feat(syntax): Nullish coalescing op ??

[bigopon]

Pull Request

📖 Description

Add support for ?? in the expression parser, and template. Now it's possible to do this

<el value.bind="prop1 ?? prop2">

An issue with Aurelia expression behavior is that Aurelia automatically coerces null/undefined to empty string in non strict mode, which won't work with ??. We will probably need to start thinking of adding safe navigation operator ?. and revisit strict vs non-strict mode soon.

Thanks @fkleuver for the parser work.

[HamedFathi]

Can we have ?. too? (for array and ...) related to this issue

[bigopon]

@HamedFathi thanks for reminding about that issue. That probably should be added too. I'm not sure whether it should be added together with this, or a separate one. cc @fkleuver

[AureliaEffect]

The benchmarking comparison can be found here: https://au2-bench-viewer.azurewebsites.net/compare?branch=master&commit=13a730fabf730bd59498ad4d84fc14361fd922b3&branch=nullish-coalescing-op&commit=ea3371707d9156d9d798b2f602449ba0921b7b4f

[codecov[bot]]

Codecov Report

Merging #1202 (8731a6c) into master (3d468c9) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1202      +/-   ##
==========================================
+ Coverage   87.94%   87.95%   +0.01%     
==========================================
  Files         159      159              
  Lines       17224    17241      +17     
  Branches     3588     3598      +10     
==========================================
+ Hits        15147    15164      +17     
  Misses       2077     2077              

Impacted Files	Coverage Δ
packages/runtime/src/binding/ast.ts	76.61% <100.00%> (+0.05%)	:arrow_up:
packages/runtime/src/binding/expression-parser.ts	100.00% <100.00%> (ø)

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[fkleuver]

I'm not sure whether it should be added together with this, or a separate one.

Doesn't matter, there's no breaking changes or conflicts etc either way. I can't predict very well when I'll have time though at the moment.

[milkshakeuk]

are these things not going to make it into v2?

[bigopon]

are these things not going to make it into v2?

@milkshakeuk null/undefined are often treated as an empty string, hence the mention of strict vs non strict mode (default). Applications normally prefer non strict mode I'd assume, so while it could be useful, it's not a lot more meaningful yet. Though eventually this and ?. should be in.