Support ?? in observable bindings

[m-akinc]

Pull Request

Motivation

We are enabling a new eslint rule (@typescript-eslint/strict-boolean-expressions) in Nimble that disallows using non-booleans in boolean expressions. We have some template bindings like x.foo || 'default_val' which are flagged by this rule and would ideally be rewritten as x.foo ?? 'default_val'. However, without proper support for ?? in binding expressions, we would have to rewrite them as x.foo !== undefined ? x.foo : 'default_value', which is excessively verbose.

📖 Description

See ni/nimble#2127

The observable binding logic uses a regex pattern to determine which kinds of expressions in a binding should cause the binding to be marked as volatile. The regex did not include ??, presumably because the FAST libraries targeted es2015, in which a binding like (x) => x.trigger ?? x.value would be transpiled to something like (x) => { var _a; return (_a = x.trigger) !== null && _a !== void 0 ? _a : x.value; }. However, a client library like Nimble, which targets a later version of the language that directly supports ?? requires FAST to include ?? in its regex.

As part of this change, I've:

• updated the regex to include ??
• changed the target of the FAST libraries to es2022 (for consistency with our client libraries)
• fixed an unrelated issue with aggregate bindings never being marked as volatile
• added test cases

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Looks like most of the test suite was accidentally disabled. If enabling the tests causes additional changes to be needed would like to take a look, please reset. If enabling the tests allows them to pass without further changes then feel free to merge