Skip parsing undefined values

src/core/createSystem.ts

@@ -55,6 +55,12 @@ export const createParser = (
     const parseEntry = (obj: SomeObject, key: string) => {
       const systemConfig = config[key];
       let propValue: any = obj[key];
+
+      // See issue #35
+      if (propValue === undefined) {
+        return {};
+      }
+
       const scale = get(props.theme, systemConfig.scale);
 
       if (typeof propValue === 'function') {

src/core/tests/parser.test.ts

@@ -6,6 +6,7 @@ const theme = {
     secondary: 'papayawhip',
   },
   fontSize: [0, 4, 8, 16],
+  space: [0, 4, 8, 16],
   breakpoints: [40, 52, 64].map((n) => n + 'em'),
   systemPropsId: false,
 };
@@ -34,6 +35,8 @@ const parser = system({
   },
   fontSize: true,
   border: true,
+  mx: { properties: ['marginLeft', 'marginRight'], scale: 'space' },
+  marginLeft: { property: 'marginLeft', scale: 'space' },
 });
 
 test('parses pseudo selectors', () => {
@@ -233,3 +236,13 @@ test('parses raw function values at each breakpoint', () => {
     },
   });
 });
+
+test('ignores undefined values - issue #35', () => {
+  const styles = parser({
+    theme,
+    color: undefined,
+    mx: '$2',
+    marginLeft: undefined,
+  });
+  expect(styles).toMatchObject({ marginLeft: 8, marginRight: 8 });
+});