## Adapter

### ary

Creates a function that accepts up to `n` arguments, ignoring any additional arguments.

Call the provided function, `fn`, with up to `n` arguments, using `Array.slice(0,n)` and the spread operator (`...`).

    const ary = (fn, n) => (...args) => fn(...args.slice(0, n));

Show examples

    const firstTwoMax = ary(Math.max, 2);
    [[2, 6, 'a'], [8, 4, 6], [10]].map(x => firstTwoMax(...x)); // [6, 8, 10]

### call

Given a key and a set of arguments, call them when given a context. Primarily useful in composition.

Use a closure to call a stored key with stored arguments.

    const call = (key, ...args) => context => context[key](...args);

Show examples

    Promise.resolve([1, 2, 3])
      .then(call('map', x => 2 * x))
      .then(console.log); //[ 2, 4, 6 ]
    const map = call.bind(null, 'map');
    Promise.resolve([1, 2, 3])
      .then(map(x => 2 * x))
      .then(console.log); //[ 2, 4, 6 ]

### collectInto

Changes a function that accepts an array into a variadic function.

Given a function, return a closure that collects all inputs into an array-accepting function.

    const collectInto = fn => (...args) => fn(args);

Show examples

    const Pall = collectInto(Promise.all.bind(Promise));
    let p1 = Promise.resolve(1);
    let p2 = Promise.resolve(2);
    let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
    Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)

### flip

Flip takes a function as an argument, then makes the first argument the last.

Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest.

    const flip = fn => (first, ...rest) => fn(...rest, first);

Show examples

    let a = { name: 'John Smith' };
    let b = {};
    const mergeFrom = flip(Object.assign);
    let mergePerson = mergeFrom.bind(null, a);
    mergePerson(b); // == b
    b = {};
    Object.assign(b, a); // == b

### over

Creates a function that invokes each provided function with the arguments it receives and returns the results.

Use `Array.map()` and `Function.apply()` to apply each function to the given arguments.

    const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));

Show examples

    const minMax = over(Math.min, Math.max);
    minMax(1, 2, 3, 4, 5); // [1,5]

### overArgs

Creates a function that invokes the provided function with its arguments transformed.

Use `Array.map()` to apply `transforms` to `args` in combination with the spread operator (`...`) to pass the transformed arguments to `fn`.

    const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val)));

Show examples

    const square = n => n * n;
    const double = n => n * 2;
    const fn = overArgs((x, y) => [x, y], [square, double]);
    fn(9, 3); // [81, 6]

### pipeAsyncFunctions

Performs left-to-right function composition for asynchronous functions.

Use `Array.reduce()` with the spread operator (`...`) to perform left-to-right function composition using `Promise.then()`. The functions can return a combination of: simple values, `Promise`'s, or they can be defined as `async` ones returning through `await`. All functions must be unary.

    const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));

Show examples

    const sum = pipeAsyncFunctions(
      x => x + 1,
      x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
      x => x + 3,
      async x => (await x) + 4
    );
    (async () => {
      console.log(await sum(5)); // 15 (after one second)
    })();

### pipeFunctions

Performs left-to-right function composition.

Use `Array.reduce()` with the spread operator (`...`) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

    const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));

Show examples

    const add5 = x => x + 5;
    const multiply = (x, y) => x * y;
    const multiplyAndAdd5 = pipeFunctions(multiply, add5);
    multiplyAndAdd5(5, 2); // 15

### promisify

Converts an asynchronous function to return a promise.

Use currying to return a function returning a `Promise` that calls the original function. Use the `...rest` operator to pass in all the parameters.

In Node 8+, you can use `util.promisify`

    const promisify = func => (...args) =>
      new Promise((resolve, reject) =>
        func(...args, (err, result) => (err ? reject(err) : resolve(result)))
      );

Show examples

    const delay = promisify((d, cb) => setTimeout(cb, d));
    delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s

### rearg

Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.

Use `Array.reduce()` and `Array.indexOf()` to reorder arguments based on `indexes` in combination with the spread operator (`...`) to pass the transformed arguments to `fn`.

    const rearg = (fn, indexes) => (...args) =>
      fn(
        ...args.reduce(
          (acc, val, i) => ((acc[indexes.indexOf(i)] = val), acc),
          Array.from({ length: indexes.length })
        )
      );

Show examples

    var rearged = rearg(
      function(a, b, c) {
        return [a, b, c];
      },
      [2, 0, 1]
    );
    rearged('b', 'c', 'a'); // ['a', 'b', 'c']

### spreadOver

Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function.

Use closures and the spread operator (`...`) to map the array of arguments to the inputs of the function.

    const spreadOver = fn => argsArr => fn(...argsArr);

Show examples

    const arrayMax = spreadOver(Math.max);
    arrayMax([1, 2, 3]); // 3

### unary

Creates a function that accepts up to one argument, ignoring any additional arguments.

Call the provided function, `fn`, with just the first argument given.

    const unary = fn => val => fn(val);

Show examples

    ['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]