ECMAScript was not the first non-Schemey language implemented by Guile, but it was the first implemented for Guile’s bytecode compiler. The goal was to support ECMAScript version 3.1, a relatively small language, but the implementer was completely irresponsible and got distracted by other things before finishing the standard library, and even some bits of the syntax. So, ECMAScript does deserve a mention in the manual, but it doesn’t deserve an endorsement until its implementation is completed, perhaps by some more responsible hacker.
In the meantime, the charitable user might investigate such invocations
as ,L ecmascript and cat test-suite/tests/ecmascript.test.