# 20个 Javascript 技巧，提高我们的摸鱼时间！

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### 1.声明和初始化数组

``````const array = Array(5).fill('');
// 输出
(5) ["", "", "", "", ""]

const matrix = Array(5).fill(0).map(() => Array(5).fill(0))
// 输出
(5) [Array(5), Array(5), Array(5), Array(5), Array(5)]
0: (5) [0, 0, 0, 0, 0]
1: (5) [0, 0, 0, 0, 0]
2: (5) [0, 0, 0, 0, 0]
3: (5) [0, 0, 0, 0, 0]
4: (5) [0, 0, 0, 0, 0]
length: 5

### 2. 求和，最小值和最大值

``````const array  = [5,4,7,8,9,2];

``````array.reduce((a,b) => a+b);
// 输出: 35

``````array.reduce((a,b) => a>b?a:b);
// 输出: 9

``````array.reduce((a,b) => a<b?a:b);
// 输出: 2

### 3.排序字符串，数字或对象等数组

``````const stringArr = ["Joe", "Kapil", "Steve", "Musk"]
stringArr.sort();
// 输出
(4) ["Joe", "Kapil", "Musk", "Steve"]

stringArr.reverse();
// 输出
(4) ["Steve", "Musk", "Kapil", "Joe"]

``````const array  = [40, 100, 1, 5, 25, 10];
array.sort((a,b) => a-b);
// 输出
(6) [1, 5, 10, 25, 40, 100]

array.sort((a,b) => b-a);
// 输出
(6) [100, 40, 25, 10, 5, 1]

``````const objectArr = [
{ first_name: 'Lazslo', last_name: 'Jamf'     },
{ first_name: 'Pig',    last_name: 'Bodine'   },
{ first_name: 'Pirate', last_name: 'Prentice' }
];
objectArr.sort((a, b) => a.last_name.localeCompare(b.last_name));
// 输出
(3) [{…}, {…}, {…}]
0: {first_name: "Pig", last_name: "Bodine"}
1: {first_name: "Lazslo", last_name: "Jamf"}
2: {first_name: "Pirate", last_name: "Prentice"}
length: 3

#### 4.从数组中过滤到虚值

`0`, `undefined`, `null`, `false`, `""`, `''`这样的假值可以通过下面的技巧轻易地过滤掉。

``````const array = [3, 0, 6, 7, '', false];
array.filter(Boolean);

// 输出
(3) [3, 6, 7]

### 5. 使用逻辑运算符处理需要条件判断的情况

``````function doSomething(arg1){
arg1 = arg1 || 10;
// 如果arg1没有值，则取默认值 10
}

let foo = 10;
foo === 10 && doSomething();
// 如果 foo 等于 10，刚执行 doSomething();
// 输出: 10

foo === 5 || doSomething();
// is the same thing as if (foo != 5) then doSomething();
// Output: 10

### 6. 去除重复值

``````const array  = [5,4,7,8,9,2,7,5];
array.filter((item,idx,arr) => arr.indexOf(item) === idx);
// or
const nonUnique = [...new Set(array)];
// Output: [5, 4, 7, 8, 9, 2]

### 7. 创建一个计数器对象或 Map

``````let string = 'kapilalipak';

const table={};
for(let char of string) {
table[char]=table[char]+1 || 1;
}
// 输出
{k: 2, a: 3, p: 2, i: 2, l: 2}

``````const countMap = new Map();
for (let i = 0; i < string.length; i++) {
if (countMap.has(string[i])) {
countMap.set(string[i], countMap.get(string[i]) + 1);
} else {
countMap.set(string[i], 1);
}
}
// 输出
Map(5) {"k" => 2, "a" => 3, "p" => 2, "i" => 2, "l" => 2}

### 8. 三元运算符很酷

``````function Fever(temp) {
return temp > 97 ? 'Visit Doctor!'
: temp < 97 ? 'Go Out and Play!!'
: temp === 97 ? 'Take Some Rest!': 'Go Out and Play!';;
}

// 输出
Fever(97): "Take Some Rest!"
Fever(100): "Visit Doctor!"

### 9. 循环方法的比较

• `for``for..in` 默认获取索引，但你可以使用`arr[index]`

• `for..in`也接受非数字，所以要避免使用。

• `forEach`, `for...of` 直接得到元素。

• forEach 也可以得到索引，但 `for...of` 不行。

### 10. 合并两个对象

``````const user = {
name: 'Kapil Raghuwanshi',
gender: 'Male'
};
const college = {
primary: 'Mani Primary School',
secondary: 'Lass Secondary School'
};
const skills = {
programming: 'Extreme',
swimming: 'Average',
sleeping: 'Pro'
};

const summary = {...user, ...college, ...skills};

// 合并多个对象
gender: "Male"
name: "Kapil Raghuwanshi"
primary: "Mani Primary School"
programming: "Extreme"
secondary: "Lass Secondary School"
sleeping: "Pro"
swimming: "Average"

### 11. 箭头函数

``````const person = {
name: 'Kapil',
sayName() {
return this.name;
}
}
person.sayName();
// 输出
"Kapil"

``````const person = {
name: 'Kapil',
sayName : () => {
return this.name;
}
}
person.sayName();
// Output
"

### 13. 可选的链

``````const user = {
employee: {
name: "Kapil"
}
};
user.employee?.name;
// Output: "Kapil"
user.employ?.name;
// Output: undefined
user.employ.name
// 输出: VM21616:1 Uncaught TypeError: Cannot read property 'name' of undefined

### 13.洗牌一个数组

``````const list = [1, 2, 3, 4, 5, 6, 7, 8, 9];
list.sort(() => {
return Math.random() - 0.5;
});
// 输出
(9) [2, 5, 1, 6, 9, 8, 4, 3, 7]
// 输出
(9) [4, 1, 7, 5, 3, 8, 2, 9, 6]

### 14.双问号语法

``````const foo = null ?? 'my school';
// 输出: "my school"

const baz = 0 ?? 42;
// 输出: 0

### 剩余和展开语法

``````function myFun(a,  b, ...manyMoreArgs) {
return arguments.length;
}
myFun("one", "two", "three", "four", "five", "six");

// 输出: 6

``````const parts = ['shoulders', 'knees'];
const lyrics = ['head', ...parts, 'and', 'toes'];

lyrics;
// 输出:
(5) ["head", "shoulders", "knees", "and", "toes"]

### 16.默认参数

``````const search = (arr, low=0,high=arr.length-1) => {
return high;
}
search([1,2,3,4,5]);

// 输出: 4

### 17. 将十进制转换为二进制或十六进制

``````const num = 10;

num.toString(2);
// 输出: "1010"
num.toString(16);
// 输出: "a"
num.toString(8);
// 输出: "12"

### 18. 使用解构来交换两个数

``````let a = 5;
let b = 8;
[a,b] = [b,a]

[a,b]
// 输出
(2) [8, 5]

### 19. 单行的回文数检查

``````function checkPalindrome(str) {
return str == str.split('').reverse().join('');
}
checkPalindrome('naman');
// 输出: true

### 20.将Object属性转换为属性数组

``````const obj = { a: 1, b: 2, c: 3 };

Object.entries(obj);
// Output
(3) [Array(2), Array(2), Array(2)]
0: (2) ["a", 1]
1: (2) ["b", 2]
2: (2) ["c", 3]
length: 3

Object.keys(obj);
(3) ["a", "b", "c"]

Object.values(obj);
(3) [1, 2, 3]

~完，我是小智，我们下期见！