Add simple example of isNaN()

.gitignore

@@ -1,2 +1,3 @@
 node_modules
-.DS_Store
+.DS_Store
+package-lock.json

implementations/isNaN.js

@@ -0,0 +1,53 @@
+/**
+ *
+  The value of NaN is NaN. This property has the attributes { 
+      [[Writable]]: false, 
+      [[Enumerable]]: false, 
+      [[Configurable]]: false 
+    }
+
+    The Number type has exactly 18437736874454810627 (that is, 264−253+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard 
+    for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 253−2) distinct “Not-a-Number” values of the IEEE Standard 
+    are represented in ECMAScript as a single special NaN value.
+
+    NaN is a property of the global object.
+
+    The initial value of NaN is Not-A-Number — the same as the value of Number.NaN. 
+    In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.
+
+    It is rather rare to use NaN in a program. It is the returned value when Math functions fail (Math.sqrt(-1)) or when 
+    a function trying to parse a number fails (parseInt("blabla"))
+
+    isNaN(NaN);       // true
+    isNaN(undefined); // true
+    isNaN({});        // true
+
+    isNaN(true);      // false
+    isNaN(null);      // false
+    isNaN(37);        // false
+
+    ### Examples
+    // strings
+    isNaN('37');      // false: "37" is converted to the number 37 which is not NaN
+    isNaN('37.37');   // false: "37.37" is converted to the number 37.37 which is not NaN
+    isNaN("37,5");    // true
+    isNaN('123ABC');  // true:  parseInt("123ABC") is 123 but Number("123ABC") is NaN
+    isNaN('');        // false: the empty string is converted to 0 which is not NaN
+    isNaN(' ');       // false: a string with spaces is converted to 0 which is not NaN
+
+    // dates
+    isNaN(new Date());                // false
+    isNaN(new Date().toString());     // true
+
+    // This is a false positive and the reason why isNaN is not entirely reliable
+    isNaN('blabla');    // true: "blabla" is converted to a number. 
+                        // Parsing this as a number fails and returns NaN
+
+    For more references see:
+        https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN
+        https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN
+*/
+
+Number.prototype.isNaN = function(testValue){
+  return isNaN(testValue);
+};