Don't require manually running npm install before the JS tests

.travis.yml

@@ -1,6 +1,4 @@
 language: java
-install:
-  - npm install
 os:
   - osx
 script:

DEVELOPMENT.md

@@ -90,14 +90,9 @@ $ sbt
 
 ## Run unit tests (JS)
 
-The JS tests require node to be installed, and a few dependencies to have been
+The JS tests require node to be installed. They automatically run `npm install` from the root of the coursier sources if needed.
-fetched with
-```
-$ npm install
-```
-(run from the root of the coursier sources).
 
-JS tests can then be run like JVM tests, like
+JS tests can be run like JVM tests, like
 ```
 $ sbt
 > ++2.12.4
@@ -190,4 +185,4 @@ we need to put 2) and 3) into `message`:
 
 Typically there needs to be at least 2 minor versions between since-version and to-be-removed-version to help migration.
 
-For example, if since version is 1.1.0, then deprecation can be removed in 1.3.0
+For example, if since version is 1.1.0, then deprecation can be removed in 1.3.0

build.sbt

@@ -41,7 +41,8 @@ lazy val tests = crossProject
   .disablePlugins(ScriptedPlugin)
   .dependsOn(core, cache % "test", scalaz)
   .jsSettings(
-    scalaJSStage.in(Global) := FastOptStage
+    scalaJSStage.in(Global) := FastOptStage,
+    testOptions := testOptions.dependsOn(runNpmInstallIfNeeded).value
   )
   .configs(Integration)
   .settings(

project/Settings.scala

@@ -1,4 +1,6 @@
 
+import java.nio.file.Files
+
 import sbt._
 import sbt.Keys._
 import sbt.ScriptedPlugin.autoImport.{sbtLauncher, scriptedBufferLog, ScriptedLaunchConf, scriptedLaunchOpts}
@@ -45,6 +47,28 @@ object Settings {
     javacOptions.in(Keys.doc) := Seq()
   )
 
+  val runNpmInstallIfNeeded = Def.task {
+    val baseDir = baseDirectory.in(ThisBuild).value
+    val evFile = baseDir / "node_modules" / ".npm_run"
+    val log = streams.value.log
+    if (!evFile.exists()) {
+      val cmd = Seq("npm", "install")
+      val b = new ProcessBuilder(cmd: _*)
+      b.directory(baseDir)
+      b.inheritIO()
+      log.info(s"Running  ${cmd.mkString(" ")}")
+      val p = b.start()
+      val retCode = p.waitFor()
+      if (retCode == 0)
+        log.info(s"${cmd.mkString(" ")}  ran successfully")
+      else
+        sys.error(s"${cmd.mkString(" ")}  failed (return code $retCode)")
+
+      // Parent dir should have been created by npm install
+      Files.write(evFile.toPath, Array.emptyByteArray)
+    }
+  }
+
   lazy val shared = javaScalaPluginShared ++ Seq(
     scalaVersion := scala212,
     libs ++= {