Create factorial function and add all tests

src/my_factorial.js

@@ -5,8 +5,35 @@
  * Distributed under terms of the MIT license.
  */
 
-function factorial() {
-    return 1;
+function factorial(n) {
+    var e = new Error('Argument should be a positive number')
+
+    if(!isNumeric(n)) {
+        throw e;
+    }    
+
+    if(!isInt(n)) {
+        throw e;
+    }
+
+    if(n < 0) {
+        throw e;
+    }
+
+    var value = 1;
+    for(var i = 2; i <= n; i++) {
+        value *= i;
+    }
+
+    return value;
+}
+
+function isNumeric(n) {
+    return !isNaN(parseFloat(n)) && isFinite(n);
+}
+
+function isInt(n){
+    return Number(n)===n && n%1===0;
 }
 
 module.exports = factorial;

test/main_test.js

@@ -10,7 +10,49 @@ var my_factorial = require('../src/my_factorial');
 exports.testCalculatesCorrectly = function(test){
 
     test.equal(1, my_factorial(0));
+    test.equal(1, my_factorial(1));
+    test.equal(2, my_factorial(2));
+    test.equal(6, my_factorial(3));
+    test.equal(120, my_factorial(5));
+    
+    test.done();
+
+}
+
+exports.testCalculatesCorrectlyForBigNumbers = function(test) {
+
+    test.equal(355687428096000, my_factorial(17));
+    test.equal(620448401733239439360000, my_factorial(24));
+    test.equal(15511210043330985984000000, my_factorial(25));
 
     test.done();
+}
+
+exports.testThrowsWithIncorrectArgument = function(test) {
+    test.throws(
+        function() {
+            my_factorial("10");
+        },
+        Error,
+        "Fail for non-numeric argument"
+
+    );
 
+    test.throws(
+        function() {
+            my_factorial(2.5);
+        },
+        Error,
+        "Fail for float argument"        
+    );
+
+    test.throws(
+        function() {
+            my_factorial(-3);
+        },
+        Error,
+        "Fail for negative argument"     
+    );
+    
+    test.done();
 }