javascript 开发网站（帮你提高开发效率的JavaScript20个技巧）

const array = Array(5).fill(''); 
// Output 
(5) ["", "", "", "", ""]
const matrix = Array(5).fill(0).map(()=>Array(5).fill(0)); 
// Output
(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

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

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

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

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

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

const array  = [40, 100, 1, 5, 25, 10];
array.sort((a,b) => a-b);
// Output
(6) [1, 5, 10, 25, 40, 100]
array.sort((a,b) => b-a);
// Output
(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));
// Output
(3) [{…}, {…}, {…}]
0: {first_name: "Pig", last_name: "Bodine"}
1: {first_name: "Lazslo", last_name: "Jamf"}
2: {first_name: "Pirate", last_name: "Prentice"}
length: 3

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

function doSomething(arg1){ 
    arg1 = arg1 || 10; 
// set arg1 to 10 as a default if it's not already set
return arg1;
}
let foo = 10;  
foo === 10 && doSomething(); 
// is the same thing as if (foo == 10) then doSomething();
// Output: 10
foo === 5 || doSomething();
// is the same thing as if (foo != 5) then doSomething();
// Output: 10

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]

let string = 'kapilalipak';
const table={}; 
for(let char of string) {
  table[char]=table[char]+1 || 1;
}
// Output
{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);
    }
  }
// Output
Map(5) {"k" => 2, "a" => 3, "p" => 2, "i" => 2, "l" => 2}

function Fever(temp) {
    return temp > 97 ? 'Visit Doctor!'
      : temp < 97 ? 'Go Out and Play!!'
      : temp === 97 ? 'Take Some Rest!';
}
// Output
Fever(97): "Take Some Rest!" 
Fever(100): "Visit Doctor!"

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};
// Output 
gender: "Male"
name: "Kapil Raghuwanshi"
primary: "Mani Primary School"
programming: "Extreme"
secondary: "Lass Secondary School"
sleeping: "Pro"
swimming: "Average"

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

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

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

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

const foo = null ?? 'my school';
// Output: "my school"
const baz = 0 ?? 42;
// Output: 0

function myFun(a,  b, ...manyMoreArgs) {
   return arguments.length;
}
myFun("one", "two", "three", "four", "five", "six");
// Output: 6

const parts = ['shoulders', 'knees']; 
const lyrics = ['head', ...parts, 'and', 'toes']; 
lyrics;
// Output: 
(5) ["head", "shoulders", "knees", "and", "toes"]

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

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

let a = 5;
let b = 8;
[a,b] = [b,a]
[a,b]
// Output
(2) [8, 5]

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

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]