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LibJS: Implement Iterator.prototype.flatMap

This prototype is a bit tricky in that we need to maintain the iteration
state of the mapped iterator's inner iterator as we return values to the
caller. To do this, we create a FlatMapIterator helper to perform the
steps that apply to the current iteration state.
This commit is contained in:
Timothy Flynn 2023-06-25 13:16:10 -04:00 committed by Andreas Kling
parent 67028ee3a3
commit ad42b4ea67
3 changed files with 277 additions and 0 deletions

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describe("errors", () => {
test("called with non-callable object", () => {
expect(() => {
Iterator.prototype.flatMap(Symbol.hasInstance);
}).toThrowWithMessage(TypeError, "mapper 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().flatMap(() => 0);
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().flatMap(() => 0);
iterator.next();
}).toThrow(TestError);
});
test("mapper function throws", () => {
function TestError() {}
class TestIterator extends Iterator {
next() {
return {
done: false,
value: 1,
};
}
}
expect(() => {
const iterator = new TestIterator().flatMap(() => {
throw new TestError();
});
iterator.next();
}).toThrow(TestError);
});
test("inner mapper value is not an object", () => {
function* generator() {
yield "Well hello";
yield "friends :^)";
}
expect(() => {
const iterator = generator().flatMap(() => Symbol.hasInstance);
iterator.next();
}).toThrowWithMessage(TypeError, "obj is not an object");
});
});
describe("normal behavior", () => {
test("length is 1", () => {
expect(Iterator.prototype.flatMap).toHaveLength(1);
});
test("mapper function sees every value", () => {
function* generator() {
yield "Well hello";
yield "friends :^)";
}
let count = 0;
const iterator = generator().flatMap((value, index) => {
++count;
switch (index) {
case 0:
expect(value).toBe("Well hello");
break;
case 1:
expect(value).toBe("friends :^)");
break;
default:
expect().fail(`Unexpected mapper invocation: value=${value} index=${index}`);
break;
}
return value.split(" ").values();
});
for (const i of iterator) {
}
expect(count).toBe(2);
});
test("inner values are yielded one at a time", () => {
function* generator() {
yield "Well hello";
yield "friends :^)";
}
const iterator = generator().flatMap(value => value.split(" ").values());
let value = iterator.next();
expect(value.value).toBe("Well");
expect(value.done).toBeFalse();
value = iterator.next();
expect(value.value).toBe("hello");
expect(value.done).toBeFalse();
value = iterator.next();
expect(value.value).toBe("friends");
expect(value.done).toBeFalse();
value = iterator.next();
expect(value.value).toBe(":^)");
expect(value.done).toBeFalse();
value = iterator.next();
expect(value.value).toBeUndefined();
expect(value.done).toBeTrue();
});
});