compose.magic_reduce.js

const compose = (...fns) => (x) => fns.reduce( (v, f) => v.then ? v.then(f) : f(v), x)

compose_simple.es6.js

function compose(...fns) {
    var left = fns.shift();
    return fns.length ? wrap(compose.apply(null, fns), left) : left;

    function wrap(inner, outer) {
        return function wrapped(...args) {
            var innerResult = inner.apply(null, args);
            return innerResult.then ? innerResult.then(outer) : outer(innerResult);
        }
    }
}

compose_simple.js

function compose() {
    var functions = [].slice.call(arguments);
    var composed = functions.pop();
 
    if (functions.length == 0)
        return composed;
 
    var current;
    while (current = functions.pop()) {
        composed = wrap(composed, current);
    }
    return composed;
 
    function wrap(inner, outer) {
        return function() {
            var args = [].slice.call(arguments);
            var innerResult = inner.apply(null, args);
            return innerResult.then ? innerResult.then(outer) : outer(innerResult);
        }
    }
}

compose_simple_tests.js

describe('compose', function() {
  it('can compose 2 functions', function() {
    var lowerback = compose(lower, reverse);
    expect(lowerback('BAR')).toBe('rab');
  });

  it('can compose functions with same input params', function() {
    var lower_back_double = compose(lower, reverse, repeat);
    expect(lower_back_double('BAR')).toBe('rraabb');
  });

  it('can compose functions with different input params', function() {
    // order matters in this ecample
    var lower_back_double = compose(lower, join, split, reverse, repeat);
    expect(lower_back_double('BAR')).toBe('rraabb');
  });

  it('should handle flowing a promise', function(done) {
    var asyncTest = compose(lower, reverseAsync);
    asyncTest('BAR').then(function(result){
        expect(result).toBe('rab');
        done();
    })
  });

  it('should handle flowing multiple promises', function(done) {
    var asyncTest = compose(reverseAsync, lower, reverseAsync);
    asyncTest('BAR').then(function(result){
        expect(result).toBe('bar');
        done();
    })
  });
});

function lower(str) { return str.toLowerCase(); }

function split(str) { return str.split(''); }

function join(strArray){ return strArray.join(''); }

function reverse(str) { return join(split(str).reverse()); }
 
function reverseAsync(str){ return Promise.resolve(reverse(str)); }

function repeat(str) {
    return str.split('').map(function(letter) {
        return letter + letter
    }).join('');
}