Enable strict null checking and fix...everything

src/core/assert.shame.d.ts

@@ -1,6 +1,7 @@
 declare module '#core/assert' {
-  function devAssert<T>(T, string?, ...any): T;
-  function userAssert<T>(T, string?, ...any): T;
-  function devAssertElement<T>(T, string?, ...any): HTMLElement;
-  function devAssertString<T>(T, string?, ...any): string;
+  function devAssert<T>(subject: T | null | undefined, msg?: string, ...any): T;
+  function userAssert<T>(subject: T | null | undefined, msg?: string, ...any): T;
+  function devAssertElement<T>(subject: T, msg?: string, ...any): HTMLElement;
+  function devAssertString<T>(subject: T, msg?: string, ...any): string;
+  function devAssertNumber<T>(subject: T, msg?: string, ...any): number;
 }

src/core/data-structures/lru-cache.js

@@ -36,7 +36,7 @@ export class LruCache {
 
   /**
    * @param {number|string} key
-   * @return {T} The cached payload.
+   * @return {T|undefined} The cached payload.
    */
   get(key) {
     const cacheable = this.cache_[key];

src/core/data-structures/observable.js

@@ -10,13 +10,13 @@ export class Observable {
    * Creates an instance of Observable.
    */
   constructor() {
-    /** @type {?Array<function(TYPE):void>} */
+    /** @type {?Array<function(TYPE=):void>} */
     this.handlers_ = null;
   }
 
   /**
    * Adds the observer to this instance.
-   * @param {function(TYPE):void} handler Observer's handler.
+   * @param {function(TYPE=):void} handler Observer's handler.
    * @return {UnlistenCallback}
    */
   add(handler) {
@@ -31,7 +31,7 @@ export class Observable {
 
   /**
    * Removes the observer from this instance.
-   * @param {function(TYPE):void} handler Observer's instance.
+   * @param {function(TYPE=):void} handler Observer's instance.
    */
   remove(handler) {
     if (!this.handlers_) {

src/core/data-structures/priority-queue.js

@@ -13,7 +13,7 @@ export class PriorityQueue {
 
   /**
    * Returns the max priority item without dequeueing it.
-   * @return {T}
+   * @return {?T}
    */
   peek() {
     const l = this.length;
@@ -79,7 +79,7 @@ export class PriorityQueue {
   /**
    * Dequeues the max priority item.
    * Items with the same priority are dequeued in FIFO order.
-   * @return {T}
+   * @return {?T}
    */
   dequeue() {
     if (!this.length) {

src/core/dom/event-helper-listen.js

@@ -68,8 +68,8 @@ export function internalListenImplementation(
       optsSupported ? opt_evtListenerOpts : capture
     );
     // Ensure these are GC'd
-    localListener = null;
-    localElement = null;
+    /** @type {?} */ (localListener) = null;
+    /** @type {?} */ (localElement) = null;
     wrapped = null;
   };
 }
@@ -95,8 +95,20 @@ export function detectEvtListenerOptsSupport() {
         return false;
       },
     };
-    self.addEventListener('test-options', null, options);
-    self.removeEventListener('test-options', null, options);
+    self.addEventListener(
+      'test-options',
+      /** @type {EventListenerOrEventListenerObject} */ (
+        /** @type {?} */ (null)
+      ),
+      options
+    );
+    self.removeEventListener(
+      'test-options',
+      /** @type {EventListenerOrEventListenerObject} */ (
+        /** @type {?} */ (null)
+      ),
+      options
+    );
   } catch (err) {
     // EventListenerOptions are not supported
   }
@@ -132,8 +144,20 @@ export function supportsPassiveEventListener(win) {
       },
     });
 
-    win.addEventListener('test-options', null, options);
-    win.removeEventListener('test-options', null, options);
+    win.addEventListener(
+      'test-options',
+      /** @type {EventListenerOrEventListenerObject} */ (
+        /** @type {?} */ (null)
+      ),
+      options
+    );
+    win.removeEventListener(
+      'test-options',
+      /** @type {EventListenerOrEventListenerObject} */ (
+        /** @type {?} */ (null)
+      ),
+      options
+    );
   } catch (err) {
     // EventListenerOptions are not supported
   }

src/core/dom/event.d.ts

@@ -2,6 +2,8 @@ export {};
 
 declare global {
   interface Event {
-    data?: Object;
+    // We assign an `Object` at times, though Typescript's dom lib supports
+    // string or null, so here we allow all three (plus unedfined).
+    data?: Object | string | null;
   }
 }

src/core/dom/get-html.js

@@ -64,12 +64,12 @@ function appendToResult(rootNode, attrs, result) {
   const allowedAttrs = attrs.filter((attr) => allowedAttributes.includes(attr));
 
   while (stack.length > 0) {
-    const node = stack.pop();
+    const node = /** @type {string|Element} */ (stack.pop());
 
     if (isString(node)) {
       result.push(node);
     } else if (node.nodeType === Node.TEXT_NODE) {
-      result.push(node.textContent);
+      result.push(node.textContent ?? '');
     } else if (isElement(node) && isApplicableNode(node)) {
       appendOpenTag(node, allowedAttrs, result);
       stack.push(`</${node.tagName.toLowerCase()}>`);

