Building strings with ${}

Interpolation is always on: drop any expression into a string with ${}.

fix age = 30;
"next year you'll be ${age + 1}"

That’s a computed value, not just a bare variable; anything that fits inside the braces goes.

The one thing you can’t do is + two strings together.

"a" + "b"

+ stays purely numeric, on purpose: no JavaScript-style 1 + "2" → "12" coercion waiting to surprise you.