# string-format

string-format is a small JavaScript library for formatting strings, based on
Python's [`str.format()`][1]. For example:

```javascript
'"{firstName} {lastName}" <{email}>'.format(user)
// => '"Jane Smith" <jsmith@example.com>'
```

The equivalent concatenation:

```javascript
'"' + user.firstName + ' ' + user.lastName + '" <' + user.email + '>'
// => '"Jane Smith" <jsmith@example.com>'
```

### Installation

#### Node

1.  Install:

    ```console
    $ npm install string-format
    ```

2.  Require:

    ```javascript
    const format = require('string-format')
    ```

#### Browser

1.  Define `window.format`:

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

### Modes

string-format can be used in two modes: [function mode](#function-mode) and
[method mode](#method-mode).

#### Function mode

```javascript
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

```javascript
'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`](#formatextendprototype-transformers):

```javascript
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:

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

Unmatched placeholders produce no output:

```javascript
'{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:

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

Positional arguments may be referenced implicitly:

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

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

```javascript
'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 `}`:

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

Dot notation may be used to reference object properties:

```javascript
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}`:

```javascript
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:

```javascript
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.

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

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

const restaurant = {name: 'Anchor & Hope', url: 'http://anchorandhopesf.com/'}

fmt('<a href="{url!escape}">{name!escape}</a>', restaurant)
// => '<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.

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

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

const restaurant = {name: 'Anchor & Hope', url: 'http://anchorandhopesf.com/'}

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

### Running the test suite

```console
$ npm install
$ npm test
```


[1]: http://docs.python.org/library/stdtypes.html#str.format