Explain the code around 25 mins into https://www.youtube.com/watch?v=4ECoOX5QZEg&lc=z13jjdxj3waze5aam04cdre5llyaspvi25c.1486236089300698

explaination.js

/* Explaination for the code around 25mins of 
 *  https://www.youtube.com/watch?v=4ECoOX5QZEg&lc=z13jjdxj3waze5aam04cdre5llyaspvi25c.1486236089300698
 */
// We have 4 helper functions
const mult = (a, b) => a * b;
const sum  = (a, b) => a + b;
// this is how the curried "apply" behaves.
const apply = (fn) => (...args) => fn.apply(null, args);
// we don't need to mess up the gist with zip.
const zip = `fahghetttaahhh booowwwwttt it`;

// We have an array with prices of items, and another with a 
// number, how many of each item we want to purchase.
const prices     = [ 99.99, 12.50, 2.00 ];
const quantities = [ 1    , 2    , 5    ];

`So we want to figure out: 
    1 item that costs  99.99  ( 1 * 99.99 ===  99.99 )
    2 items that cost  12.50  ( 2 * 12.50 ===  25.00 )
    5 items that cost  2.00   ( 5 * 2.00  ===  10.00 )
                                  SUBTOTAL:   134.99 `


" This was the result of the chained functions in video to get the subtotal"
let subtotals = zip( prices, quantities ).map( apply(mult) ).reduce( sum, 0 );

" But... wait, what does it all mean? "
" .. Let's break it down shall we... "

" zip takes 2 arrays ( A and B ) to a single 2D-array --> [ [A[0], B[0]] , [A[1], B[1]] , [A[2], B[2] ]... ]"
const priceQty = zip( prices, quantities ) //--> [ [ 99.99, 1 ] , [ 12.50, 2 ] , [ 2.00, 5 ] ]


` Let's simplify subtotals a bit, 
" zip(prices, quantities).map( apply(mult) ).reduce( sum )" is equal to: `
subtotals = priceQty.map( mult.apply(null) ).reduce( sum, 0 );

` 
priceQty.map( mult.apply(null) ) is equal to: `
[ mult.apply(null, [ 99.99, 1 ]) , mult.apply(null, [ 12.50, 2 ]) , mult.apply(null, [ 2.00, 5 ]) ].reduce( sum, 0 );
` 
This is equivelant to writing `
[ mult( 99.99, 1 ) , mult( 12.50, 2 ) , mult( 2.00, 5 ) ].reduce( sum, 0 );
` 
Which in turn is equivelant to writing `
[ 99.99, 25, 10 ].reduce( sum, 0 );

" The above reduction calls sum 3 times... Now for the play by play: (// code down here won't run //) "
[ 99.99 , 25 , 10 ].reduce(sum, 0 );          // --> priceQtys.reduce((a, b) => a + b, 0)
[ /*****/ 25 , 10 ].reduce(sum(0, 99.99 ));   // --> 0 + 99.99   === 99.99
[ /**********/ 10 ].reduce(sum(99.99, 25 ));  // --> 99.99 + 25  === 124.99
[ /*************/ ].reduce(sum(124.99, 10 )); // --> 124.99 + 10 === 134.99

` and now we have 134.99 just like we wanted `
` hope this helped you a little bit `