[added] 'elementType' custom prop type validator

The code has been borrowed from
the https://github.com/jquense/react-widgets project

src/utils/CustomPropTypes.js

@@ -1,3 +1,5 @@
+import React from 'react';
+
 const ANONYMOUS = '<<anonymous>>';
 
 const CustomPropTypes = {
@@ -15,6 +17,20 @@ const CustomPropTypes = {
    */
   mountable: createMountableChecker(),
 
+  /**
+   * Checks whether a prop provides a type of element.
+   *
+   * The type of element can be provided in two forms:
+   * - tag name (string)
+   * - a return value of React.createClass(...)
+   *
+   * @param props
+   * @param propName
+   * @param componentName
+   * @returns {Error|undefined}
+   */
+  elementType: createElementTypeChecker(),
+
   /**
    * Checks whether a prop matches a key of an associated object
    *
@@ -135,4 +151,24 @@ function all(propTypes) {
   };
 }
 
+function createElementTypeChecker() {
+  function validate(props, propName, componentName) {
+    let errMsg = `Invalid prop '${propName}' specified in '${componentName}'.` +
+      ' Expected an Element `type`';
+
+    if (typeof props[propName] !== 'function') {
+      if (React.isValidElement(props[propName])) {
+        return new Error(errMsg + ', not an actual Element');
+      }
+
+      if (typeof props[propName] !== 'string') {
+        return new Error(errMsg +
+          ' such as a tag name or return value of React.createClass(...)');
+      }
+    }
+  }
+
+  return createChainableTypeChecker(validate);
+}
+
 export default CustomPropTypes;

test/utils/CustomPropTypesSpec.js

@@ -26,6 +26,34 @@ describe('CustomPropTypes', function() {
     });
   });
 
+  describe('elementType', function () {
+    function validate(prop) {
+      return CustomPropTypes.elementType({p: prop}, 'p', 'TestComponent');
+    }
+
+    it('Should validate OK with undifined or null values', function() {
+      assert.isUndefined(validate());
+      assert.isUndefined(validate(null));
+    });
+
+    it('Should validate OK with elementType values', function() {
+      assert.isUndefined(validate('span'));
+      assert.isUndefined(validate(function(){}));
+    });
+
+    it('Should return error with not a string or function values', function() {
+      let err = validate({});
+      assert.instanceOf(err, Error);
+      assert.include(err.message, 'Expected an Element `type` such as a tag name or return value of React.createClass(...)');
+    });
+
+    it('Should return error with react element', function() {
+      let err = validate(React.createElement('span'));
+      assert.instanceOf(err, Error);
+      assert.include(err.message, 'Expected an Element `type`, not an actual Element');
+    });
+  });
+
   describe('keyOf', function () {
     let obj = {'foo': 1};
     function validate(prop) {