_template.js

/** Adding underscores template to the String object */

// Source: http://github.com/documentcloud/underscore 

 var _ = self._ || {};

  // JavaScript templating a-la ERB, pilfered from John Resig's
  // "Secrets of the JavaScript Ninja", page 83.
  // Single-quote fix from Rick Strahl's version.
  _.template = function(str, data) {
    var fn = new Function('obj',
      'var p=[],print=function(){p.push.apply(p,arguments);};' +
      'with(obj){p.push(\'' +
      str.replace(/[\r\t\n]/g, " ")
         .replace(/'(?=[^%]*%>)/g,"\t")
         .split("'").join("\\'")
         .split("\t").join("'")
         .replace(/<%=(.+?)%>/g, "',$1,'")
         .split("<%").join("');")
         .split("%>").join("p.push('")
         + "');}return p.join('');");
    return data ? fn(data) : fn;
  };

// Then you can either provide a context:

var str = "The <%= b.join('') %> jumps over the <%= a %>";
var c = _.template(str,  { 
a : "dog",
b : ['f','o','x']
})

// Or use global scope (self or this):

a = "dog";
b = ['f','o','x'];
str = "The <%= b.join('') %> jumps over the <%= a %>";
var c = _.template(str, self);

// To make it easier to use, you can add it to the String prototype:

String.prototype.template = function(context) { return _.template(this, context || self); }

// and call it like this:
"The <%= a %> jumps over the <%= b.join('') %>!".template()
// to use global variables, or:
"The <%= a %> jumps over the <%= b.join('') %>!".template({ a: "cow", b: ['m','o','o','n' ]})
// to use variables from a context.