fix(List): preserve undefined values when grown past 32 elements by jdeniau · Pull Request #2235 · immutable-js/immutable-js · GitHub
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fix(List): preserve undefined values when grown past 32 elements #2235

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jdeniau merged 1 commit into from
Jun 29, 2026
+135 −6
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fix(List): preserve undefined values when grown past 32 elements #2235

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7 changes: 4 additions & 3 deletions CHANGELOG.md
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94 changes: 94 additions & 0 deletions __tests__/List.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -935,6 +935,100 @@ describe('List', () => {
);
});

// A List grown past 32 elements while every value is `undefined` must read
// back `undefined` at every index, never `null`.
describe('preserves stored undefined (never reads back as null)', () => {
// Asserts every in-bounds slot reads as `undefined` (not `null`) through
// every read path: get, toArray, the values iterator (both directions),
// forEach, and the structural equals / hashCode.
function expectAllUndefined(list: List<number | undefined>, size: number) {
const expected = List<number | undefined>(
new Array(size).fill(undefined)
);

expect(list.size).toBe(size);
for (let i = 0; i < size; i++) {
expect(list.get(i)).toBeUndefined();
}
expect(list.toArray()).toEqual(new Array(size).fill(undefined));
expect([...list.values()]).toEqual(new Array(size).fill(undefined));
expect([...list.toSeq().reverse().values()]).toEqual(
new Array(size).fill(undefined)
);
list.forEach((value) => expect(value).toBeUndefined());
// @ts-expect-error null should not be here
expect(list.includes(null)).toBe(false);
expect(list.equals(expected)).toBe(true);
expect(list.hashCode()).toBe(expected.hashCode());
}

it('setSize(32).setSize(33) — minimal repro', () => {
expectAllUndefined(
List<number | undefined>().setSize(32).setSize(33),
33
);
});

it('setSize(1).setSize(33) — grows a tiny List across the boundary', () => {
expectAllUndefined(List<number | undefined>().setSize(1).setSize(33), 33);
});

it('setSize(31).setSize(33)', () => {
expectAllUndefined(
List<number | undefined>().setSize(31).setSize(33),
33
);
});

it('setSize(32).setSize(65) — undefined region spanning two leaf nodes', () => {
expectAllUndefined(
List<number | undefined>().setSize(32).setSize(65),
65
);
});

it('setSize(32).push(undefined) — grow via push, not setSize', () => {
expectAllUndefined(
List<number | undefined>().setSize(32).push(undefined),
33
);
});

it('writing undefined past index 32 in a mutation', () => {
const list = List<number | undefined>().withMutations((m) => {
for (let i = 0; i < 33; i++) {
m.set(i, undefined);
}
});
expectAllUndefined(list, 33);
});

// The sequence reported in #2230, reaching the same all-undefined state
// through unshift/delete/shift rather than setSize.
it('unshift/delete/setSize/shift sequence (issue #2230)', () => {
const list = List<number | undefined>()
.unshift(86)
.unshift(87)
.unshift(54) // [54, 87, 86]
.delete(1) // [54, 86]
.setSize(6) // [54, 86, undefined, undefined, undefined, undefined]
.shift()
.shift() // [undefined, undefined, undefined, undefined]
.delete(1); // -> [undefined, undefined, undefined]

expectAllUndefined(list, 3);
});

// A genuine `null` value is a valid element and must round-trip as `null`,
// distinct from an absent / undefined slot.
it('keeps an explicitly stored null as null', () => {
const list = List<number | null>().setSize(32).setSize(33).set(0, null);
expect(list.get(0)).toBeNull();
expect(list.get(1)).toBeUndefined();
expect(list.toArray()[0]).toBeNull();
});
});

it('can be efficiently sliced', () => {
const v1 = Range(0, 2000).toList();
const v2 = v1.slice(100, -100).toList();
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7 changes: 6 additions & 1 deletion eslint.config.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -144,7 +144,12 @@ export default tseslintConfig(
'jest/expect-expect': [
'error',
{
assertFunctionNames: ['expect', 'expectIs', 'expectIsNot'],
assertFunctionNames: [
'expect',
'expectIs',
'expectIsNot',
'expectAllUndefined',
],
additionalTestBlockFunctions: [],
},
],
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33 changes: 31 additions & 2 deletions src/List.js
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