Ignore children with clashing keys

Fixes #566.

src/utils/ReactChildren.js

@@ -20,8 +20,8 @@
 
 var PooledClass = require('PooledClass');
 
-var invariant = require('invariant');
 var traverseAllChildren = require('traverseAllChildren');
+var warning = require('warning');
 
 var twoArgumentPooler = PooledClass.twoArgumentPooler;
 var threeArgumentPooler = PooledClass.threeArgumentPooler;
@@ -86,16 +86,21 @@ PooledClass.addPoolingTo(MapBookKeeping, threeArgumentPooler);
 function mapSingleChildIntoContext(traverseContext, child, name, i) {
   var mapBookKeeping = traverseContext;
   var mapResult = mapBookKeeping.mapResult;
-  var mappedChild =
-    mapBookKeeping.mapFunction.call(mapBookKeeping.mapContext, child, i);
-  // We found a component instance
-  invariant(
-    !mapResult.hasOwnProperty(name),
+
+  var keyUnique = !mapResult.hasOwnProperty(name);
+  warning(
+    keyUnique,
     'ReactChildren.map(...): Encountered two children with the same key, ' +
-    '`%s`. Children keys must be unique.',
+    '`%s`. Child keys must be unique; when two children share a key, only ' +
+    'the first child will be used.',
     name
   );
-  mapResult[name] = mappedChild;
+
+  if (keyUnique) {
+    var mappedChild =
+      mapBookKeeping.mapFunction.call(mapBookKeeping.mapContext, child, i);
+    mapResult[name] = mappedChild;
+  }
 }
 
 /**

src/utils/__tests__/ReactChildren-test.js

@@ -320,17 +320,19 @@ describe('ReactChildren', function() {
     }).not.toThrow();
   });
 
-  it('should throw if key provided is a dupe with explicit key', function() {
+  it('should warn if key provided is a dupe with explicit key', function() {
     var zero = <div key="something"/>;
-    var one = <div key="something" />;
+    var one = <span key="something" />;
 
-    var mapFn = function() {return null;};
+    var mapFn = function(component) { return component; };
     var instance = (
       <div>{zero}{one}</div>
     );
 
-    expect(function() {
-      ReactChildren.map(instance.props.children, mapFn);
-    }).toThrow();
+    spyOn(console, 'warn');
+    var mapped = ReactChildren.map(instance.props.children, mapFn);
+
+    expect(console.warn.calls.length).toEqual(1);
+    expect(mapped).toEqual({'.$something': zero});
   });
 });

src/utils/flattenChildren.js

@@ -18,8 +18,8 @@
 
 "use strict";
 
-var invariant = require('invariant');
 var traverseAllChildren = require('traverseAllChildren');
+var warning = require('warning');
 
 /**
  * @param {function} traverseContext Context passed through traversal.
@@ -29,13 +29,15 @@ var traverseAllChildren = require('traverseAllChildren');
 function flattenSingleChildIntoContext(traverseContext, child, name) {
   // We found a component instance.
   var result = traverseContext;
-  invariant(
-    !result.hasOwnProperty(name),
-    'flattenChildren(...): Encountered two children with the same key, `%s`. ' +
-    'Children keys must be unique.',
+  var keyUnique = !result.hasOwnProperty(name);
+  warning(
+    keyUnique,
+    'flattenChildren(...): Encountered two children with the same key, ' +
+    '`%s`. Child keys must be unique; when two children share a key, only ' +
+    'the first child will be used.',
     name
   );
-  if (child != null) {
+  if (keyUnique && child != null) {
     result[name] = child;
   }
 }