Add getDefaultProps()

As it turns out, default values are very useful. This implements
getDefaultProps(), a hook for components to provide prop values when
a prop is not specified by the user.

src/core/ReactCompositeComponent.js

@@ -91,6 +91,19 @@ var ReactCompositeComponentInterface = {
 
   // ==== Definition methods ====
 
+  /**
+   * Invoked when the component is mounted and whenever new props are received.
+   * Values in the returned mapping will be set on `this.props` if that prop is
+   * not specified (i.e. using an `in` check).
+   *
+   * This method is invoked before `getInitialState` and therefore cannot rely
+   * on `this.state` or use `this.setState`.
+   *
+   * @return {object}
+   * @optional
+   */
+  getDefaultProps: SpecPolicy.DEFINE_ONCE,
+
   /**
    * Invoked once before the component is mounted. The return value will be used
    * as the initial value of `this.state`.
@@ -419,9 +432,7 @@ var ReactCompositeComponentMixin = {
     this._lifeCycleState = ReactComponent.LifeCycle.UNMOUNTED;
     this._compositeLifeCycleState = CompositeLifeCycle.MOUNTING;
 
-    if (this.constructor.propDeclarations) {
-      this._assertValidProps(this.props);
-    }
+    this._processProps(this.props);
 
     if (this.__reactAutoBindMap) {
       this._bindAutoBindMethods();
@@ -489,9 +500,7 @@ var ReactCompositeComponentMixin = {
    * @internal
    */
   receiveProps: function(nextProps, transaction) {
-    if (this.constructor.propDeclarations) {
-      this._assertValidProps(nextProps);
-    }
+    this._processProps(nextProps);
     ReactComponent.Mixin.receiveProps.call(this, nextProps, transaction);
 
     this._compositeLifeCycleState = CompositeLifeCycle.RECEIVING_PROPS;
@@ -573,6 +582,35 @@ var ReactCompositeComponentMixin = {
     }
   },
 
+  /**
+   * Processes props by setting default values for unspecified props and
+   * asserting that the props are valid.
+   *
+   * @param {object} props
+   * @private
+   */
+  _processProps: function(props) {
+    var propName;
+    if (this.getDefaultProps) {
+      var defaultProps = this.getDefaultProps();
+      for (propName in defaultProps) {
+        if (!(propName in props)) {
+          props[propName] = defaultProps[propName];
+        }
+      }
+    }
+    var propDeclarations = this.constructor.propDeclarations;
+    if (propDeclarations) {
+      var componentName = this.constructor.displayName;
+      for (propName in propDeclarations) {
+        var checkProp = propDeclarations[propName];
+        if (checkProp) {
+          checkProp(props, propName, componentName);
+        }
+      }
+    }
+  },
+
   /**
    * Receives next props and next state, and negotiates whether or not the
    * component should update as a result.
@@ -696,21 +734,6 @@ var ReactCompositeComponentMixin = {
     return renderedComponent;
   },
 
-  /**
-   * @param {object} props
-   * @private
-   */
-  _assertValidProps: function(props) {
-    var propDeclarations = this.constructor.propDeclarations;
-    var componentName = this.constructor.displayName;
-    for (var propName in propDeclarations) {
-      var checkProp = propDeclarations[propName];
-      if (checkProp) {
-        checkProp(props, propName, componentName);
-      }
-    }
-  },
-
   /**
    * @private
    */

src/core/__tests__/ReactCompositeComponent-test.js

@@ -194,6 +194,53 @@ describe('ReactCompositeComponent', function() {
     expect(retValAfterMountWithCrazyScope).toBe(RETURN_VALUE_AFTER_MOUNT);
   });
 
+  it('should normalize props with default values', function() {
+    var Component = React.createClass({
+      props: {key: ReactProps.string.isRequired},
+      getDefaultProps: function() {
+        return {key: 'testKey'};
+      },
+      getInitialState: function() {
+        return {key: this.props.key + 'State'};
+      },
+      render: function() {
+        return <span>{this.props.key}</span>;
+      }
+    });
+
+    var instance = <Component />;
+    ReactTestUtils.renderIntoDocument(instance);
+    reactComponentExpect(instance).scalarPropsEqual({key: 'testKey'});
+    reactComponentExpect(instance).scalarStateEqual({key: 'testKeyState'});
+
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(<Component key={null} />);
+    }).toThrow(
+      'Invariant Violation: Required prop `key` was not specified in ' +
+      '`Component`.'
+    );
+  });
+
+  it('should check default prop values', function() {
+    var Component = React.createClass({
+      props: {key: ReactProps.string.isRequired},
+      getDefaultProps: function() {
+        return {key: null};
+      },
+      render: function() {
+        return <span>{this.props.key}</span>;
+      }
+    });
+
+    var instance = <Component />;
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(instance);
+    }).toThrow(
+      'Invariant Violation: Required prop `key` was not specified in ' +
+      '`Component`.'
+    );
+  });
+
   it('should check declared prop types', function() {
     var Component = React.createClass({
       props: {