JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
<h1>Test Results</h1>
<ul>
</ul>

CSS

* { font-family: sans-serif; line-height: 150% }
li { margin-bottom: 20px }
li.success { color: green }
li.error { color: red }

JavaScript

// ------------------------------------------
// FILL IN YOUR ANSWERS HERE AND CLICK RUN!
// ------------------------------------------

function doubleInteger(i) {
    // i will be an integer. Double it and return it.
    return i * 2;
}

function isNumberEven(i) {
    // i will be an integer. Return true if it's even, and false if it isn't.
    return i % 2 === 0;
}

function getFileExtension(fileName) {
  const index = fileName.indexOf(".");
  if (index === -1) return false;
  const afterDot = fileName.slice(index + 1, fileName.length);
  const latterExtension = getFileExtension(afterDot);
  if (latterExtension) {
  	return latterExtension;
  } else {
  	return afterDot;
  }
  // JSFiddle doesnt like it, but I prefer:
  // return latterExtension ? latterExtension : afterDot;
}

function longestString(i) {
    // i will be an array.
    // return the longest string in the array
    const stringLengths = i.map((str)=>{
    	if (typeof str === 'string') {
      	return {str: str, length: str.length};
      } else {
      	return {str: "", length: 0}
      }
    });
    
    const sorted = stringLengths.sort((el1,el2)=>{
    	return el1.length < el2.length;
    });
    
    return sorted[0].str;
}

function arraySum(i) {
  // i will be an array, containing integers, strings and/or arrays like itself.
  // Sum all the integers you find, anywhere in the nest of arrays.
	return i.reduce((sum, value)=>{
  	if (Array.isArray(value)) sum += arraySum(value);
  	if (Number.isInteger(value)) sum += value;
    return sum;
  },0);
}

// ------------------------------------------
// TESTS BELOW - DO NOT WRITE UNDER THIS LINE
// ------------------------------------------

expect('doubleInteger(5)', doubleInteger(5), 10);
expect('doubleInteger(7)', doubleInteger(-10), -20);
expect('doubleInteger(0)', doubleInteger(0), 0);

expect('isNumberEven(4)', isNumberEven(4), true);
expect('isNumberEven(7)', isNumberEven(7), false);
expect('isNumberEven(0)', isNumberEven(0),...