Fix #41: NaN value when reducing division with custom property

Author: Semigradsky

src/__tests__/index.js

@@ -154,6 +154,13 @@ test(
   'calc(-90px + var(--mouseX))'
 )
 
+test(
+  'should ignore calc with css variables (6)',
+  testFixture,
+  'calc(var(--popupHeight) / 2)',
+  'calc(var(--popupHeight) / 2)'
+)
+
 test(
   'should reduce calc with newline characters',
   testFixture,

src/lib/reducer.js

@@ -223,10 +223,12 @@ function reduceDivisionExpression(node, precision) {
     }
     return node
   }
-
-  // value / value
-  node.left.value /= node.right.value
-  return node.left
+  // something / value
+  else if (isValueType(node.left.type)) {
+    node.left.value /= node.right.value
+    return node.left
+  }
+  return node
 }
 
 function reduceMultiplicationExpression(node) {