string");

console.time("array no init");
    for (var i = n; i >= 0; i--) { var a; a = [42]; };
console.timeEnd("array no init");

console.time("array init to null");
    for (var i = n; i >= 0; i--) { var a = null; a = [42]; };
console.timeEnd("array init to null");

console.time("array init to array");
    for (var i = n; i >= 0; i--) { var a = []; a = [42]; };
console.timeEnd("array init to array");

console.time("object no init");
    for (var i = n; i >= 0; i--) { var a; a = {n:42}; };
console.timeEnd("object no init");

console.time("object init to null");
    for (var i = n; i >= 0; i--) { var a = null; a = {n:42}; };
console.timeEnd("object init to null");

console.time("object init to object");
for (var i = n; i >= 0; i--) { var a = {}; a = {n:42}; };
console.timeEnd("object init to object");

console.log('~~~~~~~~~~~~~~~~~~~~~~~~~~~');