src/core/dom/img.js

@@ -6,7 +6,7 @@
 export function guaranteeSrcForSrcsetUnsupportedBrowsers(img) {
   // The <img> tag does not have a src and does not support srcset
   if (!img.hasAttribute('src') && 'srcset' in img == false) {
-    const srcset = img.getAttribute('srcset');
+    const srcset = img.getAttribute('srcset') || '';
     const matches = /\S+/.exec(srcset);
     if (matches == null) {
       return;

src/core/dom/index.js

@@ -1,3 +1,4 @@
+import {devAssert} from '#core/assert';
 import * as mode from '#core/mode';
 import {dict} from '#core/types/object';
 import {parseJson} from '#core/types/object/json';
@@ -205,9 +206,11 @@ export function isConnectedNode(node) {
  * @return {ShadowRoot|Document}
  */
 export function rootNodeFor(node) {
+  // https://developer.mozilla.org/en-US/docs/Web/API/Node/getRootNode
+  // @ts-ignore this always is truthy according to TS, but isn't in IE 11.
   if (Node.prototype.getRootNode) {
     // Type checker says `getRootNode` may return null.
-    return /** @type {ShadowRoot|Document} */ (node.getRootNode() || node);
+    return /** @type {ShadowRoot|Document} */ (node.getRootNode());
   }
   let n;
   // Check isShadowRoot() is only needed for the polyfill case.
@@ -277,6 +280,7 @@ export function getDataParamsFromAttributes(
  * @return {boolean}
  */
 export function hasNextNodeInDocumentOrder(element, opt_stopNode) {
+  /** @type {?Element} */
   let currentElement = element;
   do {
     if (currentElement.nextSibling) {
@@ -504,7 +508,7 @@ export function getVerticalScrollbarWidth(win) {
 export function dispatchCustomEvent(node, name, opt_data, opt_options) {
   const data = opt_data || {};
   // Constructors of events need to come from the correct window. Sigh.
-  const event = node.ownerDocument.createEvent('Event');
+  const event = devAssert(node.ownerDocument).createEvent('Event');
 
   // Technically .data is not a property of Event.
   event.data = data;
@@ -545,7 +549,7 @@ export function getChildJsonConfig(element) {
   }
 
   try {
-    return parseJson(script.textContent);
+    return parseJson(script.textContent ?? '');
   } catch {
     throw new Error('Failed to parse <script> contents. Is it valid JSON?');
   }
@@ -572,6 +576,6 @@ export function propagateNonce(doc, scriptEl) {
   const currentScript = doc.head.querySelector('script[nonce]');
   if (currentScript) {
     const nonce = currentScript.nonce || currentScript.getAttribute('nonce');
-    scriptEl.setAttribute('nonce', nonce);
+    scriptEl.setAttribute('nonce', nonce ?? '');
   }
 }

src/core/dom/layout/index.js

@@ -158,7 +158,9 @@ export function parseLength(s) {
  */
 export function assertLength(length) {
   userAssert(
-    /^\d+(\.\d+)?(px|em|rem|vh|vw|vmin|vmax|cm|mm|q|in|pc|pt)$/.test(length),
+    /^\d+(\.\d+)?(px|em|rem|vh|vw|vmin|vmax|cm|mm|q|in|pc|pt)$/.test(
+      length ?? ''
+    ),
     'Invalid length value: %s',
     length
   );
@@ -189,7 +191,7 @@ export function assertLengthOrPercent(length) {
 export function getLengthUnits(length) {
   assertLength(length);
   const m = userAssert(
-    /[a-z]+/i.exec(length),
+    /[a-z]+/i.exec(length ?? ''),
     'Failed to read units from %s',
     length
   );

src/core/dom/layout/intersection-no-root.js

@@ -1,3 +1,4 @@
+import {devAssert} from '#core/assert';
 import {Deferred} from '#core/data-structures/promise';
 import {createViewportObserver} from '#core/dom/layout/viewport-observer';
 import {getWin} from '#core/window';
@@ -12,10 +13,10 @@ import {getWin} from '#core/window';
  * support is better.
  */
 
-/** @type {WeakMap<Element, Deferred<IntersectionObserverEntry>>} */
+/** @type {undefined|WeakMap<Element, Deferred<IntersectionObserverEntry>>} */
 let intersectionDeferreds;
 
-/** @type {WeakMap<Window, IntersectionObserver>} */
+/** @type {undefined|WeakMap<Window, IntersectionObserver>} */
 let intersectionObservers;
 
 /**
@@ -28,7 +29,7 @@ function getInOb(win) {
     intersectionObservers = new WeakMap();
   }
 
-  let observer = intersectionObservers.get(win);
+  let observer = devAssert(intersectionObservers).get(win);
   if (!observer) {
     observer = createViewportObserver(
       (entries) => {
@@ -40,15 +41,15 @@ function getInOb(win) {
           }
           seen.add(target);
 
