final class ESFeatures extends AnyRef

ECMAScript features to use when linking to JavaScript.

The options in ESFeatures specify what features of modern versions of JavaScript are used by the Scala.js linker.

  • Options whose name is of the form useX *force* the linker to use the corresponding features, guaranteeing that the specific semantics that they provide will be used.
  • Options whose name is of the form allowX *allow* the linker to use the corresponding features if it supports them. Support for such options can be dropped in any subsequent version of the linker, including patch versions.
  • Options whose name is of the form avoidX *hint* at the linker to avoid the corresponding features *when it does not affect observable semantics*. They are related to optimizations (for performance or code size). The linker is free to ignore those options.
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  1. final def !=(arg0: Any): Boolean
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  2. final def ##(): Int
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  3. final def ==(arg0: Any): Boolean
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  4. val allowBigIntsForLongs: Boolean

    EXPERIMENTAL: Primitive Longs *may* be compiled as primitive JavaScript bigints.

    EXPERIMENTAL: Primitive Longs *may* be compiled as primitive JavaScript bigints.

    Default: false

    Future versions of Scala.js may decide to ignore this setting.

  5. final def asInstanceOf[T0]: T0
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  6. val avoidClasses: Boolean

    Avoid class'es when using functions and prototypes has the same observable semantics.

    Avoid class'es when using functions and prototypes has the same observable semantics.

    Default: true

    SpiderMonkey is known to exhibit terrible performance with JavaScript class'es, with up to an order of magnitude of performance degradation.

    Setting this option to true provides a hint to the Scala.js linker to avoid using class'es when using other JavaScript features (typically functions and prototypes) has the same observable semantics, in order to improve expected performance. Setting it to false provides a hint not to avoid class'es. Either way, the linker is free to ignore this option.

    Avoiding class'es has a negative impact on code size. If the code is only targeted at engines that are known to have good performance with class'es, it is desirable to set this option to false. If the code is targeted at browsers (among others), it is recommended to set it to true.

    This option never affects the code emitted for JavaScript classes (classes extending js.Any), since that would have an impact on observable semantics.

    This option is always ignored when useECMAScript2015 is false.

  7. val avoidLetsAndConsts: Boolean

    Avoid lets and consts when using vars has the same observable semantics.

    Avoid lets and consts when using vars has the same observable semantics.

    Default: true

    Due to their semantics in JavaScript (their Temporal Dead Zone, TDZ), lets and consts are more difficult for engines to optimize than vars. There have been known cases of dramatic performance issues with them, such as the Webkit issue https://bugs.webkit.org/show_bug.cgi?id=199866.

    Setting this option to true provides a hint to the Scala.js linker to avoid using them when using a var has the same observable semantics, in order to improve expected performance. Setting it to false provides a hint not to avoid lets and consts. Either way, the linker is free to ignore this option.

    Using lets and consts has benefits for humans writing code as they help readability and debugging, but there is little to no benefit in using them when the code is compiler-generated.

    This option is always ignored when useECMAScript2015 is false.

  8. def clone(): AnyRef
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  9. final def eq(arg0: AnyRef): Boolean
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  10. def equals(that: Any): Boolean
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  11. def finalize(): Unit
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  12. final def getClass(): Class[_]
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  13. def hashCode(): Int
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  14. final def isInstanceOf[T0]: Boolean
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  15. final def ne(arg0: AnyRef): Boolean
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  16. final def notify(): Unit
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  17. final def notifyAll(): Unit
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  18. final def synchronized[T0](arg0: ⇒ T0): T0
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  19. def toString(): String
    Definition Classes
    ESFeatures → AnyRef → Any
  20. val useECMAScript2015: Boolean

    Use ECMAScript 2015 features, such as classes and arrow functions.

    Use ECMAScript 2015 features, such as classes and arrow functions.

    Default: true

    When true, the following behaviors are guaranteed:

    • JavaScript classes are true class'es, therefore a) they can extend native JavaScript class'es and b) they inherit static members from their parent class.
    • Throwable classes are proper JavaScript error classes, recognized as such by debuggers.
    • In Script (NoModule) mode, top-level exports are defined as lets rather than vars, and behave as such.
  21. final def wait(): Unit
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  22. final def wait(arg0: Long, arg1: Int): Unit
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  23. final def wait(arg0: Long): Unit
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  24. def withAllowBigIntsForLongs(allowBigIntsForLongs: Boolean): ESFeatures
  25. def withAvoidClasses(avoidClasses: Boolean): ESFeatures
  26. def withAvoidLetsAndConsts(avoidLetsAndConsts: Boolean): ESFeatures
  27. def withUseECMAScript2015(useECMAScript2015: Boolean): ESFeatures

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