在前端開發過程、或者面試過程當中,別人問你數組經常使用的操做,你也許馬上立刻回答for
循環、forEach
、for..of
、 map
、some
......reduce
等方法。我相信前端開發的小夥伴,10我的中有8個對reduce
僅僅是停留在數據累加上, 下面本人介紹一些reduce
函數的妙用之處。前端
reduce
函數的參數問題一、reduce
函數面試
Array.prototype.reduce(function(previousVal, currentVal, index, _this) {
// 函數體
}, initVal);
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二、理解上面的函數後端
reduce
函數也是對數組迭代的過程previousVal==initVal
的previousVal==reducer
函數中的返回值了index
表示當前數組的序列化_this
表示當前的數組reduce
函數一、本身擴展數組的方法數組
Array.prototype.myReduce = function(fn, initVal) {
if (Object.prototype.toString.call(this) != '[object Array]') {
throw new Error('當前是數組的方法,不能使用到別的上面');
}
let total;
if (initVal != undefined) {
total = initVal;
} else {
total = this[0];
}
if (initVal === undefined) {
for (let i = 1; i < this.length; i++) {
total = fn(total, this[i], i, this);
}
} else {
for (let [index, val] of this.entries()) {
total = fn(total, val, index, this);
}
}
return total;
};
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一、數據原函數
let ary1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
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二、數據疊加工具
const result = ary1.reduce((pre, cur) => pre + cur);
console.log(result);
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一、實現代碼ui
const userList = [
{
id: 1,
username: 'john',
sex: 1,
email: 'john@163.com'
},
{
id: 2,
username: 'jerry',
sex: 1,
email: 'jerry@163.com'
},
{
id: 3,
username: 'nancy',
sex: 0,
email: ''
}
];
let result = userList.reduce((pre, current) => {
return {...pre, [current.id]: current};
}, {});
console.log(result);
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一、實現代碼this
const fileLines = [
'Inspector Algar,Inspector Bardle,Mr. Barker,Inspector Barton',
'Inspector Baynes,Inspector Bradstreet,Inspector Sam Brown',
'Monsieur Dubugue,Birdy Edwards,Inspector Forbes,Inspector Forrester',
'Inspector Gregory,Inspector Tobias Gregson,Inspector Hill',
'Inspector Stanley Hopkins,Inspector Athelney Jones'
];
let result = fileLines.reduce((pre, current) => {
return pre.concat(current.split(/,/g));
}, []);
console.log(result);
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一、實現代碼spa
const arr = ["今每天氣不錯", "", "早上好"];
const arr2 = arr.reduce((pre, cur) => {
return pre.concat(cur.split(''));
}, []);
console.log(arr2);
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一、實現代碼.net
const readings = [0.3, 1.2, 3.4, 0.2, 3.2, 5.5, 0.4];
let minValue = readings.reduce((x,y) => Math.min(x,y));
console.log(minValue);
let maxValue = readings.reduce((x,y) => Math.max(x,y));
console.log(maxValue);
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reduce
寫一個工具方法,用來提取對象中的數據一、後端返回的一個對象
let obj1 = {
"result": {
"report": {
"contactInfo": {
"callsNumber": 0,
"contactsNumber": 0,
"emergencyContactHasOverdue": "No",
"sameLiainson": {
"isSame": "Yes",
"accounts": "aa"
}
}
}
}
};
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二、使用reduce
的工具方法
const objectGetVal = (obj, expr) => {
if (!Object.is(Object.prototype.toString.call(obj), '[object Object]')) {
throw new Error(`${obj}不是對象`);
}
if (!Object.is(Object.prototype.toString.call(expr), '[object String]')) {
throw new Error(`${expr}必須是字符串`);
}
return expr.split('.').reduce((prev, next) => {
if (prev) {
return prev[next]
} else {
return undefined;
}
}, obj)
};
console.log(objectGetVal(obj1, 'result.report.contactInfo.emergencyContactHasOverdue'));
console.log(objectGetVal(obj1, 'result.report.emergencyContactHasOverdue'));
// 輸出結果
// No
// undefined
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reduce
生成樹型菜單參考