js中reduce函数
定义：
reduce() 方法接收一个函数作为累加器，数组中的每个值（从左到右）开始缩减，最终计算为一个值。对空数组是不会执行回调函数的。
1.数组求和
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// 1.数组求和var arr = [1,5,8,6,15,78,65,25,48,55]var sum = arr.reduce(function(total,currentValue){ return total+currentValue;});console.log(sum);//306var eachSum = 0;arr.forEach(function(currentValue){ eachSum += currentValue;})console.log(eachSum);//306 |
2.合并二维数组
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//2.合并二维数组var twoArr = [['mu','zi'],['dig','big'],['lucky','jiji']];var oneArr = twoArr.reduce(function(total,currentValue){ // console.log(total) return total.concat(currentValue);})console.log(oneArr);//["mu", "zi", "dig", "big", "lucky", "jiji"] |
3.统计一个数组中有多少个不重复的单词：
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//3.统计一个数组中有多少个不重复的单词：// 不用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());//{apple: 2, orange: 3, pear: 1}// 用reduce时： 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());//{apple: 2, orange: 3, pear: 1} |
4.对reduce的理解：
reduce(callback,initiaValue)会传入两个变量，回调函数(callback)和初始值(initiaValue)。
假设函数有个传入参数，prev和next，index和array。 Prev和next是你必须要了解的。
当没有传入初始值时，prev是从数组中第一个元素开始的，next是第二个函数。
但是当传入初始值（initiaValue）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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// 4.对reduce的理解：var arr = ["apple","orange"]; function noPassValue(){ return arr.reduce(function(prev,next){ console.log("prev:",prev); console.log("next:",next); return prev; }); } 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()); /*VM415673:4 prev: apple VM415673:5 next: orange VM415673:4 prev: apple VM415673:5 next: orange VM415673:19 No Additional parameter: apple VM415673:20 ---------------- VM415673:13 prev: {} VM415673:14 next: apple VM415673:13 prev: {apple: 1} VM415673:14 next: orange VM415673:21 With {} as an additional parameter: {apple: 1, orange: 1 |
