"Cannot find an implicit ExecutionContext" error in scala.js example app.

Question

Here is an example from the Hands-on Scala.js ebook:

package webpage  import org.scalajs.dom.ext.Ajax  import scala.scalajs.js import scala.scalajs.js.annotation.JSExport import scalatags.JsDom.all._ import org.scalajs.dom import dom.html  @JSExport object HelloWorld1 extends {   @JSExport   def main(target: html.Div) = {      lazy val box = input(       `type`:="text",       placeholder:="Type here!"     ).render      lazy val output = div(       height:="400px",       overflowY:="scroll"     ).render      box.onkeyup = (e: dom.Event) => {       output.innerHTML = "Loading..."       fetchWeather(box.value)     }      target.appendChild(       div(         h1("Weather Search"),         p(           "Enter the name of a city to pull the ",           "latest weather data from api.openweathermap.com!"         ),         p(box),         hr, output, hr       ).render     )      def fetchWeather(query: String) = {       val searchUrl =         "http://api.openweathermap.org/data/" +           "2.5/find?type=like&mode=json&q=" +           query        for{         xhr <- Ajax.get(searchUrl)         if query == box.value       } js.JSON.parse(xhr.responseText).list match{         case jsonlist: js.Array[js.Dynamic] =>           output.innerHTML = ""           showResults(jsonlist, query)         case _ =>           output.innerHTML = "No Results"       }     }     def showResults(jsonlist: js.Array[js.Dynamic], query: String) = {       for (json <- jsonlist) {         val name = json.name.toString         val country = json.sys.country.toString         val weather = json.weather.pop().main.toString          def celsius(kelvins: js.Dynamic) = {           kelvins.asInstanceOf[Double] - 273.15         }.toInt          val min = celsius(json.main.temp_min)         val max = celsius(json.main.temp_max)         val humid = json.main.humidity.toString         val (first, last) = name.splitAt(query.length)         output.appendChild(           div(             b(span(first, backgroundColor:="yellow"), last, ", ", country),             ul(               li(b("Weather "), weather),               li(b("Temp "), min, " - ", max),               li(b("Humidity "), humid, "%")             )           ).render         )       }     }   } } 

Got an error when compiling:

[info] Compiling 1 Scala source to /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/target/scala-2.11/classes... [error] /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/src/main/scala/HelloWorld1.scala:51: Cannot find an implicit ExecutionContext. You might pass [error] an (implicit ec: ExecutionContext) parameter to your method [error] or import scala.concurrent.ExecutionContext.Implicits.global. [error]         if query == box.value [error]                  ^ [warn] /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/src/main/scala/HelloWorld1.scala:53: non-variable type argument scala.scalajs.js.Dynamic in type pattern scala.scalajs.js.Array[scala.scalajs.js.Dynamic] is unchecked since it is eliminated by erasure [warn]         case jsonlist: js.Array[js.Dynamic] => [warn]                           ^ [warn] one warning found [error] one error found [info] workbench: Checking example-fastopt.js [info] workbench: Checking webpage-fastopt.js [error] (compile:compile) Compilation failed [error] Total time: 0 s, completed 7 sept. 2015 12:51:38 78. Waiting for source changes... (press enter to interrupt) 

What went wrong here?

Answer 1

As the error suggests, add the following imports:

import scala.concurrent._ import ExecutionContext.Implicits.global