Variable-argument string formatter with {token} syntax that supports object properties and argument indices

annotated.js

function (
    f // fhe format specifier
    // followed by any number of arguments
) {
    var a = arguments; // store outer arguments
    return ("" + f) // force format specifier to String
        .replace( // replace tokens in format specifier
            /\{(?:(\d+)|(\w+))\}/g, // match {token} references
            function (
                s, // the matched string (ignored)
                i, // an argument index
                p // a property name
            ) {
                return p && a[1] // if property name and first argument exist
                    ? a[1][p] // return property from first argument
                    : a[i] // assume argument index and return i-th argument
            })
}

index.js

function(f){var a=arguments;return(""+f).replace(/\{(?:(\d+)|(\w+))\}/g,function(s,i,p){return p&&a[1]?a[1][p]:a[i]})}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Ates Goral <http://magnetiq.com>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "format",
  "keywords": [ "format", "string", "token", "template" ]
}

test.js

var format = function(f){var a=arguments;return(""+f).replace(/\{(?:(\d+)|(\w+))\}/g,function(s,i,p){return p&&a[1]?a[1][p]:a[i]})};

console.log(format("{1} and {2}", "apples", "pears")); // apples and pears
console.log(format("{1} {1} {2}", "hip", "hooray")); // hip hip hooray
console.log(format("{key}: {value}", { key: "life", value: 42 })); // life: 42
console.log(format("mixing {prop} and {2}", { prop: "property" }, "index"));
    // mixing property and index

The case in which the selected item is not present in the Arguments returns "undefined" into the string.

@atk, I think that is acceptable behaviour. Do you have an alternative suggestion?

There are 2 possibilities to change this:

1. if you accept to reduce the replacements to truthy items (anything that is true on a boolean level), you could add "||s" to the end of the return statement of the callback to insert the original statement in this cases.
2. if you add another undefined argument "u", you could check if the found statement is undefined, but I don't know if this is possible within the restriction of 140 characters.

Regards, atk

You can save 6 bytes if you like. Omit one of the three parentheses. Omit the backslash escaping (because { is not followed by a number).

function(f){var a=arguments;return(""+f).replace(/{(\d+|(\w+))}/g,function(s,i,p){return p&&a[1]?a[1][p]:a[i]})}

Also see @tkissing's mustacheStyleTemplatesCompressed that does similar string formatting.