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    strat · NPM License Travis CI JavaScript Standard Style

    strat is a modern, dependency-free JavaScript library for formatting strings. It takes inspiration from Python's str.format() but is focused more on being at home in ES2015+ JavaScript than strictly adhering to Python's conventions.

    If you want stricter Python conventions, check out string-format, on which this project was based.

    try it live on runkit


    • formatting can be partially applied, allowing for reusable template functions
    • reference object properties as replacement values
    • object methods are called and can be passed arguments
    • TypeScript friendly by shipping with definitions



    1. Install

      Using Yarn:


      Or using npm:

      npm i strat
    2. Import

      import strat from 'strat'
      // commonjs / ES5
      const strat = require('strat')


    Usage in TypeScript is the same as ES2015+ with the addition of all exported types:

    import strat, { Transformer, FormatPartial /*...*/ } from 'strat'


    Just drop this repo's index.js in as a script tag to expose the strat function:

    <script src="path/to/strat/index.js"></script>
    strat('{} {}!', ['Hello', 'world'])

    If you prefer a CDN, you can use unpkg:

    <script src=""></script>
    <!-- or, with a version: -->
    <script src=""></script>

    NOTE: strat requires an environment supporting ES2015 syntax like let and arrow functions. For Node, this is Node v4.0 or greater. See for Node compatibility, or compat-table for other environments.


    view from the top

    strat('{name}, {ultimate}, {catchphrase}', hero)
    // -> 'Reinhardt, Earthshatter, "Hammer DOWN!"'

    Compare that to the equivalent string concatenation in ES5: + '' + hero.ultimate + ', "' + hero.catchphrase + '"'

    Or the more modern template literals from ES2015:

    `${}${hero.ultimate}, "${hero.catchphrase}"`

    But the strat function can also be partially applied to create reusable template functions. Just leave out the replacements like so:

    const template = strat('{name}, {ultimate}, {catchphrase}')
    // -> 'Reinhardt, Earthshatter, "Honor! Glory! REINHARDT!"'
    // -> 'Tracer, Pulse Bomb, "Cheers, love! The cavalry's here!"'
    // -> 'McCree, Deadeye, "Ain't I killed you somewhere before?"'


    strat can actually be used in two modes:

    function mode

    strat('You got- you gotta run {}.', 'Morty')
    // -> 'You got- you gotta run Morty.'
    strat(`When you come to a {} in the road, {}.`, ['fork', 'take it'])
    // -> 'When you come to a fork in the road, take it.'

    This is the recommended and standard way to use strat. Here, the first argument is your template string. The second argument is an array of replacement values, or just a single value.

    The second argument can optionally be left out, in which case a new function will be returned that you can call with your replacement parameters.

    const template = strat('Like {} and {}')
    template(['salt', 'pepper'])
    // -> 'Like salt and pepper'
    template(['peanut butter', 'jelly'])
    // -> 'Like peanut butter and jelly'

    method mode

    'Yall got anymore of them... {}?'.format('prototypes')
    // -> 'Yall got anymore of them... prototypes?'
    `Gets hard out here for a {}, {}`.format(['prototype', 'son'])
    // -> 'Gets hard out here for a prototype, son.'

    This mode is not enabled by default and is not recommended - extending built in prototypes with non-standard methods is generally considered a bad practice and it even hinders the progress of the language. If you want to use it as shown above, you must first use strat.extend:


    strat(template, [...values]) and template.format([...values]) can then be used interchangeably.

    Important Note

    You should probably not use this unless you are developing an application. If you are developing a library this will affect end users who have no control over your extension of the built-in String.prototype.


    strat(template: string, replacements: any | [...values]): string

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

    Placeholders may contain numbers which refer to positional arguments:

    strat('{0}, you have {1} unread message{2}', ['Holly', 2, 's'])
    // -> 'Holly, you have 2 unread messages'

    Unmatched placeholders produce no output:

    strat('{0}, you have {1} unread message{2}', ['Steve', 1])
    // -> 'Steve, you have 1 unread message'

    A format string may reference a positional argument multiple times:

    strat(`The name's {1}. {0} {1}.`, ['James', 'Bond'])
    // -> "The name's Bond. James Bond."

