README.md

ecma262#3195

This is the repo for the nomative change of tc39/ecma262#3195.

Status: Stage 2

Champions: Chengzhong Wu (@legendecas)

Motivation

The goal is avoid revealing internal slot [[PromiseState]] with promise.then(onFulfill, onRejection) to the promise handlers by host hook requirement.

ECMA262 HostEnqueuePromiseJob defines that a host must implement it as:

• Let scriptOrModule be GetActiveScriptOrModule() at the time HostEnqueuePromiseJob is invoked. If realm is not null, each time job is invoked the implementation must perform implementation-defined steps such that scriptOrModule is the active script or module at the time of job's invocation.

The timing of HostEnqueuePromiseJob depends on internal Promise [[PromiseState]]. The motivation instead expect the ActiveScriptOrModule been determined AT the time promise.then() is invoked.

// module A
const {promise, resolve} = Promise.withResolvers();
promise.then(handler); // (1)

// module B
resolve(); // HostEnqueuPromiseJob for (1)
promise.then(handler); // (2) HostEnqueuPromiseJob

That is, in the above example, the ActiveScriptOrModule for the handler at call site (1) should be "module A", and at call site (2) should be "module B".

HTML integration

HTML HostEnqueuePromiseJob doesn't follow the requirements of ecma262.

However, HTML spec defines that the active ScriptOrModule is saved at the time of HostMakeJobCallback is invoked rather than at the time of HostEnqueuePromiseJob is invoked. The two host hooks are invoked with the same active script or module consecutively.

This change also mandates that the active scripts are propagated through promise jobs and FinalizationRegistry cleanup callbacks.

For example, the following example uses HTML APIs that depends on the original script's incumbent document URL.

const frame = frames[0];
const setLocationHref = Object
  .getOwnPropertyDescriptor(frame.location, "href")
  .set
  .bind(frame.location);

framePromise.resolve("./page-1").then(setLocationHref);

This proposed behavior exists in the HTML spec since HTML HostMakeJobCallback already propagates the active script.

Proposed change

Checkout tc39/ecma262#3195

The reality of the implementation status is:

.	resolved.then(f)	resolve.then(v => f(v))	pending.then(f)	pending.then(v => f(v))
HTML	outer	outer	outer	outer
ECMA-262	outer	outer	☹️ inner	☹️ inner
Chrome/Firefox	☹️ inner	outer	☹️ inner	outer
Safari	outer	outer	☹️ inner	☹️ inner

• pending is resolve in inner iframe.
• f is iframe setLocationHref.
• outer: url is resolved to outer page.
• inner: url is resolved to inner iframe.

Conclusion:

• ECMA-262 and Safari reveal the [[PromiseState]].
• Chrome/Firefox don't respect .then(f) equals to .then(v => f(v))
• HTML and the proposed behavior have neither of those problems

The proposed change also fixes the inconsistent requirement on ecma262 HostEnqueuePromiseJob and HTML HostMakeJobCallback.