5个数组Array方法: indexOf、filter、forEach、map、reduce使用实例
1) indexOf
indexOf()方法返回在该数组中第一个找到的元素位置,如果它不存在则返回-1。
不使用indexOf时
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var arr = [ 'apple' , 'orange' , 'pear' ], found = false ; for ( var i= 0, l = arr.length; i< l; i++){ if (arr[i] === 'orange' ){ found = true ; } } console.log( "found:" ,found); |
使用后
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var arr = [ 'apple' , 'orange' , 'pear' ]; console.log( "found:" , arr.indexOf( "orange" ) != -1); |
2) filter
该filter()方法创建一个新的匹配过滤条件的数组。
不用 filter() 时
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var arr = [ { "name" : "apple" , "count" : 2}, { "name" : "orange" , "count" : 5}, { "name" : "pear" , "count" : 3}, { "name" : "orange" , "count" : 16}, ]; var newArr = []; for ( var i= 0, l = arr.length; i< l; i++){ if (arr[i].name === "orange" ){ newArr.push(arr[i]); } } console.log( "Filter results:" ,newArr); |
用了 filter():
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var arr = [ { "name" : "apple" , "count" : 2}, { "name" : "orange" , "count" : 5}, { "name" : "pear" , "count" : 3}, { "name" : "orange" , "count" : 16}, ]; var newArr = arr.filter( function (item){ return item.name === "orange" ; }); console.log( "Filter results:" ,newArr); |
3) forEach()
forEach为每个元素执行对应的方法
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var arr = [1,2,3,4,5,6,7,8]; // Uses the usual "for" loop to iterate for ( var i= 0, l = arr.length; i< l; i++){ console.log(arr[i]); } console.log( "========================" ); //Uses forEach to iterate arr.forEach( function (item,index){ console.log(item); }); |
forEach是用来替换for循环的
4) map()
map()对数组的每个元素进行一定操作(映射)后,会返回一个新的数组,
不使用map
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var oldArr = [{first_name: "Colin" ,last_name: "Toh" },{first_name: "Addy" ,last_name: "Osmani" },{first_name: "Yehuda" ,last_name: "Katz" }]; function getNewArr(){ var newArr = []; for ( var i= 0, l = oldArr.length; i< l; i++){ var item = oldArr[i]; item.full_name = [item.first_name,item.last_name].join( " " ); newArr[i] = item; } return newArr; } console.log(getNewArr()); |
使用map后
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var oldArr = [{first_name: "Colin" ,last_name: "Toh" },{first_name: "Addy" ,last_name: "Osmani" },{first_name: "Yehuda" ,last_name: "Katz" }]; function getNewArr(){ return oldArr.map( function (item,index){ item.full_name = [item.first_name,item.last_name].join( " " ); return item; }); } console.log(getNewArr()); |
map()是处理服务器返回数据时是一个非常实用的函数。
5) reduce()
reduce()可以实现一个累加器的功能,将数组的每个值(从左到右)将其降低到一个值。
说实话刚开始理解这句话有点难度,它太抽象了。
场景: 统计一个数组中有多少个不重复的单词
不使用reduce时
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var arr = [ "apple" , "orange" , "apple" , "orange" , "pear" , "orange" ]; function getWordCnt(){ var obj = {}; for ( var i= 0, l = arr.length; i< l; i++){ var item = arr[i]; obj[item] = (obj[item] +1 ) || 1; } return obj; } console.log(getWordCnt()); |
使用reduce()后
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var arr = [ "apple" , "orange" , "apple" , "orange" , "pear" , "orange" ]; function getWordCnt(){ return arr.reduce( function (prev,next){ prev[next] = (prev[next] + 1) || 1; return prev; },{}); } console.log(getWordCnt()); |
让我先解释一下我自己对reduce的理解。reduce(callback, initialValue)会传入两个变量。回调函数(callback)和初始值(initialValue)。假设函数它有个传入参数,prev和 next,index和array。prev和next你是必须要了解的。
一般来讲prev是从数组中第一个元素开始的,next是第二个元素。但是当你传入初始值(initialValue)后,第一个prev将是initivalValue,next将是数组中的第一个元素。
比如:
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/* * 二者的区别,在console中运行一下即可知晓 */ var arr = [ "apple" , "orange" ]; function noPassValue(){ return arr.reduce( function (prev,next){ console.log( "prev:" ,prev); console.log( "next:" ,next); return prev + " " +next; }); } function passValue(){ return arr.reduce( function (prev,next){ console.log( "prev:" ,prev); console.log( "next:" ,next); prev[next] = 1; return prev; },{}); } console.log( "No Additional parameter:" ,noPassValue()); console.log( "----------------" ); console.log( "With {} as an additional parameter:" ,passValue()); |