% fix percent sign behavior

 - it was modulus but it should be divide by 100

src/logic/calculate.js

@@ -1,3 +1,5 @@
+import Big from 'big.js';
+
 import operate from './operate';
 import isNumber from './isNumber';
 
@@ -43,6 +45,23 @@ export default function calculate(obj, buttonName) {
     };
   }
 
+  if (buttonName === '%') {
+    if (obj.operation && obj.next) {
+      const result = operate(obj.total, obj.next, obj.operation);
+      return {
+        total: Big(result).div(Big('100')).toString(),
+        next: null,
+        operation: null,
+      }
+    }
+    if (obj.next) {
+      return {
+        next: Big(obj.next).div(Big('100')).toString(),
+      }
+    }
+    return {};
+  }
+
   if (buttonName === '.') {
     if (obj.next) {
       // ignore a . if the next number already has one

src/logic/calculate.test.js

@@ -1,5 +1,10 @@
 import calculate from './calculate';
-import { expect } from 'chai';
+import chai from 'chai';
+
+// https://github.com/chaijs/chai/issues/469
+chai.config.truncateThreshold = 0
+
+const expect = chai.expect;
 
 function pressButtons(buttons) {
   const value = {};
@@ -129,3 +134,22 @@ test(['.', '4', '-', '.', '2', '='], {
 // should clear the operator when AC is pressed
 test(['1', '+', '2', 'AC'], {});
 test(['+', '2', 'AC'], {});
+
+test(['4', '%'], {
+  next: '0.04',
+});
+
+test(['4', '%', 'x', '2', '='], {
+  total: '0.08',
+});
+
+test(['4', '%', 'x', '2'], {
+  total: '0.04',
+  operation: 'x',
+  next: '2',
+});
+
+// the percentage sign should also act as '='
+test(['2', 'x', '2', '%'], {
+  total: '0.04',
+});

src/logic/operate.js

@@ -15,8 +15,5 @@ export default function operate(numberOne, numberTwo, operation) {
   if (operation === '÷') {
     return one.div(two).toString();
   }
-  if (operation === '%') {
-    return one.mod(two).toString();
-  }
   throw Error(`Unknown operation '${operation}'`);
 }