Js filter() function

Definition and usage

The filter() method creates a new array whose elements are checked by checking all elements in the specified array that meet the criteria.

Note: filter() does not check for empty arrays.

Note: filter() does not alter the original array

The filter() function is a higher-order function used to filter some elements and then return some elements

Let's not talk much, just go to the code

			// filter函数的参数
            let arr = ['a', 'b', 'c'] 
            let array = arr.filter((item, index, self) => {
    
    
            console.log(item, index, self) 
			})
             //依次返回   a 0 (3) ["a", "b", "c"]           b 1 (3) ["a", "b", "c"]         c 2 (3) ["a", "b", "c"]
             //即第⼀个参数是⾥⾯的元素,第⼆个参数为元素索引值,第三个参数为数组本⾝

	// 1.⽤filter筛选出数组⾥所有偶数
	    let arr1 = [1, 2, 3, 4, 5, 6]
	    let arr2 = arr1.filter(item => {
    
    
	        return item % 2 == 0
	    })
	    console.log(arr2) // [2, 4, 6]
	
	
	    // 3.filter数组去重
	    let arr5 = ['11', 'aaa', 'bb', 'aaa', 'bb']
	    let arr6 = []
	    arr6 = arr5.filter((item, index, self) => {
    
    
	        return self.indexOf(item) === index
	    })
	    console.log(arr6) // ["11", "aaa", "bb"]
	
	
	    // 4.过滤掉数组⾥的⼩值:
	    let arr7 = [1, 2, 3, 4, 5, 7, 60]
	    let arr8 = arr7.filter((item) => {
    
    
	        return item >= 4
	    })
	    console.log(arr8) //[4, 5, 7, 60]
	    

example:

var properties =[
       '1','2','3','4','undefined','undefined',undefined,null
]

// let aa = properties.filter((item,index) => { 
//     if( item != 'undefined' && item != null && item != undefined){
    
    
//         return  item
//     }    
// })
let aa = properties.filter(item => item != 'undefined' && item != null && item != undefined)