diff --git a/.travis.yml b/.travis.yml
index d0e905188e1f8..93b2254db071d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,8 +28,7 @@ script:
   - npm run ava
   - gulp lint
   - gulp build --fortesting --css-only
-  # TODO(jridgewell, #4347) Enable once assertions prove truthiness again.
-  # - gulp check-types
+  - gulp check-types
   - gulp dist --fortesting
   - gulp presubmit
   # dep-check needs to occur after build since we rely on build to generate
diff --git a/ads/ads.extern.js b/ads/ads.extern.js
index e1a50a7db9eae..99bb8593ab235 100644
--- a/ads/ads.extern.js
+++ b/ads/ads.extern.js
@@ -148,10 +148,3 @@ data.antid;
 data.anapiid;
 window.MADSAdrequest = {};
 window.MADSAdrequest.adrequest;
-/**
- * @constructor
- * @param {!Window} global
- * @param {!Object} data
- */
-window.EzoicAmpAd = function(global, data) {};
-window.EzoicAmpAd.prototype.createAd;
diff --git a/src/experiments.js b/src/experiments.js
index a28305869e93b..e4afe7ee7b401 100644
--- a/src/experiments.js
+++ b/src/experiments.js
@@ -36,7 +36,7 @@ const COOKIE_EXPIRATION_INTERVAL = COOKIE_MAX_AGE_DAYS * 24 * 60 * 60 * 1000;
 /** @const {string} */
 const CANARY_EXPERIMENT_ID = 'dev-channel';
 
-/** @type {Object<string, boolean>} */
+/** @type {Object<string, boolean>|undefined} */
 let toggles_;
 
 /**
diff --git a/src/log.js b/src/log.js
index dfbe4e119923d..d4ca7e615c248 100644
--- a/src/log.js
+++ b/src/log.js
@@ -219,7 +219,6 @@ export class Log {
    * @param {...*} var_args Arguments substituted into %s in the message.
    * @return {T} The value of shouldBeTrueish.
    * @template T
-   * @throws {Error}
    */
   /*eslint "google-camelcase/google-camelcase": 0*/
   assert(shouldBeTrueish, opt_message, var_args) {
diff --git a/src/observable.js b/src/observable.js
index f0621d221dc94..3ad72313b95a8 100644
--- a/src/observable.js
+++ b/src/observable.js
@@ -53,13 +53,13 @@ export class Observable {
 
   /**
    * Fires an event. All observers are called.
-   * @param {TYPE} event
+   * @param {TYPE=} opt_event
    */
-  fire(event) {
+  fire(opt_event) {
     const handlers = this.handlers_;
     for (let i = 0; i < handlers.length; i++) {
       const handler = handlers[i];
-      handler(event);
+      handler(opt_event);
     }
   }
 
diff --git a/src/service/viewer-impl.js b/src/service/viewer-impl.js
index 9d167686c0bf2..ffdf245131c97 100644
--- a/src/service/viewer-impl.js
+++ b/src/service/viewer-impl.js
@@ -125,7 +125,7 @@ export class Viewer {
     /** @private {number} */
     this.prerenderSize_ = 1;
 
-    /** @private {string} */
+    /** @private {ViewportType} */
     this.viewportType_ = ViewportType.NATURAL;
 
     /** @private {number} */
@@ -188,7 +188,7 @@ export class Viewer {
     this.isRuntimeOn_ = !parseInt(this.params_['off'], 10);
     dev().fine(TAG_, '- runtimeOn:', this.isRuntimeOn_);
 
-    this.overtakeHistory_ = parseInt(this.params_['history'], 10) ||
+    this.overtakeHistory_ = !!(parseInt(this.params_['history'], 10)) ||
         this.overtakeHistory_;
     dev().fine(TAG_, '- history:', this.overtakeHistory_);
 
@@ -237,6 +237,9 @@ export class Viewer {
     // Wait for document to become visible.
     this.docState_.onVisibilityChanged(this.recheckVisibilityState_.bind(this));
 
+    /** @private {function()|undefined} */
+    this.messagingReadyResolver_ = undefined;
+
     /**
      * This promise will resolve when communications channel has been
      * established or timeout in 20 seconds. The timeout is needed to avoid
@@ -248,10 +251,10 @@ export class Viewer {
         timerFor(this.win).timeoutPromise(
             20000,
             new Promise(resolve => {
-              /** @private @const {function()|undefined} */
               this.messagingReadyResolver_ = resolve;
             })).catch(reason => {
-              throw getChannelError(reason);
+              throw getChannelError(/** @type {!Error|string|undefined} */ (
+                  reason));
             }) : null;
 
     /**
@@ -265,7 +268,8 @@ export class Viewer {
         this.messagingReadyPromise_
             .catch(reason => {
               // Don't fail promise, but still report.
-              reportError(getChannelError(reason));
+              reportError(getChannelError(
+                  /** @type {!Error|string|undefined} */ (reason)));
             }) : null;
 
