string format

format.js

function format(fmt, args) {
  if (!fmt) return '';

  var toString = Object.prototype.toString;
  if (toString.call(args) === '[object Object]') {
    return fmt.replace(/%{?([a-z]*)}?/g, function(all, m1){
      return args[m1] ? args[m1] : all;
    });
  } else {
    if (toString.call(args) !== '[object Array]') {
      args = Array.prototype.slice.call(arguments, 1);
    }
    return fmt.replace(/%([sd])/g, function(all, m1){
      if (!args.length) return all;
      var arg = args.shift();
      return m1 === 's' ? String(arg) : Number(arg);
    });
  }
}

test.js

assert(format('a%s:%s', 's'), 'as:%s');
assert(format('a%s:%s', 's', 'd', 't'), 'as:d');
assert(format('a%s:%s', ['s', 'd', 't']), 'as:d');
assert(format('a%s', 1), 'a1');
assert(format('a%d', '3d'), 'aNaN');
assert(format('a%q', 's'), 'a%q');
assert(format('a%{a}b%b%a', {a:1, b:2}), 'a1b21');

function assert(expect, truth) {
  console.log(expect === truth);
}