context.js

{
  "helper": function(chunk, context, bodies, params) {
    if (typeof params.foo === "function") {
      // if params.foo is a function body, render it
      return params.foo(chunk, context);
    }
    // otherwise just echo the string
    return chunk.write(params.foo);
  },
  "what": "pie"
}

output

I like pie

template

{#helper foo="I like {what}"/}