[BUGFIX beta] Align EmberArray.reduce with native semantics, related with issue #21005.

packages/@ember/-internals/runtime/tests/array/reduce-test.js

@@ -8,6 +8,12 @@ class ReduceTests extends AbstractTestCase {
     this.assert.equal(res, 6);
   }
 
+  '@test uses the first item as the initial accumulator when not provided'() {
+    let obj = this.newObject([1, 2, 3]);
+    let res = obj.reduce((previousValue, item) => previousValue + item);
+    this.assert.equal(res, 6);
+  }
+
   '@test passes index of item to callback'() {
     let obj = this.newObject([1, 2, 3]);
     let res = obj.reduce((previousValue, item, index) => previousValue + index, 0);
@@ -19,6 +25,15 @@ class ReduceTests extends AbstractTestCase {
     let res = obj.reduce((previousValue, item, index, enumerable) => enumerable, 0);
     this.assert.equal(res, obj);
   }
+
+  '@test throws when called on an empty array without an initial value'() {
+    let obj = this.newObject([]);
+
+    this.assert.throws(
+      () => obj.reduce(() => 0),
+      /Reduce of empty array with no initial value/
+    );
+  }
 }
 
 runArrayTests('reduce', ReduceTests);

packages/@ember/array/index.ts

@@ -1380,19 +1380,32 @@ const EmberArray = Mixin.create(Enumerable, {
     return any(this, callback);
   },
 
-  // FIXME: When called without initialValue, behavior does not match native behavior
   reduce<T, V>(
     this: EmberArray<T>,
     callback: (summation: V, current: T, index: number, arr: EmberArray<T>) => V,
-    initialValue: V
+    initialValue?: V
   ) {
     assert('`reduce` expects a function as first argument.', typeof callback === 'function');
 
-    let ret = initialValue;
+    let length = this.length;
+    let startIndex = 0;
+    let ret: V;
+
+    if (arguments.length > 1) {
+      ret = initialValue as V;
+    } else {
+      if (length === 0) {
+        throw new TypeError('Reduce of empty array with no initial value');
+      }
+
+      ret = objectAt(this, 0) as unknown as V;
+      startIndex = 1;
+    }
 
-    this.forEach(function (item, i) {
-      ret = callback(ret, item, i, this);
-    }, this);
+    for (let index = startIndex; index < length; index++) {
+      let item = objectAt(this, index) as T;
+      ret = callback(ret, item, index, this);
+    }
 
     return ret;
   },