Create a new DEMA indicator

Fix issue #1812

strategies/indicators/doubleEMA.js

@@ -0,0 +1,33 @@
+// Richard King, 2 Feb
+// Built as a fix to the DEMA.js indicator that wasn't accurate.
+
+// required indicators
+var EMA = require('./EMA.js');
+
+var Indicator = function(period) {
+  this.input = 'price'	
+  this.result = false;
+  this.ema = new EMA(period);
+  this.doubleEMA = new EMA(period);
+}
+
+// add a price and calculate the EMAs and
+// the diff for that price
+Indicator.prototype.update = function(price) {
+  this.ema.update(price);
+  var EMA = this.ema.result;
+  this.doubleEMA.update(EMA);
+  this.calculateDoubleEMA();
+}
+
+// Formula https://www.investopedia.com/ask/answers/121814/what-double-exponential-moving-average-dema-formula-and-how-it-calculated.asp
+Indicator.prototype.calculateDoubleEMA = function() {
+
+  var EMA = this.ema.result;
+  var doubleEMA = this.doubleEMA.result;
+
+  this.result = (2 * EMA) - doubleEMA;
+
+}
+
+module.exports = Indicator;