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LibJS: Implement Iterator.prototype.filter
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3 changed files with 224 additions and 0 deletions
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describe("errors", () => {
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test("called with non-callable object", () => {
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expect(() => {
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Iterator.prototype.filter(Symbol.hasInstance);
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}).toThrowWithMessage(TypeError, "predicate is not a function");
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});
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test("iterator's next method throws", () => {
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function TestError() {}
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class TestIterator extends Iterator {
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next() {
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throw new TestError();
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}
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}
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expect(() => {
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const iterator = new TestIterator().filter(() => true);
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iterator.next();
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}).toThrow(TestError);
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});
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test("value returned by iterator's next method throws", () => {
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function TestError() {}
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class TestIterator extends Iterator {
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next() {
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return {
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done: false,
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get value() {
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throw new TestError();
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},
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};
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}
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}
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expect(() => {
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const iterator = new TestIterator().filter(() => true);
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iterator.next();
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}).toThrow(TestError);
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});
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test("predicate function throws", () => {
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function TestError() {}
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class TestIterator extends Iterator {
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next() {
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return {
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done: false,
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value: 1,
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};
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}
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}
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expect(() => {
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const iterator = new TestIterator().filter(() => {
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throw new TestError();
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});
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iterator.next();
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}).toThrow(TestError);
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});
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});
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describe("normal behavior", () => {
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test("length is 1", () => {
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expect(Iterator.prototype.filter).toHaveLength(1);
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});
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test("predicate function sees every value", () => {
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function* generator() {
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yield "a";
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yield "b";
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}
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let count = 0;
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const iterator = generator().filter((value, index) => {
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++count;
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switch (index) {
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case 0:
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expect(value).toBe("a");
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break;
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case 1:
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expect(value).toBe("b");
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break;
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default:
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expect().fail(`Unexpected predicate invocation: value=${value} index=${index}`);
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break;
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}
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return true;
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});
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for (const i of iterator) {
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}
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expect(count).toBe(2);
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});
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test("predicate function can select values", () => {
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function* generator() {
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yield 1;
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yield 0;
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yield 2;
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yield 0;
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}
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const iterator = generator().filter(value => value > 0);
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let value = iterator.next();
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expect(value.value).toBe(1);
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expect(value.done).toBeFalse();
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value = iterator.next();
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expect(value.value).toBe(2);
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expect(value.done).toBeFalse();
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value = iterator.next();
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expect(value.value).toBeUndefined();
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expect(value.done).toBeTrue();
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});
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test("predicates can be chained", () => {
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function* generator() {
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yield 1;
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yield 0;
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yield 2;
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yield 0;
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}
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let firstFilterCount = 0;
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let secondFilterCount = 0;
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const iterator = generator()
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.filter(value => {
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++firstFilterCount;
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return value > 0;
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})
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.filter(value => {
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++secondFilterCount;
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return value > 1;
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});
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let value = iterator.next();
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expect(value.value).toBe(2);
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expect(value.done).toBeFalse();
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value = iterator.next();
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expect(value.value).toBeUndefined();
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expect(value.done).toBeTrue();
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expect(firstFilterCount).toBe(4);
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expect(secondFilterCount).toBe(2);
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});
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});
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