format.js

if (!String.prototype.format) {
  String.prototype.format = function() {
    var me = this;
    
    function replacer4Array(a) {
      return me.replace(/{(\d+)}/g, function(match, number) {
        var n = parseInt(number, 10);
        
        if (isNaN(n) || typeof a[n - 1] === "undefined") {
          return match;
        } else {
          return a[n - 1];
        }
      });
    }
    
    function replacer4Object(o) {
      return me.replace(/{(\S+?)}/g, function(match, name) {
        return typeof o[name] !== "undefined" ? o[name] : match;
      });
    }
  
    var args = arguments;
    
    switch (args.length) {
      case 0: break;
      case 1: 
        var a = args[0];
        
        if (a instanceof Object) {
          if (a instanceof Array) {
            return replacer4Array(a);
          } else {
            return replacer4Object(a);
          }
        } else {
          return replacer4Array([a]);
        }
        break;
        
      default:
        return replacer4Array(args);
    }
    
    return this;
  }
}

/* Example

console.log("{1}{2}{1}".format([111, 222]));
console.log("{abc}{dfg}{abc}".format({abc: 123, dfg: 345}));
console.log("{1}{2}{1}".format(111, 222));
console.log("{1}{2}{1}".format(111));
console.log("{1}{2}{1}".format("111"));
console.log("{abc}{dfg}{abc}".format({abc: 123}));

*/