Partial

partial.js

const partial = (fn, oldArgs = []) => (...newArgs) => {
    const totalNumberOfArgsRequired = fn.length;
    const args = oldArgs.concat(newArgs);
    const numberOfRemainingArgs = totalNumberOfArgsRequired - args.length;
    return (numberOfRemainingArgs > 0) ?
        partial(fn, args) :
        fn(...args);
}

export default partial;

partialTest.js

/*globals describe, it */
import partial from '../partial';
import assert from 'assert';

describe('partial()', () => {
    it('partially applies argument', () => {
        const add = (a, b) => a + b;
        const addThree = partial(add)(3);
        const expected = add(3, 7);
        const actual = addThree(7)
        assert.equal(actual, expected);
    });
    it('curries a function', () => {
        const add = (a, b, c, d) => a + b + c + d;
        const partialAdd = partial(add);
        const expected = add(1, 2, 3, 4);
        const actual = partialAdd(1)(2)(3)(4)
        assert.equal(actual, expected);
    });
    it('works more than once', () => {
        const add = (a, b, c, d) => a + b + c + d;
        const partialAddOne = partial(add)(1);
        const actualOne = partialAddOne(2, 3, 4);
        const actualTwo = partialAddOne(1, 1, 1);
        const expectedOne = add(1, 2, 3, 4);
        const expectedTwo = add(1, 1, 1, 1);
        assert.equal(actualOne, expectedOne);
        assert.equal(actualTwo, expectedTwo);
    });
    it('allows function with no arguments', () => {
        const noArgs = () => 15;
        const partialNoArgs = partial(noArgs);
        const expected = noArgs();
        const actual = partialNoArgs()
        assert.equal(actual, expected);
    });
    it('still works when invoked upon already partially applied functions', () => {
        const add = (a, b, c, d) => a + b + c + d;
        const partialAdd = partial(add);
        const partialPartialAdd = partial(partialAdd);
        const expected = add(1, 2, 3, 4);
        const actual = partialPartialAdd(1)(2)(3)(4)
        assert.equal(actual, expected);
    });
});