Contract types for scriptc: type aliases with no meaning in TypeScript or Node.js that scriptc checks as extra promises. Import them with import type, which TypeScript erases, so Node.js never loads this package.

import type { Immutable, Int32, NoThrow } from "@scriptc/types";

const KEYWORDS: Immutable<Map<string, number>> = new Map([["const", 1]]);

class Token {
  pos: Int32 = 0;
}

function isLetter(ch: number): NoThrow<boolean> {
  return (ch >= 97 && ch <= 122) || (ch >= 65 && ch <= 90);
}

Contracts

NoThrow<T> on a function's return type: the function never throws. Callers need no exception checks.
Int32 on a parameter, variable, class instance field or function result: every stored value is a signed 32-bit integer by construction (an integer literal in range, a bitwise result such as x | 0, Math.imul, another Int32, or arithmetic scriptc can bound). The slot is stored as a 32-bit integer.
Immutable<T> on a parameter, variable, class instance field or function result: a deep read-only view, and no code writes to the value through any reference. A module constant built from literals becomes immortal data.
Acyclic on a class (class C implements Acyclic), interface (interface I extends Acyclic) or object type alias (type T = Acyclic<{ ... }>): no chain of references can lead from such an object back to itself. Its objects carry no cycle-collector header.
ProgramLifetime in a class's implements clause: instances of the class and its subclasses are never freed, so scriptc skips reference counting and cycle collection for them. Memory only: discarded instances are not reclaimed.

scriptc proves each contract or rejects the program with a diagnostic (SC7001-SC7008) that explains why it does not hold. A program that compiles behaves exactly as under Node.js. See https://scriptc.dev/docs/contracts.