diff --git a/src/renderers/dom/fiber/ReactDOMFiber.js b/src/renderers/dom/fiber/ReactDOMFiber.js
index cebe041481..42899b7b2d 100644
--- a/src/renderers/dom/fiber/ReactDOMFiber.js
+++ b/src/renderers/dom/fiber/ReactDOMFiber.js
@@ -196,8 +196,9 @@ var DOMRenderer = ReactFiberReconciler({
         typeof props.children === 'string' ||
         typeof props.children === 'number'
       ) {
+        const string = '' + props.children;
         const ownAncestorInfo = updatedAncestorInfo(hostContextDev.ancestorInfo, type, null);
-        validateDOMNesting(null, String(props.children), null, ownAncestorInfo);
+        validateDOMNesting(null, string, null, ownAncestorInfo);
       }
       parentNamespace = hostContextDev.namespace;
     } else {
@@ -237,8 +238,9 @@ var DOMRenderer = ReactFiberReconciler({
         typeof newProps.children === 'string' ||
         typeof newProps.children === 'number'
       )) {
+        const string = '' + newProps.children;
         const ownAncestorInfo = updatedAncestorInfo(hostContextDev.ancestorInfo, type, null);
-        validateDOMNesting(null, String(newProps.children), null, ownAncestorInfo);
+        validateDOMNesting(null, string, null, ownAncestorInfo);
       }
     }
     return diffProperties(domElement, type, oldProps, newProps, rootContainerInstance);
@@ -411,9 +413,9 @@ var ReactDOM = {
     }
 
     if (__DEV__) {
-      const isRootRenderedBySomeReact = Boolean(container._reactRootContainer);
+      const isRootRenderedBySomeReact = !!container._reactRootContainer;
       const rootEl = getReactRootElementInContainer(container);
-      const hasNonRootReactChild = Boolean(rootEl && ReactDOMComponentTree.getInstanceFromNode(rootEl));
+      const hasNonRootReactChild = !!(rootEl && ReactDOMComponentTree.getInstanceFromNode(rootEl));
 
       warning(
         !hasNonRootReactChild ||
diff --git a/src/renderers/dom/fiber/wrappers/ReactDOMFiberSelect.js b/src/renderers/dom/fiber/wrappers/ReactDOMFiberSelect.js
index 88853b3014..6e874d85f0 100644
--- a/src/renderers/dom/fiber/wrappers/ReactDOMFiberSelect.js
+++ b/src/renderers/dom/fiber/wrappers/ReactDOMFiberSelect.js
@@ -134,7 +134,7 @@ var ReactDOMSelect = {
     var value = props.value;
     node._wrapperState = {
       initialValue: value != null ? value : props.defaultValue,
-      wasMultiple: Boolean(props.multiple),
+      wasMultiple: !!props.multiple,
     };
 
     if (
@@ -153,11 +153,11 @@ var ReactDOMSelect = {
       didWarnValueDefaultValue = true;
     }
 
-    node.multiple = Boolean(props.multiple);
+    node.multiple = !!props.multiple;
     if (value != null) {
-      updateOptions(node, Boolean(props.multiple), value);
+      updateOptions(node, !!props.multiple, value);
     } else if (props.defaultValue != null) {
-      updateOptions(node, Boolean(props.multiple), props.defaultValue);
+      updateOptions(node, !!props.multiple, props.defaultValue);
     }
   },
 
@@ -168,18 +168,18 @@ var ReactDOMSelect = {
     node._wrapperState.initialValue = undefined;
 
     var wasMultiple = node._wrapperState.wasMultiple;
-    node._wrapperState.wasMultiple = Boolean(props.multiple);
+    node._wrapperState.wasMultiple = !!props.multiple;
 
     var value = props.value;
     if (value != null) {
-      updateOptions(node, Boolean(props.multiple), value);
-    } else if (wasMultiple !== Boolean(props.multiple)) {
+      updateOptions(node, !!props.multiple, value);
+    } else if (wasMultiple !== !!props.multiple) {
       // For simplicity, reapply `defaultValue` if `multiple` is toggled.
       if (props.defaultValue != null) {
-        updateOptions(node, Boolean(props.multiple), props.defaultValue);
+        updateOptions(node, !!props.multiple, props.defaultValue);
       } else {
         // Revert the select back to its default unselected state.
-        updateOptions(node, Boolean(props.multiple), props.multiple ? [] : '');
+        updateOptions(node, !!props.multiple, props.multiple ? [] : '');
       }
     }
   },
@@ -189,7 +189,7 @@ var ReactDOMSelect = {
     var value = props.value;
 
