ECMAScript Proposal, specs, and reference implementation for Error.prototype.stack/System.getStack/System.getStackString

Spec drafted by @ljharb along with @erights.

This proposal is currently stage 0 of the process.

Errors have never had a stack trace attached in the language spec — however, implementations have very consistently provided one on a property named “stack” on instantiated Error objects. There has long been concern about standardizing stack traces improperly - such that implementations could not claim to be fully compliant while also providing security guarantees. This proposal is an attempt to standardize the intersection of existing browser behavior, in a way that addresses these security concerns.

Object.getOwnPropertyDescriptor(new Error(), 'stack');
Object.getOwnPropertyDescriptor(Error.prototype, 'stack');

node/Chrome: stack is an own property on Error instances with a getter/setter: { configurable: true, enumerable: false }

• The getter returns undefined with a non-Error receiver, and returns the stack property (whatever its current value may be) otherwise.
• The setter is a no-op with a non-Error object receiver, and sets the return of the stack getter otherwise.

Safari: stack is an own string data property on Error instances: { configurable: true, enumerable: false, writable: true }

Firefox: stack is an own property on Error.prototype: { configurable: true, enumerable: false }

• The getter throws on a non-Error receiver, and returns the original stack trace otherwise.
• The setter sets the stack property on a non-Error receiver, and is a no-op on an Error receiver.

Error.prototype.stack is set in stone - this is a defacto reality, and will be implemented in Annex B.

System is the current proposed place to stick a privilege-granting method like getStack/getStackString. This discussion isn‘t particularly important until attempting to reach stage 3 (in other words, while in stage 2) so hopefully we can avoid the distraction until that time.

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