js中的reduce()函式
阿新 • • 發佈:2018-12-22
定義:
reduce() 方法接收一個函式作為累加器,陣列中的每個值(從左到右)開始縮減,最終計算為一個值。對空陣列是不會執行回撥函式的。
案例
1.陣列求和
// 1.陣列求和 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.合併二維陣列
//2.合併二維陣列
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.統計一個數組中有多少個不重複的單詞:
//3.統計一個數組中有多少個不重複的單詞: // 不用reduce時: 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時: 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.對reduce的理解:
reduce(callback,initiaValue)會傳入兩個變數,回撥函式(callback)和初始值(initiaValue)。
假設函式有個傳入引數,prev和next,index和array。 Prev和next是你必須要了解的。
當沒有傳入初始值時,prev是從陣列中第一個元素開始的,next是第二個函式。
但是當傳入初始值(initiaValue)後,第一個prev將是initivalValue,next將是陣列中的第一個元素。
比如:
// 4.對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}
*/
若有不足請多多指教!希望給您帶來幫助!參考文獻:連結