[fix] StyleSheet handling of default 'borderWidth'

Problem:

The default border width should be '0px', but setting a border width
value to 'null' would override the default and result in no border width
being applied to the element. This could cause unwanted user agent
styles to be applied.

Solution:

createReactDOMStyle now replaces border width values of 'null' with
'0px'.

Fix #697

src/apis/StyleSheet/__tests__/createReactDOMStyle-test.js

@@ -22,6 +22,23 @@ describe('apis/StyleSheet/createReactDOMStyle', () => {
     expect(firstStyle).toEqual(secondStyle);
   });
 
+  describe('borderWidth styles', () => {
+    test('defaults to 0 when "null"', () => {
+      expect(createReactDOMStyle({ borderWidth: null })).toEqual({
+        borderTopWidth: '0px',
+        borderRightWidth: '0px',
+        borderBottomWidth: '0px',
+        borderLeftWidth: '0px'
+      });
+      expect(createReactDOMStyle({ borderWidth: 2, borderRightWidth: null })).toEqual({
+        borderTopWidth: '2px',
+        borderRightWidth: '0px',
+        borderBottomWidth: '2px',
+        borderLeftWidth: '2px'
+      });
+    });
+  });
+
   describe('flexbox styles', () => {
     test('flex defaults', () => {
       expect(createReactDOMStyle({ display: 'flex' })).toEqual({

src/apis/StyleSheet/createReactDOMStyle.js

@@ -52,6 +52,14 @@ const colorProps = {
   color: true
 };
 
+const borderWidthProps = {
+  borderWidth: true,
+  borderTopWidth: true,
+  borderRightWidth: true,
+  borderBottomWidth: true,
+  borderLeftWidth: true
+};
+
 const systemFontStack =
   '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Ubuntu, "Helvetica Neue", sans-serif';
 
@@ -141,13 +149,26 @@ const createReducer = (style, styleProps) => {
   let hasResolvedTextShadow = false;
 
   return (resolvedStyle, prop) => {
-    const value = normalizeValue(prop, style[prop]);
+    let value = normalizeValue(prop, style[prop]);
+
+    // Make sure the default border width is explicitly set to '0' to avoid
+    // falling back to any unwanted user-agent styles.
+    if (borderWidthProps[prop]) {
+      value = value == null ? normalizeValue(null, 0) : value;
+    }
+
+    // Normalize color values
+    if (colorProps[prop]) {
+      value = processColor(value);
+    }
+
+    // Ignore everything else with a null value
     if (value == null) {
       return resolvedStyle;
     }
 
     switch (prop) {
-      // ignore React Native styles
+      // Ignore some React Native styles
       case 'aspectRatio':
       case 'elevation':
       case 'overlayColor':
@@ -251,23 +272,17 @@ const createReducer = (style, styleProps) => {
       }
 
       default: {
-        // normalize color values
-        let finalValue = value;
-        if (colorProps[prop]) {
-          finalValue = processColor(value);
-        }
-
         const longFormProperties = styleShortFormProperties[prop];
         if (longFormProperties) {
           longFormProperties.forEach((longForm, i) => {
-            // the value of any longform property in the original styles takes
-            // precedence over the shortform's value
+            // The value of any longform property in the original styles takes
+            // precedence over the shortform's value.
             if (styleProps.indexOf(longForm) === -1) {
-              resolvedStyle[longForm] = finalValue;
+              resolvedStyle[longForm] = value;
             }
           });
         } else {
-          resolvedStyle[prop] = finalValue;
+          resolvedStyle[prop] = value;
         }
       }
     }