JavaScript equivalent of Python’s format() function?

Another approach, using the String.prototype.replace method, with a “replacer” function as second argument:

String.prototype.format = function () {
  var i = 0, args = arguments;
  return this.replace(/{}/g, function () {
    return typeof args[i] != 'undefined' ? args[i++] : '';
  });
};

var bar1 = 'foobar',
    bar2 = 'jumped',
    bar3 = 'dog';

'The lazy {} {} over the {}'.format(bar3, bar2, bar1);
// "The lazy dog jumped over the foobar"