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JavaScript string formatting inspired by Python’s `str.format()`
CoffeeScript Makefile
Branch: master


String::format is a small JavaScript library for formatting strings, based on Python's str.format(). For example:

'"{firstName} {lastName}" <{email}>'.format(user)
// => '"Jane Smith" <>'

The equivalent concatenation:

'"' + user.firstName + ' ' + user.lastName + '" <' + + '>'
// => '"Jane Smith" <>'



  1. Install:

    $ npm install string-format
  2. Require:

    var format = require('string-format')


  1. Define window.format:

    <script src="path/to/string-format.js"></script>


String::format can be used in two modes: function mode and method mode.

Function mode

format('Hello, {}!', 'Alice')
// => 'Hello, Alice!'

In this mode the first argument is a template string and the remaining arguments are values to be interpolated.

Method mode

'Hello, {}!'.format('Alice')
// => 'Hello, Alice!'

In this mode values to be interpolated are supplied to the format method of a template string. This mode is not enabled by default. The method must first be defined via format.extend:


format(template, $0, $1, …, $N) and template.format($0, $1, …, $N) can then be used interchangeably.

format(template, $0, $1, …, $N)

Returns the result of replacing each {…} placeholder in the template string with its corresponding replacement.

Placeholders may contain numbers which refer to positional arguments:

'{0}, you have {1} unread message{2}'.format('Holly', 2, 's')
// => 'Holly, you have 2 unread messages'

Unmatched placeholders produce no output:

'{0}, you have {1} unread message{2}'.format('Steve', 1)
// => 'Steve, you have 1 unread message'

A format string may reference a positional argument multiple times:

"The name's {1}. {0} {1}.".format('James', 'Bond')
// => "The name's Bond. James Bond."

Positional arguments may be referenced implicitly:

'{}, you have {} unread message{}'.format('Steve', 1)
// => 'Steve, you have 1 unread message'

A format string must not contain both implicit and explicit references:

'My name is {} {}. Do you like the name {0}?'.format('Lemony', 'Snicket')
// => ValueError: cannot switch from implicit to explicit numbering

{{ and }} in format strings produce { and }:

'{{}} creates an empty {} in {}'.format('dictionary', 'Python')
// => '{} creates an empty dictionary in Python'

Dot notation may be used to reference object properties:

var bobby = {firstName: 'Bobby', lastName: 'Fischer'}
var garry = {firstName: 'Garry', lastName: 'Kasparov'}

'{0.firstName} {0.lastName} vs. {1.firstName} {1.lastName}'.format(bobby, garry)
// => 'Bobby Fischer vs. Garry Kasparov'

0. may be omitted when referencing a property of {0}:

var repo = {owner: 'davidchambers', slug: 'string-format'}

// => ''

If the referenced property is a method, it is invoked with no arguments to determine the replacement:

var sheldon = {
  firstName:  'Sheldon',
  lastName:   'Cooper',
  dob:        new Date('1970-01-01'),
  fullName:   function() { return '{firstName} {lastName}'.format(this) },
  quip:       function() { return 'Bazinga!' }

'{fullName} was born at precisely {dob.toISOString}'.format(sheldon)
// => 'Sheldon Cooper was born at precisely 1970-01-01T00:00:00.000Z'

"I've always wanted to go to a goth club. {quip.toUpperCase}".format(sheldon)
// => "I've always wanted to go to a goth club. BAZINGA!"

format.extend(prototype[, transformers])

This function defines a format method on the provided prototype (presumably String.prototype). One may provide an object mapping names to transformers. A transformer is applied if its name appears, prefixed with !, after a field name in a template string.

format.extend(String.prototype, {
  escape: function(s) {
    return s.replace(/[&<>"'`]/g, function(c) {
      return '&#' + c.charCodeAt(0) + ';'
  upper: function(s) { return s.toUpperCase() }

'Hello, {!upper}!'.format('Alice')
// => 'Hello, ALICE!'

var restaurant = {
  name: 'Anchor & Hope',
  url: ''

'<a href="{url!escape}">{name!escape}</a>'.format(restaurant)
// => '<a href="">Anchor &#38; Hope</a>'

Running the test suite

$ npm install
$ npm test
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