LibJS: Implement Iterator.prototype.reduce

2025-07-27 19:17:44 +00:00 · 2023-06-25 13:41:01 -04:00 · 2023-06-25 13:41:01 -04:00 · acc05480e8
commit acc05480e8
parent ad42b4ea67
3 changed files with 196 additions and 0 deletions
--- a/Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.reduce.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.reduce.js
@ -0,0 +1,124 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.reduce(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "reducer is not a function");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            new TestIterator().reduce(() => 0);
+        }).toThrow(TestError);
+    });
+
+    test("value returned by iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    get value() {
+                        throw new TestError();
+                    },
+                };
+            }
+        }
+
+        expect(() => {
+            new TestIterator().reduce(() => 0);
+        }).toThrow(TestError);
+    });
+
+    test("reducer function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            new TestIterator().reduce(() => {
+                throw new TestError();
+            });
+        }).toThrow(TestError);
+    });
+
+    test("no available initial value", () => {
+        function* generator() {}
+
+        expect(() => {
+            generator().reduce(() => 0);
+        }).toThrowWithMessage(TypeError, "Reduce of empty array with no initial value");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.reduce).toHaveLength(1);
+    });
+
+    test("reducer function sees every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        generator().reduce((accumulator, value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected reducer invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return value;
+        }, "");
+
+        expect(count).toBe(2);
+    });
+
+    test("reducer uses first value as initial value", () => {
+        function* generator() {
+            yield 1;
+            yield 2;
+            yield 3;
+        }
+
+        const result = generator().reduce((accumulator, value) => accumulator + value);
+        expect(result).toBe(6);
+    });
+
+    test("reducer uses provided value as initial value", () => {
+        function* generator() {
+            yield 1;
+            yield 2;
+            yield 3;
+        }
+
+        const result = generator().reduce((accumulator, value) => accumulator + value, 10);
+        expect(result).toBe(16);
+    });
+});