js：Array. Reduce cumulative calculation and array consolidation

grammar

array.reduce(function(prevValue, currentValue, currentIndex, arr), initialValue)

Parameters	describe
prevValue	It's necessary . Previous value , Or the return value after calculation .
currentValue	It's necessary . The current element
currentIndex	Optional . Index of the current element
arr	Optional . The array object to which the current element belongs .
initialValue	Optional . The initial value passed to the function

Example ： Cumulative calculation

let list = [1, 2, 3, 5];

let result = list.reduce((prevValue, currentValue)=>{
    
  return prevValue + currentValue;
}, 0);

console.log(result);
// 11

Example ： Merge array

let list = [[1, 2, 3], [], [8, 9]];

let result = list.reduce((prevArr, currentArr)=>{
    
  return prevArr.concat(currentArr);
}, []);

console.log(result);
// [ 1, 2, 3, 8, 9 ]

Reference resources
https://www.runoob.com/jsref/jsref-reduce.html