JavaScript string formatting inspired by Python’s `str.format()`
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String::format is a small JavaScript utility which adds a format method to strings. It's inspired by and modelled on Python's str.format().

When format is invoked on a string, placeholders within the string are replaced with values determined by the arguments provided. A placeholder is a sequence of characters beginning with { and ending with }.

About this Fork

dchambers's original version implemented nested variable interpolation, and included support for transformations that would provide functionality similar to the conversions in python's str.format()

It did not implement printf()-style number formatting, so here I'm attempting to do that. At the moment, only signs, integer precision, and field padding are implemented.


string.format(value1, value2, ..., valueN)

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:

"{0} x {0} x {0} = {1}".format(3, 3*3*3)
# "3 x 3 x 3 = 27"

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")
# ERROR: 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:

bobby = first_name: "Bobby", last_name: "Fischer"
garry = first_name: "Garry", last_name: "Kasparov"

"{0.first_name} {0.last_name} vs. {1.first_name} {1.last_name}".format(bobby, garry)
# "Bobby Fischer vs. Garry Kasparov"

When referencing the first positional argument, 0. may be omitted:

repo = owner: "pypy", slug: "pypy", followers: [...]

"{owner}/{slug} has {followers.length} followers".format(repo)
# "pypy/pypy has 516 followers"

If the referenced property is a method, it is invoked and the result is used as the replacement string:

me = name: "David", dob: new Date "26 Apr 1984"

"{name} was born in {dob.getFullYear}".format(me)
# "David was born in 1984"

sheldon = quip: -> "Bazinga!"

"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!"


“Transformers” can be attached to String.prototype.format.transformers:

String::format.transformers.upper = -> @toUpperCase()

"Batman's preferred onomatopoeia: {0!upper}".format("pow!")
# "Batman's preferred onomatopoeia: POW!"

Within a transformer, this is the string returned by the referenced object's toString method, so transformers may be used in conjunction with non-string objects:

peter_parker =
  first_name: "Peter"
  last_name: "Parker"
  toString: -> @first_name + " " + @last_name

"NAME: {!upper}".format(peter_parker)

A transformer could sanitizing untrusted input:

String::format.transformers.escape = ->
  @replace /[&<>"'`]/g, (chr) -> "&#" + chr.charCodeAt(0) + ";"

"<p class=status>{!escape}</p>".format("I <3 EICH")
# "<p class=status>I &#60;3 EICH</p>"

Or pluralize nouns, perhaps:

String::format.transformers.s = -> "s" unless +this is 1

"{0}, you have {1} unread message{1!s}".format("Holly", 2)
# "Holly, you have 2 unread messages"

"{0}, you have {1} unread message{1!s}".format("Steve", 1)
# "Steve, you have 1 unread message"

String::format does not currently define any transformers.


If a format string is used in multiple places, one could assign it to a variable to avoid repetition. The idiomatic alternative is to invoke String::format with no arguments, which produces a reusable function:

greet = "{0}, you have {1} unread message{1!s}".format()

greet("Holly", 2)
# "Holly, you have 2 unread messages"

greet("Steve", 1)
# "Steve, you have 1 unread message"

Running the test suite

make setup
make test