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    partial-application.macro · Version License Travis CI JavaScript Standard Style

    Partial application syntax and implicit parameters for JavaScript, inspired by Scala's _ & Kotlin's it.

    try it live on the online playground


    partial-application.macro provides two symbols - it and _.

    it can be used in an expression passed to a function which implicitly creates a lambda function in place accepting a single argument.

    The _ symbol is inspired by Scala and is used as a placeholder to signal that a function call is partially applied - the original code isn't actually called yet, but will return a new function receiving the arguments you signified as placeholders. Think of the values that aren't placeholders as being "bound", and you'll provide the rest later.

    Check out the examples section to see all the different ways these are useful.


    npm i --save-dev partial-application.macro

    Make sure you also have Babel and babel-macros installed (the following use Babel v7):

    npm i --save-dev @babel/cli @babel/core babel-macros

    ... and configured with Babel:

    module.exports = {
      presets: [],
      plugins: ['module:babel-macros']

    for usage without babel-macros, see standalone plugin

    Then just import and use:

    import { _, it } from 'partial-application.macro'

    it is also the default export, so you could also do:

    import it from 'partial-application.macro'

    The benefits of this explicit import are that linters and type systems won't have a fit over _ and it not being defined. It's also self-documenting and more easily understandable. Anyone looking at your code will know that these symbols come from partial-application.macro.

    set custom tokens

    You can set custom identifiers for these just by using an aliased import.

    import { it as IT, _ as PLACEHOLDER } from 'partial-application.macro'

    or for the default it export:

    import IT from 'partial-application.macro'


    lambda parameters

    Scala, Kotlin, etc have what's called a lambda parameter - an easy shorthand for passing unary (single-argument) functions to other functions (higher order). It's useful in higher order functions like Array#map():

    import it from 'partial-application.macro'
    const people = [
      { name: 'Jeff' },
      { name: 'Karen' },
      { name: 'Genevieve' }

    // -> ['Jeff', 'Karen', 'Genevieve']

    argument placeholders

    Transform this:

    import { _ } from 'partial-application.macro'
    function sumOfThreeNumbers (x, y, z) {
      return x + y + z
    const oneAndTwoPlusOther = sumOfThreeNumbers(1, 2, _)

    ... into this:

    function sumOfThreeNumbers (x, y, z) {
      return x + y + z
    const oneAndTwoPlusOther = _arg => {
      return sumOfThreeNumbers(1, 2, _arg)

    _ and it in assignments

    Most expressions using _ and it can also be used outside function calls and assigned to a variable. Here are some ultra simple cases to demonstrate this:

    import { _, it } from 'partial-application.macro'
    const identity = it
    const isEqualToItself = it === it
    const areSameThing = _ === _

    ... becomes:

    const identity = _it => _it
    const isEqualToItself = _it2 => _it2 === _it2
    const areSameThing = (_arg, _arg2) => _arg === _arg2

    We could implement a hasOwn() function to check if a property exists on an object like this:

    import { _ } from 'partial-application.macro'
    const hasOwn = _.hasOwnProperty(_)
    const object = { flammable: true }
    hasOwn(object, 'flammable')
    // -> true

    other expressions

    You can also put these macros to use within binary expressions, template literals, and most other expressions.

    import { it, _ } from 'partial-application.macro'
    const log = console.log(_)
    log([0, 1, 0, 1].filter(!!it))
    // -> [1, 1]
    const heroes = [
      { name: 'bob', getPower () { return { level: 9001 } } },
      { name: 'joe', getPower () { return { level: 4500 } } }
    log(heroes.find(it.getPower().level > 9000))
    // -> { name: 'bob', getPower: [Function] }
    const greet = `Hello, ${_}!`
    // -> Hello, world!


    .babelrc.js (Babel v7)

    module.exports = {
      presets: [],
      plugins: ['module:babel-macros']

    .babelrc (Babel v6)

      "presets": [],
      "plugins": ["babel-macros"]

    standalone plugin

    A standalone version is also provided for those not already using babel-macros:

    • .babelrc.js (Babel v7)

      module.exports = {
        presets: [],
        plugins: ['module:partial-application.macro/plugin']
    • .babelrc (Babel v6)

        "presets": [],
        "plugins": ["partial-application.macro/plugin"]

    differences between _ and it

    There are two separate constructs provided by partial-application.macro:

    • _: partial application symbol
    • it: implicit parameter symbol

    There are a couple of major difference between the two:


    _ will always traverse upward out of the nearest function call, while it will be transformed in place. It's easiest to see when we look at a simple example:

    import { _, it } from 'partial-application.macro'

    While these look like they might be the same, they'll come out acting very different:

    _arg => return => _it)

    argument reuse

    it always refers to the same argument even when used multiple times in an argument list. _ will always refer to the next argument.

    import { _, it } from 'partial-application.macro'
    console.log(+ _ + _)
    console.log(it + it + it)

    ... are compiled to:

    (_arg, _arg2, _arg3) => console.log(_arg + _arg2 + _arg3)
    console.log(_it => _it + _it + _it)

    caveats & limitations

    _ is a common variable name ( eg. for lodash )

    This is the most obvious potential pitfall when using this plugin. _ is commonly used as the identifier for things like lodash's collection of utilities.

    There are a few reasons this is totally fine.

    1. _ is a common symbol for partial application

      The Scala language uses the underscore as a placeholder for partially applied functions, and tons of JavaScript libraries have also used it - so it's become recognizable.

    2. Monolithic builds of packages like lodash are on the way out

      lodash v5 will be getting rid of the monolithic build in favor of explicitly imported or 'cherry-picked' utilities. So it will become less common to see the entirety of lodash imported, especially with ES module tree-shaking on the horizon.

      On top of that, babel-plugin-lodash still works effectively when you just import what you need like this:

      import { add } from 'lodash'
    3. The plugin allows for [custom symbols][#set-custom-tokens]

      If you do happen to need _ or it as identifiers, you're able to change the imported symbols (using standard aliased imports) to anything you want.

    4. Partial application with _ is damn cool

    comparison to libraries

    Lodash, Underscore, Ramda, and other libraries have provided partial application with a helper function something like _.partial(fn, _) which wraps the provided function, and basically just takes advantage of the fact that {} !== {} to recognize that the monolithic _, _.partial.placeholder, or Ramda's R.__ is a specific object deemed a placeholder.

    This Babel plugin gives you the same features at the syntax level. And on top of that, it adds features no runtime library can manage (like arbitrary expressions) and comes with zero runtime overhead. The macros are compiled away and turn into regular functions that don't have to check their arguments to see if a placeholder was provided.

    see also


    1. Clone the repo: git clone
    2. Move into the new directory: cd partial-application.macro
    3. Install dependencies: npm install
    4. Link the project to itself: npm link && npm link partial-application.macro
    5. Build the source: npm run build
    6. Run tests: npm test

    this project runs on itself, so take note of the npm link step since it's necessary to build or test


    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 build steps [above][#development] but using your forked repo
    3. Modify the source files in the src directory 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 partial-application.macro

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