     if (value != null) {
-      updateOptions(node, Boolean(props.multiple), value);
+      updateOptions(node, !!props.multiple, value);
     }
   },
 };
diff --git a/src/renderers/dom/shared/ReactBrowserEventEmitter.js b/src/renderers/dom/shared/ReactBrowserEventEmitter.js
index a191a45b14..4e994d9a03 100644
--- a/src/renderers/dom/shared/ReactBrowserEventEmitter.js
+++ b/src/renderers/dom/shared/ReactBrowserEventEmitter.js
@@ -154,7 +154,7 @@ var topEventMapping = {
 /**
  * To ensure no conflicts with other potential React instances on the page
  */
-var topListenersIDKey = '_reactListenersID' + String(Math.random()).slice(2);
+var topListenersIDKey = '_reactListenersID' + ('' + Math.random()).slice(2);
 
 function getListeningForDocument(mountAt) {
   // In IE8, `mountAt` is a host object and doesn't have `hasOwnProperty`
diff --git a/src/renderers/dom/shared/ReactDOMComponentTree.js b/src/renderers/dom/shared/ReactDOMComponentTree.js
index 78c543bea2..b843b9407a 100644
--- a/src/renderers/dom/shared/ReactDOMComponentTree.js
+++ b/src/renderers/dom/shared/ReactDOMComponentTree.js
@@ -31,7 +31,7 @@ var internalEventHandlersKey = '__reactEventHandlers$' + randomKey;
  */
 function shouldPrecacheNode(node, nodeID) {
   return (node.nodeType === 1 &&
-          node.getAttribute(ATTR_NAME) === String(nodeID)) ||
+          node.getAttribute(ATTR_NAME) === ('' + nodeID)) ||
          (node.nodeType === 8 &&
           node.nodeValue === ' react-text: ' + nodeID + ' ') ||
          (node.nodeType === 8 &&
diff --git a/src/renderers/dom/shared/validateDOMNesting.js b/src/renderers/dom/shared/validateDOMNesting.js
index 8fa3216f70..a0c2c74349 100644
--- a/src/renderers/dom/shared/validateDOMNesting.js
+++ b/src/renderers/dom/shared/validateDOMNesting.js
@@ -400,7 +400,7 @@ if (__DEV__) {
         childTag,
         ancestorInstance,
         ancestorTag,
-        Boolean(invalidParent)
+        !!invalidParent
       ) + '.';
     } else {
       addendum = getCurrentFiberStackAddendum();
diff --git a/src/renderers/dom/stack/client/ReactDOMComponent.js b/src/renderers/dom/stack/client/ReactDOMComponent.js
index 9fd3a45f92..035726db6e 100644
--- a/src/renderers/dom/stack/client/ReactDOMComponent.js
+++ b/src/renderers/dom/stack/client/ReactDOMComponent.js
@@ -185,7 +185,7 @@ if (__DEV__) {
       return;
     }
 
-    validateDOMNesting(null, String(content), this, this._ancestorInfo);
+    validateDOMNesting(null, '' + content, this, this._ancestorInfo);
     this._contentDebugID = contentDebugID;
     if (hasExistingContent) {
       ReactInstrumentation.debugTool.onBeforeUpdateComponent(contentDebugID, content);
diff --git a/src/renderers/dom/stack/client/ReactMount.js b/src/renderers/dom/stack/client/ReactMount.js
index a1554d81bd..afd78bf317 100644
--- a/src/renderers/dom/stack/client/ReactMount.js
+++ b/src/renderers/dom/stack/client/ReactMount.js
@@ -440,7 +440,7 @@ var ReactMount = {
         callback === null || typeof callback === 'function',
         'render(...): Expected the last optional `callback` argument to be a ' +
         'function. Instead received: %s.',
-        String(callback)
+        '' + callback
       );
     }
     if (!React.isValidElement(nextElement)) {
diff --git a/src/renderers/dom/stack/client/wrappers/ReactDOMSelect.js b/src/renderers/dom/stack/client/wrappers/ReactDOMSelect.js
index 6191b4999b..3bde1212c0 100644
--- a/src/renderers/dom/stack/client/wrappers/ReactDOMSelect.js
+++ b/src/renderers/dom/stack/client/wrappers/ReactDOMSelect.js
@@ -135,7 +135,7 @@ var ReactDOMSelect = {
     inst._wrapperState = {
       initialValue: value != null ? value : props.defaultValue,
       listeners: null,
-      wasMultiple: Boolean(props.multiple),
+      wasMultiple: !!props.multiple,
     };
 