    Positional arguments may be referenced implicitly:

    strat('{}, you have {} unread message{}', ['Steve', 1])
    // -> 'Steve, you have 1 unread message'

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

    strat('My name is {} {}. Do you like the name {0}?', ['Lemony', 'Snicket'])
    // -> Error: cannot mix implicit & explicit formatting

    Escape { and } characters by doubling it ( ie. {{ and }} produce { and } respectively ):

    strat('{{}} creates an empty {} {}', ['object', 'literal'])
    // -> '{} creates an empty object literal'

    Dot notation may be used to reference object properties:

    const rick = { firstName: 'Rick', lastName: 'Sanchez' }
    const morty = { firstName: 'Morty', lastName: 'Smith' }
    strat('{0.firstName} {0.lastName} and {1.firstName} {1.lastName}', [rick, morty])
    // -> 'Rick Sanchez and Morty Smith'

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

    const song = {
      title: 'Handlebars',
      artist: 'Flobots',
      album: 'Fight With Tools'
    strat('{title} | [{artist}] | {album}', song)
    // -> 'Handlebars | [Flobots] | Fight With Tools'

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

    const reacher = {
      firstName:   'Jack',
      lastName:    'Reacher',
      dob:         new Date('1960-10-29'),
      fullName:    function () { return strat('{firstName} {lastName}', this) },
      movieSequel: function () { return strat('{fullName}: never go back', this) }
    strat('{fullName} was born {dob.toISOString}.', reacher)
    // -> 'Jack Reacher was born 1960-10-29T00:00:00.000Z.'
    // ... you probably shouldn't know that by the way
    strat('Definitely watch {movieSequel.toUpperCase}', reacher)
    // -> 'Definitely watch JACK REACHER: NEVER GO BACK'

    To pass arguments to a method, pass them as a comma delimited list, with a space after the method name:

    const person = {
      react (tired, mood) {
        if (tired) {
          if (mood === 'sad') return 'cried'
          return 'rolled his eyes'
        } else {
          if (mood === 'mad') return 'broke stuff'
          return 'shook his fist'
    strat('Average Joe {react true, indifferent}.', person)
    // -> 'Average Joe rolled his eyes.'

    Note that all arguments are passed as strings, so you'll have to parse them appropriately if you need, for example, a number or boolean.

    However, you can use _ to pass the falsy null value in the argument list:

    strat('Average Joe {react _, mad}.', person)
    // -> 'Average Joe broke stuff.'

    strat.create(transformers?: { ...functions })

    You can create a new instance of strat by calling strat.create(). You may also optionally supply an object containing transformer functions that you can use in strat() to modify string replacements.

    Transformers are very similar to a function you'd pass to Array#map(). They receive three arguments: the value on which it's being used, the key, and the full collection of replacements provided to the template.

    transform(value: string, key: string, collection: [...values]): string

    To use a transformer, call it by prefixing it with ! after the field name in the template string. For example, {reaction!exclaim} where exclaim was previously passed in the transformers object.

    Here's a simple example operating only on the value argument:

    const instance = strat.create({
      exclaim: str => str.toUpperCase() + '!'
    instance('Hello, {!exclaim}', 'world')
    // -> 'Hello, WORLD!'

    And here's one that uses all three arguments to semi-intelligently pluralize units:

    const instance = strat.create({
      pluralize (str, key, col) {
        const unit = key.slice(0, -5)
        const singular = unit.slice(0, -1)
        return col[0][unit] === 1 ? singular : unit
    const template = instance('{days}{daysLabel!pluralize}')
    template({ days: 1, daysLabel: 'days' }) // -> '1day'
    template({ days: 2, daysLabel: 'days' }) // -> '2days'

    See strat.extend for a more involved example.

    strat.extend(prototype: object, transformers?: { ...functions })

    Important Note

    For most use cases it is recommended to use function mode instead, since extending built in prototypes is usually a bad idea.

    This function can be used to extend any object (usually String.prototype) with a format method. The second argument is optional, and is an object containing transformer functions.

    strat.extend(String.prototype, {
      escape (str) {
        return str.replace(/[&<>"'`]/g, v => {
          return '&#' + v.charCodeAt(0) + ';'
    const store = {
      name: 'Barnes & Noble',
      url: ''
    '<a href="{url!escape}">{name!escape}</a>'.format(store)
    // -> '<a href="">Barnes &#38; Noble</a>'

    see also

    • logger-neue – refined logging utility that utilizes strat


    1. Clone the repo: git clone
    2. Move into the new directory: cd strat
    3. Install dependencies: yarn or npm install
    4. Run tests: yarn test or npm test
    5. Test the TypeScript definitions: yarn test:types or npm run test:types


    Pull requests and any issues found are always welcome.

    1. Fork the project, and preferably create a branch named something like feat-make-better
    2. Follow the development steps above but using your forked repo
    3. Modify the source files as needed
    4. Make sure all tests continue to pass, and it never hurts to have more tests
    5. Push & pull request! 🎉


    MIT © Bo Lingen / citycide


    npm i strat

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