let render = function (str, data) {
  let tpl = str.replace(/<%=([\s\S]+?)%>/g,function (match, code) {
      return "‘ +obj."+code+"+ ‘";
  }) ;

  tpl = "var tpl=‘"+tpl+"‘\n return tpl;";
  let complied = new Function(‘obj‘,tpl);
  return complied(data);
};

let tpl = "Hello <%=username%>.";
console.log(render(tpl,{username:"dana"}));

//上面这种做法每次都要编译执行。可以简化，使得编译一次，多次执行。
//具体做法就是生成一个函数，然后返回，吧参数当做参数传递

let compile = function (str) {
    let tpl = str.replace(/<%=([\s\S]+?)%>/g,function (match, code) {
        return "‘ +obj."+code+"+‘";
    });

    tpl = "var tpl = ‘"+tpl+"‘\nreturn tpl";
    return new Function(‘obj,escape‘,tpl);
};
let render1  = function (compile, data) {
    return compile(data);
};

let res = render1(compile,"Hello <%=username%>.");
console.log(res({username:"dana1"}));

模板解析DEMO