Fix polyfill for flat and flatMap to handle sparse arrays.

PiperOrigin-RevId: 508190440

Author: Closure Team

src/com/google/javascript/jscomp/js/es6/array/flat.js

@@ -35,15 +35,17 @@ $jscomp.polyfill('Array.prototype.flat', function(orig) {
     // TODO(sdh): Consider respecting Symbol.species (b/121061255).
     depth = depth === undefined ? 1 : depth;
     var flattened = [];
-    for (var i = 0; i < this.length; i++) {
-      var element = this[i];
+    var flattenElementIntoArray = function(element) {
       if (Array.isArray(element) && depth > 0) {
         var inner = Array.prototype.flat.call(element, depth - 1);
         flattened.push.apply(flattened, inner);
       } else {
         flattened.push(element);
       }
-    }
+    };
+    // Use Array.prototype explicitly since IE11 doesn't support forEach
+    // on array-like NodeList
+    Array.prototype.forEach.call(this, flattenElementIntoArray);
     return flattened;
   };
 

src/com/google/javascript/jscomp/js/es6/array/flatmap.js

@@ -34,8 +34,8 @@ $jscomp.polyfill('Array.prototype.flatMap', function(orig) {
    */
   var flatMap = function(callback, thisArg) {
     var mapped = [];
-    for (var i = 0; i < this.length; i++) {
-      var result = callback.call(thisArg, this[i], i, this);
+    var mapAndFlattenElementIntoArray = function(element, index) {
+      var result = callback.call(thisArg, element, index, this);
       if (Array.isArray(result)) {
         mapped.push.apply(mapped, result);
       } else {
@@ -47,7 +47,10 @@ $jscomp.polyfill('Array.prototype.flatMap', function(orig) {
         // non-Array.
         mapped.push(result);
       }
-    }
+    };
+    // Use Array.prototype explicitly since IE11 doesn't support forEach
+    // on array-like NodeList
+    Array.prototype.forEach.call(this, mapAndFlattenElementIntoArray);
     return mapped;
   };