-          observer.unobserve(target);
-          intersectionDeferreds.get(target).resolve(entries[i]);
-          intersectionDeferreds.delete(target);
+          devAssert(observer).unobserve(target);
+          devAssert(intersectionDeferreds).get(target)?.resolve(entries[i]);
+          devAssert(intersectionDeferreds).delete(target);
         }
       },
       win,
       {needsRootBounds: false}
     );
-    intersectionObservers.set(win, observer);
+    devAssert(intersectionObservers).set(win, observer);
   }
   return observer;
 }
@@ -64,13 +65,13 @@ function getInOb(win) {
  */
 export function measureIntersectionNoRoot(el) {
   if (intersectionDeferreds?.has(el)) {
-    return intersectionDeferreds.get(el).promise;
+    return devAssert(intersectionDeferreds.get(el)).promise;
   }
 
   const inOb = getInOb(getWin(el));
   inOb.observe(el);
 
   const deferred = new Deferred();
-  intersectionDeferreds.set(el, deferred);
+  devAssert(intersectionDeferreds).set(el, deferred);
   return deferred.promise;
 }

src/core/dom/layout/intersection.js

@@ -1,14 +1,15 @@
+import {devAssert} from '#core/assert';
 import {Deferred} from '#core/data-structures/promise';
 import {dict} from '#core/types/object';
 import {getWin} from '#core/window';
 
 import {LayoutRectDef, layoutRectFromDomRect} from './rect';
 import {createViewportObserver} from './viewport-observer';
 
-/** @type {WeakMap<Element, Deferred<IntersectionObserverEntry>>|undefined} */
+/** @type {undefined|WeakMap<Element, Deferred<IntersectionObserverEntry>>|undefined} */
 let intersectionDeferreds;
 
-/** @type {WeakMap<Window, IntersectionObserver>|undefined} */
+/** @type {undefined|WeakMap<Window, IntersectionObserver>|undefined} */
 let intersectionObservers;
 
 /**
@@ -21,7 +22,7 @@ function getInOb(win) {
     intersectionObservers = new WeakMap();
   }
 
-  let observer = intersectionObservers.get(win);
+  let observer = devAssert(intersectionObservers).get(win);
   if (!observer) {
     observer = createViewportObserver(
       (entries) => {
@@ -33,15 +34,15 @@ function getInOb(win) {
           }
           seen.add(target);
 
-          observer.unobserve(target);
-          intersectionDeferreds.get(target).resolve(entries[i]);
-          intersectionDeferreds.delete(target);
+          devAssert(observer).unobserve(target);
+          devAssert(intersectionDeferreds).get(target)?.resolve(entries[i]);
+          devAssert(intersectionDeferreds).delete(target);
         }
       },
       win,
       {needsRootBounds: true}
     );
-    intersectionObservers.set(win, observer);
+    devAssert(intersectionObservers).set(win, observer);
   }
   return observer;
 }
@@ -56,15 +57,15 @@ function getInOb(win) {
  * @return {Promise<IntersectionObserverEntry>}
  */
 export function measureIntersection(el) {
-  if (intersectionDeferreds && intersectionDeferreds.has(el)) {
-    return intersectionDeferreds.get(el).promise;
+  if (intersectionDeferreds?.has(el)) {
+    return devAssert(intersectionDeferreds.get(el)).promise;
   }
 
   const inOb = getInOb(getWin(el));
   inOb.observe(el);
 
   const deferred = new Deferred();
-  intersectionDeferreds.set(el, deferred);
+  devAssert(intersectionDeferreds).set(el, deferred);
   return deferred.promise;
 }
 

src/core/dom/media-query-props.js

@@ -1,5 +1,6 @@
-/** @typedef {Array<{query: ?MediaQueryList, value: string}>} */
-let ExprDef;
+import {devAssert} from '#core/assert';
+
+/** @typedef {Array<{query: ?MediaQueryList, value: string}>} ExprDef */
 
 const TRUE_VALUE = '1';
 
@@ -37,7 +38,7 @@ export class MediaQueryProps {
    */
   resolveMatchQuery(queryString) {
     // This will create a list query like this:
-    // `[{query: matchMedia(queryString), value: true}, {query: null, value: false}]`
+    // `[{query: matchMedia(queryHeadString), value: true}, {query: null, value: false}]`
     return (
       this.resolve_(queryString, parseMediaQueryMatchExpr, TRUE_VALUE) ===
       TRUE_VALUE
@@ -85,7 +86,8 @@ export class MediaQueryProps {
     if (!exprString.trim()) {
       return emptyExprValue;
     }
-    let expr = this.exprMap_[exprString] || this.prevExprMap_[exprString];
+    let expr =
+      this.exprMap_[exprString] || devAssert(this.prevExprMap_)[exprString];
     if (!expr) {
       expr = parser(this.win_, exprString);
       toggleOnChange(expr, this.callback_, true);
@@ -114,7 +116,7 @@ function parseMediaQueryMatchExpr(win, queryString) {
  * @return {ExprDef}
  */
 function parseMediaQueryListExpr(win, exprString) {
-  return (
+  return /** @type {ExprDef} */ (
     exprString
       .split(',')
       .map((part) => {