# JS技巧
**
## 三元运算符
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';
```
**
**
## 数字转字符串/字符串转数字
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 + ""; // 数字转字符串
let n = +s; // 字符串转数字
```
**
**
## 填充数组
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'}));
```
```
**
**
## 对象的动态属性
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"
};
```
**
**
## 删除重复项
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))
```
**
**
## 数组到对象
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]]
```
**
**
## 短路条件
新手
```javascript
if (docs) {
goToDocs();
}
```
老手
```javascript
docs && goToDocs()
```
**
**
## 使用^检查数字是否相等
```javascript
if(a!=123) // before // 一般开发者
if(a^123) // after // B格比较高的
```
**
**
## 对象遍历
```javascript
const age = {
Rahul: 20,
max: 16
};
// 方案1：先得 key 在遍历key
const keys = Object.keys(age);
keys.forEach(key => age[key]++);
console.log(age); // { Rahul: 21, max: 16 }
// 方案2 - `for...in` 循环
for(let key in age){
age[key]++;
}
console.log(age); // { Rahul: 22, max: 18 }
```
**
**
## 获取对象的所有键
```javascript
cosnt obj = {
name: "前端小智",
age: 16,
address: "厦门",
profession: "前端开发",
};
console.log(Object.keys(obj)); // name, age, address, profession
```
**
**
## 检查值是否为数组
```javascript
const arr = [1, 2, 3];
console.log(typeof arr); // object
console.log(Array.isArray(arr)); // true
```
**
**
## 初始化大小为n的数组并填充默认值
```javascript
const size = 5;
const defaultValue = 0;
const arr = Array(size).fill(defaultValue);
console.log(arr); // [0, 0, 0, 0, 0]
```
**
**
## 真值和虚值
```javascript
虚值：false,0, "",null,undefined和NaN。
真值："Values",0",{},[]。
```
**
**
## 三等号和双等号的区别
```javascript
// 双等号 - 将两个操作数转换为相同类型,再比较
console.log(0 == 'o'); // true
// 三等号 - 不转换为相同类型
console.log(0 === '0'); // false
```
**
**
## 接收参数更好的方式
```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 =>它是一个值，而undefined不是
```javascript
const fn = (x = 'default value') => console.log(x);
fn(undefined); // default value
fn(); // default value
fn(null); // null
```
**
