Initial spec draft

[legendecas]

This is the initial spec draft of the proposal. It follows the (tentative) idea of:

1. The async context storage is a per-agent setting.
2. Specifying unhandledrejection behavior #16: HostPromiseRejectionTracker is called in the context at the time of the promise's rejection.

[jridgewell]

I pushed some changes:

• Minor grammar updates
• Simplified AsyncContextSnapshot() to just return a reference to the current
• Changed AsyncContext.p.run() to create a cloned mapping
• Small updates to the spec steps

[jridgewell]

I've been thinking about 2 extensions to the constructor:

1. A name string
2. A defaultValue value

The name string is intended to aid debugging in dev-tools. I imagine they might want to display the global context mapping, and it'd be more helpful if we could annotate that list in some way. This could be stored on the symbol's description.

The defaultValue would be returned instead of undefined whenever the context hasn't been run in this call stack. This is sometimes useful in React.

[legendecas]

I've added name as the first argument.

However, I'd find defaultValue can be a foot gun as it may return the same reference to a single object and it may be mistakenly modified. A function returning a default value might be a better choice here.

[jridgewell]

Sorry, we should bring this up in committee and add it to the spec afterwards. That with the defaultValue API.

[legendecas]

removed and opened an issue to track it: #29.

[kamilogorek]

Is there a way to see a pretty-rendered version of the current spec?

[legendecas]

Please check out https://tc39.es/proposal-async-context/ for rendered version.