ESFeatures

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

Default: false

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

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 esVersion < ESVersion.ES2015.

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 esVersion < ESVersion.ES2015.

The ECMAScript version that is assumed to be supported by the runtime.

Default: ESVersion.ES2015

The linker and the libraries may use this value to:

• provide more features that rely on recent ECMAScript language features, and/or
• dead-code-eliminate away polyfills.

Prefer reading this value over useECMAScript2015Semantics to perform feature tests.

Use the ECMAScript 2015 semantics of Scala.js language features.

Default: true

As of Scala.js 1.6.0, this is true if and only if esVersion >= ESVersion.ES2015. In the future, it might become independently configurable.

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.
• Lambdas for js.Functions that are not also js.ThisFunctions are JavaScript arrow functions (=>). Lambdas for js.ThisFunctions are function functions.
• Throwable classes are proper JavaScript error classes, recognized as such by debuggers.
• In Script (NoModule) mode, top-level exports are defined as lets.

When false, the following behaviors apply instead:

• All classes defined in Scala.js are functions instead of class'es. Non-native JS classes cannot extend native JS class'es and they do not inherit static members from their parent class.
• All lambdas for js.Functions are functions.
• Throwable classes have JavaScript's Error.prototype in their prototype chain, but they are not considered proper error classes.
• In Script (NoModule) mode, top-level exports are defined as vars.

Prefer reading this value instead of esVersion to determine which semantics apply. Doing so will be future-proof if and when this setting becomes configurable independently from esVersion.

Use ECMAScript 2015 features.

Prefer reading esVersion or useECMAScript2015Semantics instead, depending on the use case.

This is always equal to useECMAScript2015Semantics.

Specifies whether the linker should use ECMAScript 2015 features.

If false, this method sets the esVersion to ESVersion.ES5_1. Otherwise, if esVersion was below ES2015, it sets it to ES2015. Otherwise, it returns the same configuration of ESFeatures.