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[0.73] Make Hermes the default JS engine #12371

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merged 1 commit into from
Nov 10, 2023

Commits on Nov 9, 2023

  1. [0.73] Make Hermes the default JS engine

    This PR backports microsoft#12321 to 0.73.
    
    This PR makes Hermes the default JS engine for RNW going forward.
    
    For now, this is still managed by setting the `<UseHermes>` MSBuild property. However, if the property is not explicitly set, the default value in `JSEngine.props` will now be `true` instead of `false`.
    
    The property will remain exposed (and now set to `true`) in project's `ExperimentalFeatures.props` files. This will be done for new projects created with the `npx react-native-windows-init` command targeting versions with this change. (Use or omission of the `--useHermes` flag will no longer be recognized at project creation time). Hermes will also be the default for projects created with the new `npx react-native init-windows` CLI command.
    
    Current users attempting to *upgrade* their existing projects with these tools will be given a warning if they were previously using the Chakra engine, and instructions to revert back to using Chakra if necessary. However, support for Chakra will be deprecated in the future, so best to migrate sooner rather than later.
    
    In addition, this PR should also enable Hermes use when consuming the official (experimental) NuGet package binaries.
    
    Finally, this PR updates our CI infrastructure to instead denote "Chakra" (rather than "Hermes") for builds and tests.
    
    - New feature (non-breaking change which adds functionality)
    - Breaking change (fix or feature that would cause existing functionality to not work as expected)
    - This change requires a documentation update
    
    Hermes is the officially supported and maintained JS engine for React Native, and as time goes on, the cost of maintain Chakra (which hasn't been updated in years) only increases. The "new" architecture includes Hermes.
    
    Resolves microsoft#11251
    
    See above
    
    N/A
    
    Ran existing tests
    
    Should this change be included in the release notes: yes
    
    Hermes is now the default JS engine for all new projects and will eventually be the only supported JS engine.
    jonthysell committed Nov 9, 2023
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