Extended array method, such as map, reduce, fliter, forEach method

map method

This method can be seen as mapping relationships

let arr = [1,2,3,4,4,6,7]
let result = arr.map(item =>  item*3 )    
console.log(result)
// 具体例子 
let score = [50,54,60,70]
let rank = score.map(item => item>=60?'合格':'不合格')
console.log(rank)

reduce method

// 数组中的reduce方法  可以看成汇总  
let arr  = [1,3,4,5]
let result = arr.reduce((pre,next) => pre+next)
console.log(result)

filter method

Method for the filtration fliter

 // 数组中的filter 方法 
    let arr = [1,3,4,5,56,]
    let result = arr.filter(item => item>4)
    console.log(result)   
    // 可以用在过滤一些自己不要的数据上 

forEach method

This method is loop through

   let arr = [1,2,3,45,3]
    arr.forEach((item, index) => {
        console.log(item, index)
    });