Faster System.nanoTime(), both in JS and Wasm.

sjrd · sjrd · commit ae8e7608bd8f · 2025-08-28T14:49:33.000+02:00
Instead of storing a lambda returning the high precision time,
we store the global object whose `now()` method returns the high
precision time. This is possible because the only two alternatives,
`performance.now()` and `Date.now()`, happen to share their method
name and the scale of their results.

This implementation removes a JS function call dispatch, which
makes the call faster both in JS and in Wasm.
diff --git a/javalib/src/main/scala/java/lang/System.scala b/javalib/src/main/scala/java/lang/System.scala
@@ -71,18 +71,17 @@ object System {
     js.Date.now().toLong
 
   private object NanoTime {
-    val getHighPrecisionTime: js.Function0[scala.Double] = {
-      if (js.typeOf(global.performance) != "undefined" && !Utils.isUndefined(global.performance.now)) {
-        () => global.performance.now().asInstanceOf[scala.Double]
-      } else {
-        () => js.Date.now()
-      }
+    val highPrecisionTimer: js.Dynamic = {
+      if (js.typeOf(global.performance) != "undefined" && !Utils.isUndefined(global.performance.now))
+        global.performance
+      else
+        global.Date
     }
   }
 
   @inline
   def nanoTime(): scala.Long =
-    (NanoTime.getHighPrecisionTime() * 1000000).toLong
+    (NanoTime.highPrecisionTimer.now().asInstanceOf[scala.Double] * 1000000).toLong
 
   // arraycopy ----------------------------------------------------------------
 
