Function Implementation Hiding Proposal: Goals, Directives, and Alternative Approaches

The article summarizes the current status of the TC39 function-implementation-hiding proposal, which is at stage‑2 and may advance to stage‑3 after the December meeting, urging developers to consider its impact on JavaScript APIs.

It introduces two new directives— hide source and sensitive —that allow developers to hide implementation details from Function.prototype.toString and from the file‑path information exposed by Error.prototype.stack . Example usage:

function foo() { /* ... */ }
console.assert(foo.toString().includes('...'))

When the hide source directive is applied:

'hide source'
function foo() { /* ... */ }
console.assert(!foo.toString().includes('...'))

The directives help library authors avoid breaking changes caused by exposing source code, enable more faithful polyfills, and improve security for sensitive projects.

Typical stack traces before applying the directives look like:

$ node
Welcome to Node.js v12.13.0.
> console.log((new Error).stack)
Error
    at repl:1:14
    at Script.runInThisContext (vm.js:116:20)
    ...

After applying hide source (or sensitive ), the stack output no longer contains the source location of hidden functions.

$ node
Welcome to Node.js v12.13.0.
> console.log((new Error).stack)
Error
    at repl:1:14
    at Script.runInThisContext (vm.js:116:20)
    ... // locations of hidden functions are omitted

The article also discusses alternative solutions that were considered and rejected:

A one‑time hiding function such as Error.hideFromStackTraces or Function.prototype.hideSource .

A clone‑creating hiding function that returns a wrapped version of the original function.

Deleting Function.prototype.toString entirely.

Using a Symbol flag (e.g., Symbol.hideSource ) to toggle hiding.

Each alternative is evaluated for feasibility, compatibility, and tooling impact, with the directive approach favored for its file‑level granularity and backward compatibility.

Frequently asked questions address whether the directives hide function names or parameter counts (they do not), their effect on devtools, and the likelihood of widespread adoption. The proposal deliberately does not include a preserve source directive because the use‑case is rare.

Finally, the article lists relevant links to the proposal repository and discussion threads.