small fix, added tests, readme

lib/README.md

@@ -0,0 +1,14 @@
+Equation
+========
+
+Solve math expressions or create equations for repeated and complex Math tasks.
+
+```javascript
+// solve
+console.log(Equation.solve('4 * lg4 ^ 3')); // 32 
+
+// equations
+let sphereArea = Equation.equation('4 * PI * r^2');
+
+console.log(sphereArea(5)); // 314.1592653589793
+```

lib/index.js

@@ -2,6 +2,7 @@ import ReadStream from './readstream';
 import operators from './operators';
 import constants from './constats';
 import * as _ from './helpers';
+import polyfills from 'babel/polyfill';
 
 let Equation = {
   /**
@@ -49,7 +50,8 @@ let Equation = {
     let variables = [];
 
     stack.forEach(a => {
-      if (!_.isNumber(a) && !operators[a] && a === a.toLowerCase()) {
+      if (typeof a === 'string' && !_.isNumber(a) &&
+          !operators[a] && a === a.toLowerCase()) {
         // grouped variables like (y) need to have their parantheses removed
         variables.push(_.removeSymbols(a));
       }
@@ -131,7 +133,6 @@ const parseExpression = expression => {
     stack.push(record);
   }
 
-  // $0(stack);
   return parseGroups(stack);
 };
 
@@ -150,9 +151,9 @@ const parseGroups = stack => {
   // Parantheses become inner arrays which will then be processed first
   let sub = 0;
   return stack.reduce((a, b) => {
-    if (b[b.length - 1] === '(') {
+    if (b.includes('(')) {
       if (b.length > 1) {
-        _.dive(a, sub).push(b.slice(0, -1), []);
+        _.dive(a, sub).push(b.replace('(', ''), []);
       } else {
         _.dive(a, sub).push([]);
       }