Add Sum method (#34)

.gitignore

@@ -71,3 +71,6 @@ typings/
 
 # Serverless directories
 .serverless
+
+# vscode settings
+.vscode/settings.json

index.d.ts

@@ -13,4 +13,5 @@ interface Array<T> {
   TakeWhile<T>(predicate: (X: T) => boolean): Array<T>;
   Union<T>(adder: Array<T>): Array<T>;
   Where<T>(predicate: (X: T) => boolean): Array<T>;
+  Sum<number>(): number;
 }

index.js

@@ -11,6 +11,7 @@ const TakeWhile = require('./src/TakeWhile');
 const ToDictionary = require('./src/ToDictionary');
 const Union = require('./src/Union');
 const Where = require('./src/Where');
+const Sum = require('./src/Sum');
 
 const bindAll = function() {
   Array.prototype.Aggregate = Aggregate;
@@ -26,6 +27,7 @@ const bindAll = function() {
   Array.prototype.ToDictionary = ToDictionary;
   Array.prototype.Union = Union;
   Array.prototype.Where = Where;
+  Array.prototype.Sum = Sum;
 };
 
 module.exports = bindAll;

src/Sum.js

@@ -0,0 +1,13 @@
+/**
+ * @template number?
+ * @returns {number}
+ */
+function Sum() {
+  let sum = 0;
+  for (let x of this) {
+    if (x != null) sum += x;
+  }
+  return sum;
+}
+
+module.exports = Sum;

test/Sum.spec.js

@@ -0,0 +1,21 @@
+const expect = require('chai').expect;
+require('..')();
+
+describe('Array#prototype#Sum', () => {
+  const input = [1, 2, 3];
+  const input_with_null = [1, 2, null, 3];
+
+  it('should display the correct sum', () => {
+    const expected = input.reduce((prev, x) => x + prev);
+
+    const actual = input.Sum();
+    expect(actual).to.eql(expected);
+  });
+
+  it('should not include null values in sum', () => {
+    const expected = input_with_null.reduce((prev, x) => x + prev);
+
+    const actual = input_with_null.Sum();
+    expect(actual).to.eql(expected);
+  });
+});