mark js reduce usage

arr.reduce ( function (PREV, CUR, index, ARR) { 
... 
}, the init); 

// ARR  represents the original array; 
// PREV  represents the return value of the callback time of the last call, or the initial value of the init; 
// cur  represents the array element currently being processed; 
// index  represents the index of the array element currently being processed, if provided init value, then the index is 0, or index. 1; 
// init  represents an initial value.

 

reduce () method takes a function as an accumulator, each value in the array (left to right) started to shrink, as a final calculated value.

reduce () function as a higher order, for compose function.

Note:  the reduce () for an empty array will not be implemented callback function.

expand:

1 去 重
const newArr = arr.reduce(function (prev, cur) {
    prev.indexOf(cur) === -1 && prev.push(cur);
    return prev;
},[]);

2 最值
const max = arr.reduce(function (prev, cur) {
    return Math.max(prev,cur);
});

3 求和
const sum = arr.reduce(function (prev, cur) {
    return prev + cur;
},0);