JS Array ...map ,reduce , filter..
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Array共有九个方法
Refer: http://www.cnblogs.com/webapi/p/5706348.html
Array.prototype.indexOfArray.prototype.lastIndexOfArray.prototype.everyArray.prototype.someArray.prototype.forEachArray.prototype.mapArray.prototype.filterArray.prototype.reduceArray.prototype.reduceRight
常见的有五种:
.1indexOf
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
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
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
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():
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
var arr = [1,2,3,4,5,6,7,8];// Uses the usual "for" loop to iteratefor(var i= 0, l = arr.length; i< l; i++){console.log(arr[i]);}console.log("========================");//Uses forEach to iteratearr.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
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
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()
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());
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
使用reduce()后
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:
/*
* 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());
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