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); //306 var 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 |