Octopus package steps have a feature that allows you to replace Octopus Variables in any file.
Great, but I use that in JavaScript it needs to work locally before any octopus magic happens. So I use this:
var baseUrl = octoTemplate("#{portalUri}", "http://localhost:4200/");
....
function octoTemplate(value, fallback) {
if (value.substr(0, 2) === "#{") {
return fallback;
}
return value;
}