You just use the js primitives.
I'm looking more at things like the Django template library, handlebars, Go HTML templates, etc. etc. And most especially things written with the idea that code shouldn't be in templates. Template languages that were always capable of running code, and were simply separate languages so designers don't have to learn a general-purpose programming language but can learn just this focused thing they need are obviously not what I mean.
I've drunk from the "templates shouldn't have logic in them" well too, but I think as you scale up it just becomes impractical. And in that case, you're better off with a template that always knew it was going to have code in rather than a template language that was designed from the beginning to not have it, but had code sneak in through the back door anyhow. The first one will at least be designed. The second one... well....
Really what we should be doing is generating data structures in <whatever> language rather than creating new languages.
Terraform kinda took a similar path, although the reigning champion of this style of software has to be HTML templating solutions. "Templates shouldn't have code in them! Ever! Any! Totally broken bad bad bad, and that's why I started this new template language that has no code constructs in it at all! Although, uh, we do kinda need to be able to loop through things like table rows from databases... and, well, we need if statements to highlight a particular result... and, well, we need full arithmetic support so we can use the modulo operator on those rows... and we need functions to factor out reusable functionality at the template layer... and oh crap that's all you need for Turing Completeness, isn't it? Well... uhh..." "Hi, I'm another developer and I'm starting a new template language because that previous one is broken and busted and written by dum dums because it has Turing completeness in it! Templates shouldn't have any code in them, ever! Although, uh, I do kinda need for loops for table rows from databases..."