适用于您日常工作流程的 ESEST 提示、技巧、最佳实践和代码片段示例

const variablename = "value"

const variablename = "other value"     //-->syntaxerror: identifier 'variablename' has already been declaredvariablename = "other value"    //-->typeerror: assignment to constant variable.

const arrayname = [1,2,3,4]arrayname.push(5)    // output -->[1,2,3,4,5]const arrayname = [9,8,7,6]    //-->syntaxerror: identifier 'arrayname' has already been declared

const person = {name:"developer",email:"developer@developer.com",city:"new delhi"}person.name ="developer 2" //change a property person.location = "gurugram" //add a new propertyperson = {name:"dilip",email:"dilip@abc.com",city:"delhi"} //reassign it    //-->syntaxerror: identifier 'arrayname' has already been declared

var x = 1{ //this is a block scope    const x = 2}console.log(x)     //output -->1

let variablename = "value"

var x = 1{ //this is a block scope    let x = 2}console.log(x) //output -->1

let variablename = "other value"      //-->syntaxerrorvariablename = "other value"

console.log(sayhello)    //output -->undefined//variable sayhello is hoisted at the top before it was defined by var//this means that variable is there but with value of undefinedvar sayhello = "hello world" console.log(sayhello)    //output -->"hello world"

console.log(sayhello)     //-->referenceerror: cannot access 'sayhello' before initialization/definedlet sayhello = "hello world"console.log(sayhello)    //output -->"hello world"

for (var i = 0; i the number is 2  (x3)   //settimeout reference i after when the for loop endsconsole.log(i)    //--> 2//i is leaked outside the for loop

for (let i = 0; i the number is 0    //-->the number is 1    //-->the number is 2

let myfunction = (a, b) => {  sum = a * b;  return sum;}

let myfunction = (a, b) => a * b;

let myfunction = () => ("hello world");  

function functionname(param1,param2){     return param1+param2;}

let myarr = [1,2,3]let doublethenfilter = arr => arr.map((value) => (value * 2) )                                  .filter((value) => (value % 3 === 0));doublethenfilter(myarr)

function doublethenfilter(arr){    return arr.map(function(value){        return value *2;    }).filter(function(value){        return value % 3 === 0;    })};

let first = "dilip";let last = "mishra";console.log(`hello, ${first} ${last}`);//output --> "hello, dilip mishra"

let first = "dilip";let last = "mishra";var greeting = 'hello, ' + name + '!';console.log('hello, ' + first + ' ' +last);  //output --> "hello, dilip mishra"

let text ='the quickbrown foxjumps overthe lazy dog';  //output -->  "the quickbrown foxjumps overthe lazy dog"

let price = 10;let vat = 0.25;let total = 'total: ${(price * (1 + vat)).tofixed(2)}';  //output -->  "total: 12.50"

const person = { name: 'john', age: 30 };const { name, age } = person;console.log(name, age);  //output --> john 30

const person = { name: 'john', age: 30 };const { name:username, age:userage } = person;console.log(username, userage);  //output --> john 30

var person = { name: 'john', age: 30 };var name = person.name;var age = person.age;console.log(name, age);  //output --> john 30

var arr = [1,2,3];var [a,b,c] = arr;console.log(a); //-->1console.log(b); //-->2console.log(c); //-->3

function myfunction(x, y) {  var y = y || 10;  return x + y;}myfunction(5); // will return 15

function myfunction(x, y = 10) {  // y is 10 if not passed or undefined  return x + y;}myfunction(5); //output --> will return 15

var arr1 = [1, 2];var arr2 = [3, 4];var combined = arr1.concat(arr2);console.log(combined);  // [1, 2, 3, 4]

const arr1 = [1, 2];const arr2 = [3, 4];const combined = [...arr1, ...arr2];console.log(combined);  // [1, 2, 3, 4]

let nums = [4,5,6];let nums2 = [1,2,3,...nums,7,8];console.log(nums2);    //--> [1,2,3,4,5,6,7,8]

function sumvalues(a,b,c){     console.log(arguments);  //print out an array of the arguments of the functionreturn a+b+c;}let nums = [2,3,4];sumvalues(...nums); //values 2,3,4 of nums array has been passed to a,b,c parameters    //-->[2,3,4]    //-->9sumvalues(5,5,...nums); //value 2 of nums array has been passed to c parameter    //-->[5,5,2,3,4]    //-->12

let nums = [1,2,3,4];math.min(nums);    //--> nanmath.min(...nums);    //-->1

function fetchdata(callback) {  settimeout(function() {    callback('data loaded');  }, 1000);}fetchdata(function(data) {  console.log(data);  // data loaded});

function fetchdata() {  return new promise((resolve, reject) => {    settimeout(() => resolve('data loaded'), 1000);  });}fetchdata().then(data => console.log(data));  // data loaded

function person(name, age) {  this.name = name;  this.age = age;}person.prototype.greet = function() {  return 'hello, i am ' + this.name;};var john = new person('john', 30);console.log(john.greet());  // hello, i am john

class person {  constructor(name, age) {    this.name = name;    this.age = age;  }  greet() {    return `hello, i am ${this.name}`;  }}const john = new person('john', 30);console.log(john.greet());  // hello, i am john

(function() {  function add(x, y) {    return x + y;  }  window.add = add;})();console.log(add(2, 3));  // 5

// math.jsexport function add(x, y) {  return x + y;}// main.jsimport { add } from './math.js';console.log(add(2, 3));  // 5

async function myfunc(){    return "this is a promise";}myfunc().then((val)=>{console.log(val)});    //-->"this is a promise"

async function dohomework(){    let isdone = false;    if (isdone){        return("is done");    }else{        throw "is not done";    }}dohomework().then(function(homeworkresult){    console.log("the homework " + homeworkresult);}).catch(function(homeworkresult){    console.log("the homework " + homeworkresult);})    //"the homework is not done"

async function myFunc(){    let myPromise = new Promise((resolve,reject)=>{        setTimeout(()=>{resolve("done!")},1000)    });    let result = await myPromise; //wait for this promise before continuing    return result;}myFunc().then((result)=>{console.log(result)})