     // Trusted viewer and referrer.
@@ -283,10 +287,12 @@ export class Viewer {
           this.isTrustedViewerOrigin_(this.win.location.ancestorOrigins[0]));
       trustedViewerPromise = Promise.resolve(trustedViewerResolved);
     } else {
+
+      /** @private {!function(boolean)|undefined} */
+      this.trustedViewerResolver_ = undefined;
       // Wait for comms channel to confirm the origin.
       trustedViewerResolved = undefined;
       trustedViewerPromise = new Promise(resolve => {
-        /** @const @private {!function(boolean)|undefined} */
         this.trustedViewerResolver_ = resolve;
       });
     }
@@ -294,6 +300,9 @@ export class Viewer {
     /** @const @private {!Promise<boolean>} */
     this.isTrustedViewer_ = trustedViewerPromise;
 
+    /** @private {!function(string)|undefined} */
+    this.viewerOriginResolver_ = undefined;
+
     /** @const @private {!Promise<string>} */
     this.viewerOrigin_ = new Promise(resolve => {
       if (!this.isEmbedded()) {
@@ -868,7 +877,7 @@ export class Viewer {
   /**
    * Requests AMP document to receive a message from Viewer.
    * @param {string} eventType
-   * @param {*} data
+   * @param {!JSONType} data
    * @param {boolean} unusedAwaitResponse
    * @return {(!Promise<*>|undefined)}
    * @export
@@ -1065,7 +1074,7 @@ function parseParams_(str, allParams) {
 
 /**
  * Creates an error for the case where a channel cannot be established.
- * @param {!Error=} opt_reason
+ * @param {!Error|string=} opt_reason
  * @return {!Error}
  */
 function getChannelError(opt_reason) {
diff --git a/src/service/viewport-impl.js b/src/service/viewport-impl.js
index 931d22adc5635..d0da3401dd9b5 100644
--- a/src/service/viewport-impl.js
+++ b/src/service/viewport-impl.js
@@ -500,8 +500,8 @@ export class Viewport {
       return null;
     }
     if (this.viewportMeta_ === undefined) {
-      this.viewportMeta_ = this.win_.document.querySelector(
-          'meta[name=viewport]');
+      this.viewportMeta_ = /** @type {?HTMLMetaElement} */ (
+          this.win_.document.querySelector('meta[name=viewport]'));
       if (this.viewportMeta_) {
         this.originalViewportMetaString_ = this.viewportMeta_.content;
       }
@@ -1253,7 +1253,7 @@ export function updateViewportMetaString(currentValue, updateParams) {
     if (params[k] !== updateParams[k]) {
       changed = true;
       if (updateParams[k] !== undefined) {
-        params[k] = updateParams[k];
+        params[k] = /** @type {string} */ (updateParams[k]);
       } else {
         delete params[k];
       }
diff --git a/src/service/vsync-impl.js b/src/service/vsync-impl.js
index 140698b03b75d..3fef0c7a8be8f 100644
--- a/src/service/vsync-impl.js
+++ b/src/service/vsync-impl.js
@@ -125,7 +125,7 @@ export class Vsync {
    * will be undefined.
    *
    * @param {!VsyncTaskSpecDef} task
-   * @param {VsyncStateDef=} opt_state
+   * @param {!VsyncStateDef=} opt_state
    */
   run(task, opt_state) {
     this.tasks_.push(task);
diff --git a/src/service/xhr-impl.js b/src/service/xhr-impl.js
index 59c288ead1379..95c8c32bcb234 100644
--- a/src/service/xhr-impl.js
+++ b/src/service/xhr-impl.js
@@ -100,7 +100,7 @@ export class Xhr {
     if (opt_init && opt_init.responseType == 'document') {
       return fetchPolyfill(input, opt_init);
     }
-    return fetchPolyfill(input, opt_init);
+    return (this.win.fetch || fetchPolyfill).apply(null, arguments);
   }
 
   /**
diff --git a/src/url.js b/src/url.js
index bfdf8ed32f308..085b15b3bfa39 100644
--- a/src/url.js
+++ b/src/url.js
@@ -61,7 +61,7 @@ export let Location;
  */
 export function parseUrl(url) {
   if (!a) {
-    a = self.document.createElement('a');
+    a = /** @type {HTMLAnchorElement} */ (self.document.createElement('a'));
     cache = self.UrlCache || (self.UrlCache = Object.create(null));
   }
 
@@ -200,7 +200,7 @@ export function assertHttpsUrl(
     urlString, elementContext, sourceName = 'source') {
   user().assert(urlString != null, '%s %s must be available',
       elementContext, sourceName);
-  const url = parseUrl(urlString);
+  const url = parseUrl(/** @type {string} */ (urlString));
   user().assert(
       url.protocol == 'https:' || /^(\/\/)/.test(urlString) ||
       url.hostname == 'localhost' || endsWith(url.hostname, '.localhost'),
@@ -208,7 +208,7 @@ export function assertHttpsUrl(
       '"https://" or "//" or be relative and served from ' +
       'either https or from localhost. Invalid value: %s',
       elementContext, sourceName, urlString);
-  return urlString;
+  return /** @type {string} */ (urlString);
 }
 
 /**