Update isNan() to typeof (#748)

javascript/introduction-to-js-1/tasks/math/math2.html

@@ -37,7 +37,7 @@
 const para1 = document.createElement('p');
 para1.textContent = `Your finalResult is ${ finalResult }`;
 const para2 = document.createElement('p');
-const finalNumberCheck = isNaN(finalNumber) === false ? 'finalNumber is a number type. Well done!' : `Ooops! finalNumber is not a number.`;
+const finalNumberCheck = typeof finalNumber === 'number' ? 'finalNumber is a number type. Well done!' : `Ooops! finalNumber is not a number.`;
 para2.textContent = finalNumberCheck;
 
 section.appendChild(para1);