Improve error for style mutation

src/renderers/dom/shared/ReactDOMComponent.js

@@ -170,6 +170,32 @@ function legacyReplaceProps(partialProps, callback) {
   }
 }
 
+function friendlyStringify(obj) {
+  if (typeof obj === 'object') {
+    if (Array.isArray(obj)) {
+      return '[' + obj.map(friendlyStringify).join(', ') + ']';
+    } else {
+      var pairs = [];
+      for (var key in obj) {
+        if (Object.prototype.hasOwnProperty.call(obj, key)) {
+          var keyEscaped = /^[a-z$_][\w$_]*$/i.test(key) ?
+            key :
+            JSON.stringify(key);
+          pairs.push(keyEscaped + ': ' + friendlyStringify(obj[key]));
+        }
+      }
+      return '{' + pairs.join(', ') + '}';
+    }
+  } else if (typeof obj === 'string') {
+    return JSON.stringify(obj);
+  } else if (typeof obj === 'function') {
+    return '[function object]';
+  }
+  // Differs from JSON.stringify in that undefined becauses undefined and that
+  // inf and nan don't become null
+  return String(obj);
+}
+
 var styleMutationWarning = {};
 
 function checkAndWarnForMutatedStyle(style1, style2, component) {
@@ -202,8 +228,8 @@ function checkAndWarnForMutatedStyle(style1, style2, component) {
     'the `render` %s. Previous style: %s. Mutated style: %s.',
     componentName,
     owner ? 'of `' + ownerName + '`' : 'using <' + componentName + '>',
-    JSON.stringify(style1),
-    JSON.stringify(style2)
+    friendlyStringify(style1),
+    friendlyStringify(style2)
   );
 }
 

src/renderers/dom/shared/__tests__/ReactDOMComponent-test.js

@@ -128,8 +128,8 @@ describe('ReactDOMComponent', function() {
         'Warning: `div` was passed a style object that has previously been ' +
         'mutated. Mutating `style` is deprecated. Consider cloning it ' +
         'beforehand. Check the `render` of `App`. Previous style: ' +
-        '{"border":"1px solid black"}. Mutated style: ' +
-        '{"border":"1px solid black","position":"absolute"}.'
+        '{border: "1px solid black"}. Mutated style: ' +
+        '{border: "1px solid black", position: "absolute"}.'
       );
 
       style = {background: 'red'};
@@ -147,6 +147,23 @@ describe('ReactDOMComponent', function() {
       expect(console.error.argsForCall.length).toBe(2);
     });
 
+    it('should warn semi-nicely about NaN in style', function() {
+      spyOn(console, 'error');
+
+      var style = {fontSize: NaN};
+      var div = document.createElement('div');
+      ReactDOM.render(<span style={style}></span>, div);
+      ReactDOM.render(<span style={style}></span>, div);
+
+      expect(console.error.argsForCall.length).toBe(1);
+      expect(console.error.argsForCall[0][0]).toEqual(
+        'Warning: `span` was passed a style object that has previously been ' +
+        'mutated. Mutating `style` is deprecated. Consider cloning it ' +
+        'beforehand. Check the `render` using <span>. Previous style: ' +
+        '{fontSize: NaN}. Mutated style: {fontSize: NaN}.'
+      );
+    });
+
     it('should update styles if initially null', function() {
       var styles = null;
       var container = document.createElement('div');