I first learned about JScript in the mid-2000s because I often accessed the Windows Scripting Host to have access to some functions not available in the language I was working in at the time. Later, I found that you could write and execute standalone .js and .vbs files in JScript and VBScript. Since then, I have used JScript to write various scripts to automate processes in Windows including running programs that support command line access, accessing the ImageMagick ActiveX, and performing instructions more complex than those supported by Windows batch files. Examples of each of these implementations can be seen in the example scripts I have on this site.
This is the latest help documentation for Windows Scripting which implements JScript. Microsoft's latest version is 5.7, but, as far as I know, the last help documentation they released was for version 5.6.
var shell = new ActiveXObject("WScript.Shell"); shell.Popup("Hello World.");