     if (
@@ -169,18 +169,18 @@ var ReactDOMSelect = {
     inst._wrapperState.initialValue = undefined;
 
     var wasMultiple = inst._wrapperState.wasMultiple;
-    inst._wrapperState.wasMultiple = Boolean(props.multiple);
+    inst._wrapperState.wasMultiple = !!props.multiple;
 
     var value = props.value;
     if (value != null) {
-      updateOptions(inst, Boolean(props.multiple), value);
-    } else if (wasMultiple !== Boolean(props.multiple)) {
+      updateOptions(inst, !!props.multiple, value);
+    } else if (wasMultiple !== !!props.multiple) {
       // For simplicity, reapply `defaultValue` if `multiple` is toggled.
       if (props.defaultValue != null) {
-        updateOptions(inst, Boolean(props.multiple), props.defaultValue);
+        updateOptions(inst, !!props.multiple, props.defaultValue);
       } else {
         // Revert the select back to its default unselected state.
-        updateOptions(inst, Boolean(props.multiple), props.multiple ? [] : '');
+        updateOptions(inst, !!props.multiple, props.multiple ? [] : '');
       }
     }
   },
@@ -191,7 +191,7 @@ var ReactDOMSelect = {
       var value = props.value;
 
       if (value != null) {
-        updateOptions(inst, Boolean(props.multiple), value);
+        updateOptions(inst, !!props.multiple, value);
       }
     }
   },
diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index 79f80c9e40..57095c921e 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -95,7 +95,7 @@ function coerceRef(current: Fiber | null, element: ReactElement) {
         'bug in React. Please file an issue.',
         mixedRef
       );
-      const stringRef = String(mixedRef);
+      const stringRef = '' + mixedRef;
       // Check if previous string ref matches new string ref
       if (current !== null && current.ref !== null && current.ref._stringRef === stringRef) {
         return current.ref;
@@ -117,7 +117,8 @@ function coerceRef(current: Fiber | null, element: ReactElement) {
 
 function throwOnInvalidObjectType(returnFiber : Fiber, newChild : Object) {
   if (returnFiber.type !== 'textarea') {
-    const childrenString = String(newChild);
+    // $FlowFixMe - Intentional cast to string
+    const childrenString = '' + newChild;
     let addendum = '';
     if (__DEV__) {
       addendum =
diff --git a/src/renderers/shared/fiber/ReactFiberClassComponent.js b/src/renderers/shared/fiber/ReactFiberClassComponent.js
index 2856279537..15bd45bba4 100644
--- a/src/renderers/shared/fiber/ReactFiberClassComponent.js
+++ b/src/renderers/shared/fiber/ReactFiberClassComponent.js
@@ -47,7 +47,8 @@ if (__DEV__) {
       '%s(...): Expected the last optional `callback` argument to be a ' +
       'function. Instead received: %s.',
       callerName,
-      String(callback)
+      // $FlowFixMe - Intentional cast to string
+      '' + callback
     );
   };
 }
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index 3f910aa1d9..2aa797ffb4 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -167,7 +167,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         callback === null || typeof callback === 'function',
         'render(...): Expected the last optional `callback` argument to be a ' +
         'function. Instead received: %s.',
-        String(callback)
+        // $FlowFixMe - Intentional cast to string
+        '' + callback
       );
     }
     addTopLevelUpdate(current, nextState, callback, priorityLevel);
diff --git a/src/renderers/shared/fiber/ReactFiberScheduler.js b/src/renderers/shared/fiber/ReactFiberScheduler.js
index 8ab6ee177f..3f23452a0e 100644
--- a/src/renderers/shared/fiber/ReactFiberScheduler.js
+++ b/src/renderers/shared/fiber/ReactFiberScheduler.js
@@ -776,7 +776,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       'by a bug in React. Please file an issue.'
     );
     isPerformingWork = true;
-    const isPerformingDeferredWork = Boolean(deadline);
+    const isPerformingDeferredWork = !!deadline;
 
