Out-of-bounds array read/write throws RangeError instead of undefined (clean build, runtime abort)

[ryoppippi]

Out-of-bounds array access throws RangeError where Node evaluates to undefined. The build is clean; the divergence only shows up at runtime, and it hits the most ordinary JS idioms there are — destructuring a short array, probing arr[i] before checking length, growing an array by index.

This one cost me the most time while porting a real CLI: six separate sites, each compiling without a diagnostic and then aborting on the first run.

Repro

1. Destructuring past the end

export {}
function main() {
  const a = ['x']
  const [first, second] = a
  console.log(`first=${first} second=${second}`)
}
main()

$ node --experimental-strip-types oob.ts
first=x second=undefined

$ scriptc build oob.ts -o oob && ./oob
scriptc: RangeError: array index 1 out of bounds (length 1)
# exit 134

2. Indexed read past the end

export {}
function main() {
  const a: string[] = ['x']
  console.log(`read=${a[1]}`)
}
main()

Node: read=undefined — scriptc: RangeError: array index 1 out of bounds (length 1).

3. Write past the end (array growth)

export {}
function main() {
  const a: string[] = []
  a[2] = 'z'
  console.log(`len=${a.length} v=${a[2]}`)
}
main()

Node: len=3 v=z — scriptc: RangeError: array index 2 out of bounds (length 0).

Expected

Per the README's "What compiles behaves byte-for-byte like Node", an out-of-range read should evaluate to undefined and an out-of-range write should extend the array.

Why it is easy to hit

These are the shapes real code uses, and none of them look risky:

const [src, selector] = positionals          // optional second CLI argument
const peek = () => toks[pos]                 // recursive-descent parser at EOF
if (bytes[0] === 0xef && bytes[1] === 0xbb)  // BOM sniff on an empty body
while (grid[r][c] !== undefined) c++         // HTML table grid fill
argv[index - 1] === flag                     // look-behind at index 0

Under noUncheckedIndexedAccess the checker already types these as T | undefined, so the source is honest about the undefined case — the runtime just never produces it.

Suggestions

Ideally the read lowers to a bounds test yielding undefined, and the write extends. If matching Node here is genuinely out of scope for the value representation, a compile-time diagnostic would still be a large improvement over a runtime abort — the alternative today is discovering each site by running the binary.

Environment

• scriptc 0.0.17, @scriptc/compiler 0.0.17
• macOS 26.5.2, arm64, Node v24.18.0
• Static build (no --dynamic), default backend