🚀 Proposal: Cross-Context Function Annotation (for APIs like `page.evaluate` of playwright)

Acknowledgement

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Comment

Motivation

APIs such as page.evaluate in browser automation libraries (e.g. Playwright) execute a function in a different JavaScript runtime context (browser context instead of Node.js).

Example:

const myText = "Hello world";
await page.evaluate(() => {
  console.log(myText); // ❌ Runtime error (myText is not defined in browser)
});

Although this fails at runtime, TypeScript does not report an error. The compiler assumes the lambda executes in the same lexical and global environment as the caller.

This creates a mismatch between TypeScript’s static model and the actual runtime semantics of cross-context APIs.

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Problem Statement

TypeScript currently has no way to express that:

• A function executes in a separate runtime context
• The function must not capture outer-scope variables
• The function must not access Node.js globals
• The function runs against a specific global type (e.g. Window)

As a result:

• Accidental closure capture is allowed
• Incorrect global usage is not flagged
• Runtime errors occur that could be prevented at compile time

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Real-World Example

const secret = 42;

await page.evaluate(() => {
  console.log(secret); // ❌ Runtime error: secret is not defined
});

TypeScript currently allows this because it has no notion of execution context isolation.

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Proposed Solution

Introduce a way to declare that a function:

1. Executes in a separate context
2. Has no access to outer lexical scope
3. Uses a specified global type

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Option A: isolated Function Modifier (New Syntax)

const myText = "Hello world";
await page.evaluate(isolated () => {
  console.log(document.title); // ✅ OK
  console.log(myText);  // ❌ Compile error
});

Semantics:

• No closure capture allowed
• No outer-scope variable access
• Global type defined by API signature
• Treated as if compiled in a separate program with structured-clone semantics

This would be conceptually similar to --isolatedModules, but at function scope.

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Option B: CrossContext<TGlobal> Type-Level Mechanism

If new syntax is not desirable, introduce a type-level mechanism:

type CrossContext<TGlobal, TArgs extends any[], TResult> =
  (this: TGlobal, ...args: TArgs) => TResult;

Library usage example:

interface Page {
  evaluate<R, A>(
    fn: CrossContext<Window, [A], R>,
    arg: A
  ): Promise<R>;
}

With compiler rule:

• Functions of type CrossContext<...> may not reference outer lexical bindings
• Only globals available on TGlobal are allowed

Example behavior:

const x = 5;

await page.evaluate(() => {
  console.log(x);        // ❌ Error: outer scope capture not allowed
  console.log(process);  // ❌ Error: not part of Window
  console.log(document); // ✅ OK
});

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Option C: JSDoc Annotation

await page.evaluate(
  /** @crossContext Window */
  () => {
    console.log(document.title);
  }
);

Compiler behavior:

• Treat function as isolated
• Restrict global access
• Disallow closure capture

This avoids syntax changes while still enabling static safety.

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Prior Art

• Web Workers (structured cloning, no shared closures)
• postMessage semantics
• Rust’s Send / Sync closure restrictions
• Sandboxed runtimes (e.g. Electron multi-process model)

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Benefits

• Prevents a common class of runtime errors
• Improves safety of browser automation, workers, and SSR
• Makes execution semantics explicit
• Improves editor tooling and developer experience
• Enables safer multi-runtime APIs

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Non-Goals

• Not intended as a security boundary
• Not intended to restrict normal functions
• Not a replacement for ESLint, but a stronger compiler guarantee

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Why This Belongs in TypeScript (Not Just ESLint)

While ESLint could detect some closure capture cases, only the TypeScript compiler can:

• Properly reason about type environments
• Enforce global type restrictions
• Integrate deeply with existing type inference

Cross-context execution is increasingly common in:

• Browser automation
• Workers
• Hybrid runtimes
• SSR frameworks

TypeScript currently has no way to model this distinction.

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Summary

TypeScript lacks a mechanism to model execution-context isolation. Introducing an isolated function modifier or a CrossContext<TGlobal> constraint would prevent runtime errors and better reflect modern JavaScript runtime architectures.

El issue no menciona archivos fuente, pruebas ni puntos de entrada del compilador. Empieza comparando los enfoques propuestos de sintaxis, type-level y JSDoc con los ejemplos de captura de cierres y acceso global; después, define el comportamiento del compilador y las pruebas necesarias para demostrar el aislamiento entre contextos.