engine.js

function Eljs(template){

  var chaves = {};

  var pattern = /\#\{([^}]+)\}/g;
  var pattern_compiled = /\$\{([^}]+)\}/g;
  var compiled = "";

  (function(engine){
    
    compiled = template.replace(pattern_compiled, function(p, valor, index){
      chaves[index] = new Function("json", "return json." + valor + ";");

console.log( chaves[index] );

      return "#{" + index + "}";
    })

  })(this);

  this._parse = function(json){
    return compiled.replace(pattern, function(p, valor, index){
      return chaves[valor](json);
    })
  }

  console.log(chaves);

}
Eljs.prototype = {
  parse: function(json){
    return this._parse(json);
  }
}

var template = "<span>O nome completo do paciente é: ${getNome()}</span> com email ${user.email}"
var engine = new Eljs(template);
engine.parse( {  getNome: function(){ return this.user.nome; }, user: { email: "cmilfont@gmail.com", nome: "Paulo" } } );