标签:join array from dex ast hud for name 不重复
今天在网上看到一篇帖子,如题:
出处:前端开发博客 (http://caibaojian.com/5-array-methods.html)
在ES5中一共有9个Array方法,分别是:
Array.prototype.indexOf Array.prototype.lastIndexOf Array.prototype.every Array.prototype.some Array.prototype.forEach Array.prototype.map Array.prototype.filter Array.prototype.reduce Array.prototype.reduceRight
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。
via不使用indexOf时原文来自:http://caibaojian.com/5-array-methods.html
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);
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);
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 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
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。
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
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
//code from http://caibaojian.com/5-array-methods.html 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; },{}); }
最后感谢大神的指点;
出处:前端开发博客 (http://caibaojian.com/5-array-methods.html)
js数组中indexOf/filter/forEach/map/reduce详解
标签:join array from dex ast hud for name 不重复
原文地址:http://www.cnblogs.com/leungUwah/p/6017971.html