Atom doesn't have built-in support for building any type of code nor does it have built-in support for executing any kind of code other than JavaScript. Atom has a JavaScript interactive command-line (also known as a REPL) available through the Developer Tools. You can access the JavaScript REPL by using the following steps:
View > Developer > Toggle Developer Tools
If you're looking for a JavaScript execution environment beyond a REPL, Atom doesn't come with anything built-in for that purpose.
If you want to build code or execute scripts from within Atom there are a number of packages available including:
Resources on getting started with languages that are commonly asked about: