ES6 Template like strings in ES3 compatible syntax.

String.prototype.template.js

// accepts optional transformer
// now transformers are compatible with ES6
String.prototype.template = function (fn, object) {'use strict';
  // Andrea Giammarchi & me - WTFPL License
  var
    hasTransformer = typeof fn === 'function',
    stringify = JSON.stringify,
    re = /\$\{/g,
    strings = [],
    values = hasTransformer ? [] : strings,
    i = 0,
    l = this.length,
    c,
    n,
    str,
    m
  ;
  while ((m = re.exec(this))) {
    str = this.slice(i, m.index)
    if (str.length) {
      strings.push(hasTransformer && str || stringify(str));
    }
    n = 1;
    i = m.index + 2;
    while (i < l) {
      c = this[i++];
      n += ((c == '{') && 1) || ((c == '}') && -1) || 0;
      if (!n) {
        values.push('(' + this.slice(m.index + 2, i - 1) + ')')
        break;
      }
    }
  }
  if (i < l) {
    str = this.slice(i);
    strings.push(hasTransformer && str || stringify(str));
  }
  if (hasTransformer) {
    str = 'function' + (Math.random() * 1e5 | 0);
    strings = [
      str,
      'with(this)return ' + str + '(' + stringify(strings) + (
        values.length ? (',' + values.join(',')) : ''
      ) + ')'
    ];
  } else {
    strings = ['with(this)return ' + strings.join('+')];
  }
  return Function.apply(null, strings).call(
    hasTransformer ? object : fn,
    hasTransformer && fn
  );
};

// --

function uc(strings /* ...values */) {
  return ([].slice.call(arguments, 1)).reduce(function (a, v, i) {
    a.push(strings[i]);
    a.push(String(v).toUpperCase());
    return a;
  }, []).join('');
};

function f(_) { return 'foo'; }

console.log('${num} is the loneliest number'.template({num: 'One'}));
// One is the loneliest number
console.log('hm ... ${f({a: {b : 1}})}'.template({f: f, a: 1}));
// hm ... foo
console.log('hm ... ${f({a: {b : 1}})}'.template(uc, {f: f, a: 1}));
// hm ... FOO