[KevinH]

Have you enabled javascript in Sigil Preferences? It is off by default for security reasons.

Are you creating an epub3? Epub2 does not support javascript.

What e-readers have you tested it with? Not all epub3 e-readers support javascript? And no epub2 only e-readers do that I know of.

Where or how is your javascript function invoked? Defining a javascript function does not invoke it.

And fwiw, scripting the nav which is meant to always be machine parseable is probably not the best idea.