Einar Egilsson

Javascript format strings

Python has been my favorite language for a while but since I started making Mozilla Extensions I’ve started to like Javascript a lot too. One thing I miss from Python though are the format strings. In Python you can always create a string like this:

s = '%s is a %s' % ('John', 'Moron')

Since Javascript is a very flexible language and allows you to alter its built in types I decided to try to create something similar for it. Here’s what I came up with:

String.prototype.$ = function() {

  s = this;
  if (arguments.length == 1 && arguments[0].constructor == Object) {
    for (var key in arguments[0]) {
      s = s.replace(new RegExp("\$" + key, "g"), arguments[0][key]);
    }
  } else {
    for (var i = 0; i < arguments.length; i++) {
      s = s.replace(new RegExp("\$" + (i+1), "g"), arguments[i]);
    }
  }
  return s;
};

The function works in two ways.

1. You can send in 1-n parameters and then $1 – $n in the string will be replaced by the parameters values. Example

   var undef; //Undefined value
   var s = 'My name is $1 and I am $2 years old. This is a null variable: $3, this one is undefined: $4' .$ ('Einar', 27, null, undef);
   alert(s);
   

   Test

2. You can send in 1 parameter that’s a javascript object and then $keyname in the string will be replaced by the value for ‘keyname’ in the javascript object. Example

   var d; //Undefined value
   var dict = { name : 'Einar', age : 27, nullvar : null, undef : d };
   var s = 'My name is $name and I am $age years old. This is a null variable: $nullvar, this one is undefined: $undef' .$ (dict);
   alert(s);
   

   Test

This works fine in Mozilla Firefox and Internet Explorer 7, I haven’t tested it in other browsers. I know the $ is used for document.getElementById in prototype and maybe other js libraries but I thought it was the best choice here too, I don’t think it’s confusing to use it for these two things. In case the syntax highlighting stuff screwed up the code on the page you can also download it here.