Deprecate support for ECMAScript 5.1.

That support is the biggest source of alternative code paths and polyfills in our codebase. The polyfills and other alternative implementations we need to support ES 5.1 include: * `Math.fround` to support `Float` operations (in `CoreJSLib`). * Other bit manipulation magic for floating-point numbers (in `FloatingPointBits`). * Using dynamic JS `Array`s instead of typed arrays for Scala `Array`s of primitive types. * Using custom Java `Map`s instead of `js.Map`. * Various implementations of math methods in `jl.Math` (although these may come back for a Wasm-only target, along with more of them). * Using run-time generated random property names instead of `Symbol`s. * Very complex code to handle surrogate pairs in `ju.regex.*`. * (Imperfect) desugaring of `...rest` parameters and `...spread` operators. Moreover, the ES 5.1 output does not even have exactly the same semantics as later versions: * JS classes are not true `class`es. Notably, that means they cannot extend native ES classes, and they do not inherit static members. * Top-level exports are declared as `var`s instead of `let`s. 10 years after the instruction of ECMAScript 2015, I believe it is time to deprecate the support for Es 5.1, so that we can eventually remove it.
scala-js · sjrd · Feb 3, 2025 · Feb 3, 2025 · Feb 3, 2025 · Feb 3, 2025
commit 4b42414874955b46e28e12530746af9f4eb088db
diff --git a/linker-interface/shared/src/main/scala/org/scalajs/linker/interface/ESVersion.scala b/linker-interface/shared/src/main/scala/org/scalajs/linker/interface/ESVersion.scala
@@ -30,7 +30,10 @@ final class ESVersion private (val edition: Int, val name: String)
 }
 
 object ESVersion {
-  /** ECMAScrîpt 5.1. */
+  /** ECMAScript 5.1. */
+  @deprecated(
+      "Support for ECMAScript 5.1 is deprecated and will eventually be removed.",
+      since = "1.19.0")
   val ES5_1: ESVersion = new ESVersion(5, "ECMAScript 5.1")
 
   /** ECMAScript 2015 (6th edition). */

diff --git a/...ce/shared/src/test/scala/org/scalajs/linker/interface/StandardConfigFingerprintTest.scala b/...ce/shared/src/test/scala/org/scalajs/linker/interface/StandardConfigFingerprintTest.scala
@@ -42,7 +42,7 @@ class StandardConfigFingerprintTest {
   @Test
   def noFingerprintCollisionESFeatures(): Unit = {
     val sc1 = StandardConfig().withESFeatures(_.withESVersion(ESVersion.ES2015))
-    val sc2 = StandardConfig().withESFeatures(_.withESVersion(ESVersion.ES5_1))
+    val sc2 = StandardConfig().withESFeatures(_.withESVersion(ESVersion.ES2016))
     assertFingerprintsNotEquals(sc1, sc2)
   }
 

diff --git a/linker/jvm/src/main/scala/org/scalajs/linker/backend/closure/ClosureLinkerBackend.scala b/linker/jvm/src/main/scala/org/scalajs/linker/backend/closure/ClosureLinkerBackend.scala
@@ -78,7 +78,6 @@ final class ClosureLinkerBackend(config: LinkerBackendImpl.Config)
     import ClosureOptions.LanguageMode._
 
     esFeatures.esVersion match {
-      case ESVersion.ES5_1  => ECMASCRIPT5_STRICT
       case ESVersion.ES2015 => ECMASCRIPT_2015
       case ESVersion.ES2016 => ECMASCRIPT_2016
       case ESVersion.ES2017 => ECMASCRIPT_2017
@@ -87,6 +86,10 @@ final class ClosureLinkerBackend(config: LinkerBackendImpl.Config)
       case ESVersion.ES2020 => ECMASCRIPT_2020
       case ESVersion.ES2021 => ECMASCRIPT_2021
 
+      // Test for ES5_1 without triggering the deprecation warning
+      case esVersion if esVersion.edition == 5 =>
+        ECMASCRIPT5_STRICT
+
       case _ =>
         throw new AssertionError(s"Unknown ES version ${esFeatures.esVersion}")
     }

diff --git a/linker/shared/src/test/scala/org/scalajs/linker/AnalyzerTest.scala b/linker/shared/src/test/scala/org/scalajs/linker/AnalyzerTest.scala
@@ -654,6 +654,7 @@ class AnalyzerTest {
     }
   }
 
+  @deprecated("test deprecated features", "forever")
   @Test
   def newTargetWithoutES2015(): AsyncResult = await {
     val classDefs = Seq(
-Original file line number
+Diff line change
@@ Expand Up / @@ -654,6 +654,7 @@ class AnalyzerTest { @@
         }
       }
+      @deprecated("test deprecated features", "forever")
       @Test
       def newTargetWithoutES2015(): AsyncResult = await {
         val classDefs = Seq(
@@ Expand Down @@