JSFiddle - React, Tailwind, and code Playground

JavaScript

function f(x) {
  if (x === undefined) console.log('x is undefined [=== undefined].');
  else console.log('x is not undefined [=== undefined.]');

  if (typeof(x) === 'undefined') console.log('x is undefined [typeof(x) === \'undefined\'].');
  else console.log('x is not undefined [typeof(x) === \'undefined\'].');

  // This will throw exception because what the hell is j? It is nowhere to be found.
  try {
    if (j === undefined) console.log('j is undefined [=== undefined].');
    else console.log('j is not undefined [=== undefined].');
  } catch (e) {
    console.log('Error!!! Cannot use [=== undefined] because j is nowhere to be found in our source code.');
  }

  // However this will not throw exception
  if (typeof j === 'undefined') console.log('j is undefined (typeof(x) === \'undefined\'). We can use this check even though j is nowhere to be found in our source code and it will not throw.');
  else console.log('j is not undefined [typeof(x) === \'undefined\'].');
};
console.log('Output from f():');
f();
console.log('------------------------------------------');
console.log('Output from f(null):');
f(null);
console.log('------------------------------------------');
console.log('Output from f(1):');
f(1);