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string-format

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

> const user = {
.   firstName: 'Jane',
.   lastName: 'Smith',
.   email: '[email protected]',
. }
> '"{firstName} {lastName}" <{email}>'.format (user)
'"Jane Smith" <[email protected]>'

The equivalent concatenation:

> '"' + user.firstName + ' ' + user.lastName + '" <' + user.email + '>'
'"Jane Smith" <[email protected]>'

Installation

Node

  1. Install:

    $ npm install string-format

  2. Require:

    const format = require ('string-format')

Browser

  1. Define window.format:

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

Modes

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.extend (String.prototype, {})

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:

> const bobby = {firstName: 'Bobby', lastName: 'Fischer'}
> const 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}:

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

> 'https://github.com/{owner}/{slug}'.format (repo)
'https://github.com/davidchambers/string-format'

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

> const sheldon = {
.   firstName: 'Sheldon',
.   lastName: 'Cooper',
.   dob: new Date ('1970-01-01'),
.   fullName: function() { return this.firstName + ' ' + this.lastName },
.   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.create (transformers)

This function takes an object mapping names to transformers and returns a formatting function. A transformer is applied if its name appears, prefixed with !, after a field name in a template string.

> const fmt = format.create ({
.   escape: s =>
.     s.replace (/[&<>"'`]/g, c => '&#' + c.charCodeAt (0) + ';'),
.   upper: s =>
.     s.toUpperCase (),
. })

> fmt ('Hello, {!upper}!', 'Alice')
'Hello, ALICE!'

> fmt ('<a href="{url!escape}">{name!escape}</a>', {
.   name: 'Anchor & Hope',
.   url: 'http://anchorandhopesf.com/',
. })
'<a href="http://anchorandhopesf.com/">Anchor &#38; Hope</a>'

format.extend (prototype, transformers)

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

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

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

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

Running the test suite

$ npm install
$ npm test