# JS技巧
**
## 三元運(yùn)算符
新手
```javascript
let hungry = true;
let eat;
if (hungry == true) {
? ? ? eat = 'yes';
} else {
? ? ? eat = 'no';
}
```
老手
```javascript
let hungry = true;
let eat = hungry == true ? 'yes' : 'no';
```
**
**
## 數(shù)字轉(zhuǎn)字符串/字符串轉(zhuǎn)數(shù)字
新手
```javascript
let num = 15;
let s = num.toString(); // number to string
let n = Number(s); // string to number
```
老手
```javascript
let num = 15;
let s = num + ""; // 數(shù)字轉(zhuǎn)字符串
let n = +s; // 字符串轉(zhuǎn)數(shù)字
```
**
**
## 填充數(shù)組
新手
```javascript
for(let i=0; i < arraySize; i++){
? filledArray[i] {'hello' : 'goodbye'};
}
老手
```javascript
let filledArray = new Array(arraysize).fill(null).map(()=> ({'hello' : 'goodbye'}));
```
```
**
**
## 對(duì)象的動(dòng)態(tài)屬性
新手
```javascript
let dynamic = "value";
let user = {
? ? id: 1,
};
user[dynamic] = "other value";
```
老手
```javascript
let dynamic = "value";
let user = {
? ? id: 1,
? ? [dynamic] :? "other value"
};
```
**
**
## 刪除重復(fù)項(xiàng)
新手
```javascript
let array = [100, 23, 23, 23, 23, 67, 45];
let outputArray = [];
let flag = false;
for (j = 0; < array.length; j++) {
? for (k = 0; k < outputArray.length; k++) {
? ? ? if (array[j] == outputArray[k]) {
? ? ? ? flag = true;
? ? ? }
? ? }
? ? if (flag == false) {
? ? ? outputArray.push(array[j]);
? ? }
? ? flag = false;
}
// tArray = [100, 23, 67, 45]
```
老手
```javascript
let array = [100, 23, 23, 23, 23, 67, 45];
let outputArray = Array.from(new Set(array))
```
**
**
## 數(shù)組到對(duì)象
新手
```javascript
let number = {
? one: 1,
? two: 2,
};
let keys = [];
for (let numbers in numbers) {
? if (number.hasOwnProperty(number)) {
? ? keys.push(number);
? ? }
}
// key = [ 'one', 'two' ]
```
老手
```javascript
let number = {
? one: 1,
? two: 2,
};
let key = Object.keys(numbers); // key = [ 'one', 'two' ]
let value = Object.values(numbers);? // value = [ 1, 2 ]
let entry = Object.entries(numbers); // entry = [['one' : 1], ['two' : 2]]
```
**
**
## 短路條件
新手
```javascript
if (docs) {
? ? goToDocs();
}
```
老手
```javascript
docs && goToDocs()
```
**
**
## 使用^檢查數(shù)字是否相等
```javascript
if(a!=123) // before // 一般開(kāi)發(fā)者
if(a^123) // after // B格比較高的
```
**
**
## 對(duì)象遍歷
```javascript
const age = {
? Rahul: 20,?
? max: 16
};
// 方案1：先得 key 在遍歷key
const keys = Object.keys(age);
keys.forEach(key => age[key]++);
console.log(age); // { Rahul: 21, max: 16 }
// 方案2 - `for...in` 循環(huán)
for(let key in age){
? age[key]++;
}
console.log(age); // { Rahul: 22, max: 18 }
```
**
**
## 獲取對(duì)象的所有鍵
```javascript
cosnt obj = {
? name: "前端小智",
? age: 16,
? address: "廈門(mén)",
? profession: "前端開(kāi)發(fā)",
};
console.log(Object.keys(obj)); // name, age, address, profession
```
**
**
## 檢查值是否為數(shù)組
```javascript
const arr = [1, 2, 3];
console.log(typeof arr); // object
console.log(Array.isArray(arr)); // true
```
**
**
## 初始化大小為n的數(shù)組并填充默認(rèn)值
```javascript
const size = 5;
const defaultValue = 0;
const arr = Array(size).fill(defaultValue);
console.log(arr); // [0, 0, 0, 0, 0]
```
**
**
## 真值和虛值
```javascript
虛值：false,0, "",null,undefined和NaN磷瘤。
真值："Values",0",{},[]艺演。
```
**
**
## 三等號(hào)和雙等號(hào)的區(qū)別
```javascript
// 雙等號(hào) - 將兩個(gè)操作數(shù)轉(zhuǎn)換為相同類(lèi)型,再比較
console.log(0 == 'o'); // true
// 三等號(hào) - 不轉(zhuǎn)換為相同類(lèi)型
console.log(0 === '0'); // false
```
**
**
## 接收參數(shù)更好的方式
```javascript
function downloadData(url, resourceId, searchTest, pageNo, limit) {}
downloadData(...); // need to remember the order
```
或者
```javascript
function downloadData(
{ url, resourceId, searchTest, pageNo, limit } = {}
) {}
downloadData(
? { resourceId: 2, url: "/posts", searchText: "WebDev" }
);
```
**
**
## null vs undefined
null =>它是一個(gè)值遍愿，而undefined不是
```javascript
const fn = (x = 'default value') => console.log(x);
fn(undefined); // default value
fn(); // default value
fn(null); // null
```
**
