There's a much neater way to do this now and it's documented well.
https://github.com/janl/mustache.js/
var customTags = [ '<%', '%>' ];
Mustache.render(template, view, {}, customTags);
//Either tell mustache to use them each time
Mustache.render(template, view, {}, customTags);
//Or override the tags property and Mustache knows what to do until you tell it otherwise
Mustache.tags = customTags;
var updateTemplate = '<small class="text-muted">Last updated <% updated_at %> by <% updated_by %></small>';
var html = Mustache.render(updateTemplate, {updated_at:'2019-11-18 14:54:20',updated_by:'abc'});