Support for the ECMAScript 'throw' operator

[rbuckton]

This PR adds support for the stage-2 ECMAScript 'throw' operator: https://github.com/tc39/proposal-throw-expressions.

NOTE: This feature does not yet support reachability checks at the expression level (e.g. in (throw x), y, y should be considered unreachable). I can investigate this either as part of this PR or (preferably) in a follow-up PR. Our current reachability algorithm does not work well with expressions and will require some further investigation to make it more robust.

Fixes: #18535

[weswigham]

@rbuckton Can you include a link to the ECMAScript proposal in the OP, for posterity?

[rbuckton]

@weswigham, I've updated the description. I was waiting on the transfer of the proposal repository to the tc39 organization.

[weswigham]

👍

[rbuckton]

I'm going to add an --experimentalThrowExpressions option before this is merged.

[mhegazy]

Based on today's discussion (#18895), we want to hold off on this untill it makes stage 3, then include it without a flag.

[mihailik]

Are there plans for a similar try expression?

const _crypto = try { require('crypto') } catch { }

[plantain-00]

@mihailik
need to propose it tc39 first https://github.com/tc39/proposals

[mihailik]

@plantain-00 not really, in fact it often goes in the opposite direction.

[Jessidhia]

@mihailik nope, that is exactly how it is supposed to be done -- via TC39. TypeScript is not adding any additional syntax that is not being tracked by TC39, besides the current syntax that is being grandfathered in (as, namespace, enum) and JSX.

At any rate, I expect that, were you to propose it right now, it would get deferred in favor of do expressions. Alternatively, a proposal to also convert if/for/while into expressions.

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