PLS try this:
import java.text.DecimalFormat
import com.kms.katalon.core.webui.keyword.WebUiBuiltInKeywords as WebUI
WebUI.openBrowser("https://www.google.com")
def v = WebUI.executeJavaScript("return 1.2345", null)
WebUI.closeBrowser()
println "v is instance of " + v.getClass().getName()
println "v=" + v
DecimalFormat df1 = new DecimalFormat("#,###,###,##0.00");
DecimalFormat df2 = new DecimalFormat("#,###,###,##0");
println "v=" + df1.format(v)
println "v=" + df2.format(v)
this will emit
v is instance of java.lang.Double
v=1.2345
v=1.23
v=1