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fix(codemode): align array callback behavior (#36584)

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Aiden Cline 2026-07-13 00:34:18 -05:00 • committed by GitHub
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3 changed files with 77 additions and 22 deletions
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1
packages/codemode/interpreter-support.md
View file

@ -171,7 +171,6 @@ ultimate source of truth.
- [ ] `Array.prototype.toSpliced`.
- [ ] Canonical index handling: a key such as `"01"` must not alias index `1`.
- [ ] Complete sparse-array parity. Promise combinators do consume holes as `undefined` members, as in JS.
- [ ] Correct `findLast` return behavior when its predicate mutates the examined element.
## Strings

43
packages/codemode/src/interpreter/methods.ts
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@ -727,16 +727,16 @@ const invokeArrayMethod = <R>(
}
return undefined
case "reduce": {
let accumulator: unknown
let start: number
if (args.length >= 2) {
accumulator = args[1]
start = 0
} else {
if (length === 0)
throw new InterpreterRuntimeError("Array.reduce of an empty array with no initial value.", node)
accumulator = target[0]
start = 1
let start = 0
let accumulator = args[1]
if (args.length < 2) {
while (start < length && !(start in target)) start += 1
if (start === length)
throw new InterpreterRuntimeError("Array.reduce of an empty array with no initial value.", node).as(
"TypeError",
)
accumulator = target[start]
start += 1
}
for (let index = start; index < length; index += 1) {
if (!(index in target)) continue
@ -745,16 +745,16 @@ const invokeArrayMethod = <R>(
return accumulator
}
case "reduceRight": {
let accumulator: unknown
let start: number
if (args.length >= 2) {
accumulator = args[1]
start = length - 1
} else {
if (length === 0)
throw new InterpreterRuntimeError("Array.reduceRight of an empty array with no initial value.", node)
accumulator = target[length - 1]
start = length - 2
let start = length - 1
let accumulator = args[1]
if (args.length < 2) {
while (start >= 0 && !(start in target)) start -= 1
if (start < 0)
throw new InterpreterRuntimeError("Array.reduceRight of an empty array with no initial value.", node).as(
"TypeError",
)
accumulator = target[start]
start -= 1
}
for (let index = start; index >= 0; index -= 1) {
if (!(index in target)) continue
@ -764,7 +764,8 @@ const invokeArrayMethod = <R>(
}
case "findLast":
for (let index = length - 1; index >= 0; index -= 1) {
if (yield* apply([target[index], index, target])) return target[index]
const item = target[index]
if (yield* apply([item, index, target])) return item
}
return undefined
case "findLastIndex":

55
packages/codemode/test/array-callbacks-test262.test.ts
View file

@ -26,9 +26,11 @@
* - test/built-ins/Array/prototype/forEach/15.4.4.18-7-1.js
* - test/built-ins/Array/prototype/forEach/15.4.4.18-7-2.js
* - test/built-ins/Array/prototype/reduce/15.4.4.21-9-5.js
* - test/built-ins/Array/prototype/reduce/15.4.4.21-9-c-ii-20.js
* - test/built-ins/Array/prototype/reduce/15.4.4.21-9-1.js
* - test/built-ins/Array/prototype/reduce/15.4.4.21-10-1.js
* - test/built-ins/Array/prototype/reduceRight/15.4.4.22-9-5.js
* - test/built-ins/Array/prototype/reduceRight/15.4.4.22-9-c-ii-20.js
* - test/built-ins/Array/prototype/reduceRight/15.4.4.22-9-1.js
* - test/built-ins/Array/prototype/reduceRight/15.4.4.22-10-1.js
* - test/built-ins/Array/prototype/flatMap/depth-always-one.js
@ -210,6 +212,11 @@ const cases = [
code: `let calls = 0; const result = [1].reduce(() => { calls += 1; return 2 }); return [result, calls]`,
expected: [1, 0],
},
{
path: "test/built-ins/Array/prototype/reduce/15.4.4.21-9-c-ii-20.js",
code: `let accessed = false; const result = [11].reduce((previous) => { accessed = true; return previous === undefined }, undefined); return [result, accessed]`,
expected: [true, true],
},
{
path: "test/built-ins/Array/prototype/reduce/15.4.4.21-10-1.js",
code: `const input = [1, 2, 3, 4, 5]; input.reduce(() => 1); return input`,
@ -225,6 +232,11 @@ const cases = [
code: `let calls = 0; const result = [1].reduceRight(() => { calls += 1; return 2 }); return [result, calls]`,
expected: [1, 0],
},
{
path: "test/built-ins/Array/prototype/reduceRight/15.4.4.22-9-c-ii-20.js",
code: `let accessed = false; const result = [11].reduceRight((previous) => { accessed = true; return previous === undefined }, undefined); return [result, accessed]`,
expected: [true, true],
},
{
path: "test/built-ins/Array/prototype/reduceRight/15.4.4.22-10-1.js",
code: `const input = [1, 2, 3, 4, 5]; input.reduceRight(() => 1); return input`,
@ -323,3 +335,46 @@ describe("Test262 Array callback adaptations", () => {
})
}
})
describe("Array callback regressions", () => {
test("reduce and reduceRight find the first present element", async () => {
expect(
await value(`
const left = []
left[2] = 3
const right = []
right[0] = 4
right[3] = 1
right.pop()
return [left.reduce((a, b) => a + b), right.reduceRight((a, b) => a + b)]
`),
).toEqual([3, 4])
})
test("reduce and reduceRight reject arrays containing only holes", async () => {
expect(
await value(`
const values = []
values[2] = 1
values.pop()
let left
let right
try { values.reduce((a, b) => a + b) } catch (error) { left = error.name }
try { values.reduceRight((a, b) => a + b) } catch (error) { right = error.name }
return [left, right]
`),
).toEqual(["TypeError", "TypeError"])
})
test("findLast returns the value observed before predicate mutation", async () => {
expect(
await value(`
const values = [1]
return values.findLast((item, index, array) => {
array[index] = 2
return true
})
`),
).toBe(1)
})
})