     // This outer loop exists so that we can restart the work loop after
     // catching an error. It also lets us flush Task work at the end of a
@@ -1034,7 +1034,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   function hasCapturedError(fiber : Fiber) : boolean {
     // TODO: capturedErrors should store the boundary instance, to avoid needing
     // to check the alternate.
-    return Boolean(
+    return (
       capturedErrors !== null &&
       (capturedErrors.has(fiber) || (fiber.alternate !== null && capturedErrors.has(fiber.alternate)))
     );
@@ -1043,7 +1043,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   function isFailedBoundary(fiber : Fiber) : boolean {
     // TODO: failedBoundaries should store the boundary instance, to avoid
     // needing to check the alternate.
-    return Boolean(
+    return (
       failedBoundaries !== null &&
       (failedBoundaries.has(fiber) || (fiber.alternate !== null && failedBoundaries.has(fiber.alternate)))
     );
diff --git a/src/renderers/shared/fiber/ReactFiberUpdateQueue.js b/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
index f07fec3f46..dd4438ab83 100644
--- a/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
+++ b/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
@@ -341,7 +341,7 @@ function addTopLevelUpdate(
   callback : Callback | null,
   priorityLevel : PriorityLevel
 ) : void {
-  const isTopLevelUnmount = Boolean(
+  const isTopLevelUnmount = !!(
     partialState &&
     partialState.element === null
   );
@@ -470,7 +470,8 @@ function commitCallbacks(finishedWork : Fiber, queue : UpdateQueue, context : mi
       typeof callback === 'function',
       'Invalid argument passed as callback. Expected a function. Instead ' +
       'received: %s',
-      String(callback)
+      // $FlowFixMe - Intentional cast to string
+      '' + callback
     );
     callback.call(context);
   }
diff --git a/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js b/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
index a1434d7b88..8b834b7d62 100644
--- a/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
+++ b/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js
@@ -1002,7 +1002,7 @@ var ReactCompositeComponent = {
   ) {
     var inst = this._instance;
 
-    var hasComponentDidUpdate = Boolean(inst.componentDidUpdate);
+    var hasComponentDidUpdate = !!inst.componentDidUpdate;
     var prevProps;
     var prevState;
     var prevContext;
diff --git a/src/renderers/shared/stack/reconciler/ReactUpdateQueue.js b/src/renderers/shared/stack/reconciler/ReactUpdateQueue.js
index 12c7fd30f0..64253022c1 100644
--- a/src/renderers/shared/stack/reconciler/ReactUpdateQueue.js
+++ b/src/renderers/shared/stack/reconciler/ReactUpdateQueue.js
@@ -24,7 +24,7 @@ if (__DEV__) {
       '%s(...): Expected the last optional `callback` argument to be a ' +
       'function. Instead received: %s.',
       callerName,
-      String(callback)
+      '' + callback
     );
   };
 }
diff --git a/src/renderers/shared/utils/validateCallback.js b/src/renderers/shared/utils/validateCallback.js
index 657265b84f..f9f1ac6952 100644
--- a/src/renderers/shared/utils/validateCallback.js
+++ b/src/renderers/shared/utils/validateCallback.js
@@ -19,7 +19,8 @@ function validateCallback(callback: ?Function) {
     !callback || typeof callback === 'function',
     'Invalid argument passed as callback. Expected a function. Instead ' +
     'received: %s',
-    String(callback)
+    // $FlowFixMe - Intentional cast to string
+    '' + callback
   );
 }
 
diff --git a/src/shared/utils/traverseAllChildren.js b/src/shared/utils/traverseAllChildren.js
index f84a6af300..035770c1f1 100644
--- a/src/shared/utils/traverseAllChildren.js
+++ b/src/shared/utils/traverseAllChildren.js
@@ -157,7 +157,7 @@ function traverseAllChildrenImpl(
           }
         }
       }
-      var childrenString = String(children);
+      var childrenString = '' + children;
       invariant(
         false,
         'Objects are not valid as a React child (found: %s).%s',
diff --git a/src/test/reactComponentExpect.js b/src/test/reactComponentExpect.js
index 48d9092dbb..f35f255014 100644
--- a/src/test/reactComponentExpect.js
+++ b/src/test/reactComponentExpect.js
@@ -212,7 +212,7 @@ Object.assign(reactComponentExpectInternal.prototype, {
       expect(this._instance.tag === HostText).toBe(true);
       var actualVal = this._instance.memoizedProps;
       expect(typeof actualVal === 'string' || typeof actualVal === 'number').toBe(true);
-      expect(String(actualVal)).toBe(val);
+      expect('' + actualVal).toBe(val);
     } else {
       // Fiber reconciler
       var elementType = typeof this._instance._currentElement;