editing the core files
Go to current/core/server/helpers
and create handlebars helper
Best to copy exsiting helper and extend off it.
Open the index.js
in that folder:
Add helper to core
coreHelpers.my_helper = require('./my_helper');
And Register helper inside of the registerAllCoreHelpers
function
registerThemeHelper('my_helper', coreHelpers.my_helper);