JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [];  // length: 0
console.log(arr);
console.log("length: " + arr.length);

arr.push(10);  // length: 1
console.log(arr);
console.log("length: " + arr.length);

arr[1] = 20;   // length: 2
console.log(arr);
console.log("length: " + arr.length);

arr["2"] = 30; // length: 3
console.log(arr);
console.log("length: " + arr.length);

arr[10] = null; // length: 11
console.log(arr);
console.log("length: " + arr.length);

arr.foo = "bar"; // length: 11
console.log(arr);
console.log("length: " + arr.length);

arr["foo"] = 42; // length: 11
console.log(arr);
console.log("length: " + arr.length);

arr["+"] = 9001; // length: 11
console.log(arr);
console.log("length: " + arr.length);

arr["-3"] = 123; // length: 11
console.log(arr);
console.log("length: " + arr.length);

var a = 20;
arr[-a] = 7;
console.log(arr);
console.log("length: " + arr.length);

var b = -20;
arr[-b] = 8;
console.log(arr);
console.log("length: " + arr.length);