LibJS: Implement Iterator.prototype.filter

Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.filter.js

@@ -0,0 +1,156 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.filter(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "predicate is not a function");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().filter(() => true);
+            iterator.next();
+        }).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(() => {
+            const iterator = new TestIterator().filter(() => true);
+            iterator.next();
+        }).toThrow(TestError);
+    });
+
+    test("predicate function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().filter(() => {
+                throw new TestError();
+            });
+            iterator.next();
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.filter).toHaveLength(1);
+    });
+
+    test("predicate function sees every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        const iterator = generator().filter((value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected predicate invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return true;
+        });
+
+        for (const i of iterator) {
+        }
+
+        expect(count).toBe(2);
+    });
+
+    test("predicate function can select values", () => {
+        function* generator() {
+            yield 1;
+            yield 0;
+            yield 2;
+            yield 0;
+        }
+
+        const iterator = generator().filter(value => value > 0);
+
+        let value = iterator.next();
+        expect(value.value).toBe(1);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBe(2);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+
+    test("predicates can be chained", () => {
+        function* generator() {
+            yield 1;
+            yield 0;
+            yield 2;
+            yield 0;
+        }
+
+        let firstFilterCount = 0;
+        let secondFilterCount = 0;
+
+        const iterator = generator()
+            .filter(value => {
+                ++firstFilterCount;
+                return value > 0;
+            })
+            .filter(value => {
+                ++secondFilterCount;
+                return value > 1;
+            });
+
+        let value = iterator.next();
+        expect(value.value).toBe(2);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+
+        expect(firstFilterCount).toBe(4);
+        expect(secondFilterCount).toBe(2);
+    });
+});