reduce-Method

/* REDUCE METHOD */
//reduce(function (accumulator, currentValue, currentIndex, array) { /* … */ }, initialValue)


/*
const array1 = [1, 2, 3, 4];

// 0 + 1 + 2 + 3 + 4
const initialValue = 0;
const sumWithInitial = array1.reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  initialValue
);

console.log(sumWithInitial);
// expected output: 10
*/


/*
Why is important to declare initValue?

!!!
The first time that the callback is run there is no
 "return value of the previous calculation".
 If supplied, an initial value may be used in its place.
  Otherwise the array element at index 0 is used as
  the initial value and iteration starts
  from the next element (index 1 instead of index 0).
*/

//example without init value
// average calc in array with reduce


let result = [1, 2, 3].reduce((acc, x, index, arr) => (
    acc + x / arr.length
));

console.log(result); // --> 2,6,
//corret avg value should be 1+2+3=6/3=2 or 1/3+2/3+3/3=2
//but the first zero-indexed element is used as initValue
// operation start applying from the 1-indexed element
console.log(1 + 2 / 3 + 3 / 3); // --> 2,6


// with init value 0, correct way:
// 0+1/3+2/3+3/3=2

let resultOk = [1, 2, 3].reduce((acc, x, index, arr) => (
    acc + x / arr.length
), 0);
console.log(resultOk); //--> 2
//-------------------------------



//01 Returning Array:

// filter even with reduce:

let numbers = [20, 10, 5, 6, 7];

console.log(numbers.reduce((acc, el) => {

    if (el % 2 == 0) {
        acc.push(el);
    }

    return acc;
}, []))
//---------------------------------------



//02 return objet:
let n = 100;
let resObj = ['keyA', 'keyZ', 'key0', 'code00', 345].reduce((acc, curEl, ind, arr) => {

    n++;
    acc[curEl] = n;
    return acc;
}, {'000':'xxx'});

console.log(resObj); //--> {345: 105, 000: 'xxx', keyA: 101, keyZ: 102, key0: 103, code00: 104}

//----- - -- -------------------------

//03 return string, {length} is instead of Array.length

let resStr = ['how', 'to', 'use', 'reduce', 'method', '!'].reduce((acc, x, index, { length }) => {
    let space = ' ';
    if (index == length -1) {
        space='';
    }
    acc += space+x;

    return acc;
}, 'Teach');

console.log(resStr);