JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="log">
</pre>

JavaScript

// j is never defined in the global namespace
// even though the code-block that defines j
// is never reached
;(function() {
    log("before (global)", window.j) // undefined
    log("before (local)", j) // undefed
 
    // don't define j, but it gets defined anyway
    // because of function scope and var-hoisting
    if (false) {
      var j = "hi";
    } else {
      j = "what";
    }
 
    log("after (local)", j) // what
    log("after (global)", window.j) // undefined
}());

function log(a, b) {
    $("#log").append(a + ": "+ b + "\n")
}