Benchmarks: write in Scala or JS, run in your browser.

See an online demo here: https://japgolly.github.io/scalajs-benchmark/.
The tiny source code is here: gh-pages/demo.

Changelogs — Latest: 0.10.0.

1. Include scalajs-benchmark as a dependency in your Scala.JS project.

libraryDependencies += "com.github.japgolly.scalajs-benchmark" %%% "benchmark" % "0.10.0"

1. You write benchmarks.

import japgolly.scalajs.benchmark._
import japgolly.scalajs.benchmark.gui._

object Example {
  val suite = GuiSuite(
    Suite("Example Benchmarks")(

      // Benchmark #1
      Benchmark("foreach") {
        var s = Set.empty[Int]
        (1 to 100) foreach (s += _)
        s
      },

      // Benchmark #2
      Benchmark("fold") {
        (1 to 100).foldLeft(Set.empty[Int])(_ + _)
      }
    )
  )
}

(Hey, can you make that 1 to 100 configurable in the GUI? You sure can.)

1. Add a tiny loader HTML like this.

2. Create a main app and point scalajs-benchmark at your suite of benchmarks.

import org.scalajs.dom.document
import japgolly.scalajs.benchmark.gui.BenchmarkGUI

object Main {

  def main(args: Array[String]) = {
    val body = document getElementById "body"
    BenchmarkGUI.renderSuite(body)(Example.suite)
  }
}

If you have a library of different benchmarks, instead of using renderSuite, you use renderMenu to create an app that lets the user navigate the library and run benchmarks.

Example:

BenchmarkGUI.renderMenu(body)(
  suites.example.Examples.all,
  suites.scala.all)

1. Compile; run in browser. Done.

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Note: This is not affiliated with the official Scala.JS project; it's just named to be informative rather than interesting.