您可能从未使用过的鲜为人知的 Javascript 功能

let foo = null;let bar = foo ?? 'default value';console.log(bar); // output: 'default value'

const user = {    profile: {        name: 'alice'    }};const userprofilename = user.profile?.name;console.log(userprofilename); // output: 'alice'const useraccountname = user.account?.name;console.log(useraccountname); // output: undefined

const largenumber = 1_000_000;console.log(largenumber); // output: 1000000

const promises = [promise.resolve('success'), promise.reject('error'), promise.resolve('another success')];promise.allsettled(promises).then((results) => {    results.foreach((result) => console.log(result));});// output:// { status: 'fulfilled', value: 'success' }// { status: 'rejected', reason: 'error' }// { status: 'fulfilled', value: 'another success' }

class myclass {    #privatefield = 42;    getprivatefield() {        return this.#privatefield;    }}const instance = new myclass();console.log(instance.getprivatefield()); // output: 42console.log(instance.#privatefield); // uncaught private name #privatefield is not defined.

let a = true;let b = false;a &&= 'assigned if true';b ||= 'assigned if false';console.log(a); // output: 'assigned if true'console.log(b); // output: 'assigned if false'

outerloop: for (let i = 0; i <pre class="brush:php;toolbar:false">labelblock: {    console.log('this will be printed');    if (true) {        break labelblock; // exit the block    }    console.log('this will not be printed');}// output:// this will be printed

function logwithtimestamp(strings, ...values) {    const timestamp = new date().toisostring();    return (        `[${timestamp}] ` +        strings.reduce((result, str, i) => {            return result + str + (values[i] || '');        })    );}const user = 'johndoe';const action = 'logged in';console.log(logwithtimestamp`user ${user} has ${action}.`);// outputs: [2024-07-10t12:34:56.789z] user johndoe has logged in.

function validate(strings, ...values) {    values.foreach((value, index) => {        if (typeof value !== 'string') {            throw new error(`invalid input at position ${index + 1}: expected a string`);        }    });    return strings.reduce((result, str, i) => {        return result + str + (values[i] || '');    });}try {    const validstring = validate`name: ${'alice'}, age: ${25}`;    console.log(validstring); // this will throw an error} catch (error) {    console.error(error.message); // outputs: invalid input at position 2: expected a string}

const iseven = (num) => (num & 1) === 0;const isodd = (num) => (num & 1) === 1;console.log(iseven(4)); // outputs: trueconsole.log(isodd(5)); // outputs: true

const multiplybytwo = (num) => num  num >> 1;console.log(multiplybytwo(5)); // outputs: 10console.log(dividebytwo(10)); // outputs: 5

const ispoweroftwo = (num) => num > 0 && (num & (num - 1)) === 0;console.log(ispoweroftwo(16)); // outputs: trueconsole.log(ispoweroftwo(18)); // outputs: false

const obj = { name: 'alice', age: 25 };console.log('name' in obj); // output: trueconsole.log('height' in obj); // output: false

function checkvalue(value) {    debugger; // execution will pause here if a debugger is available    return value > 10;}checkvalue(15);

let a, b, c;a = b = c = 10;console.log(a, b, c); // output: 10 10 10

const funcname = 'dynamicfunction';const obj = {    [funcname]() {        return 'this is a dynamic function';    }};console.log(obj.dynamicfunction()); // output: 'this is a dynamic function'

function sum() {    let total = 0;    for (let i = 0; i <p>使用参数对象来访问传递给函数的所有参数，这对于具有可变长度参数的函数很有用。</p><h3>      15. 一元+运算符</h3><p>一元运算符 (+) 将其操作数转换为数字。</p><h4>      代码示例：</h4><pre class="brush:php;toolbar:false">console.log(+'abc'); // outputs: nanconsole.log(+'123'); // outputs: 123console.log(+'45.67'); // outputs: 45.67 (converted to a number)console.log(+true); // outputs: 1console.log(+false); // outputs: 0console.log(+null); // outputs: 0console.log(+undefined); // outputs: nan

const base = 2;const exponent = 3;const result = base ** exponent;console.log(result); // output: 8

function myfunction() {}myfunction.description = 'this is a function with a property';console.log(myfunction.description); // output: 'this is a function with a property'

function trackcount() {    if (!trackcount.count) {        trackcount.count = 0;    }    trackcount.count++;    console.log(`function called ${trackcount.count} times.`);}trackcount(); // outputs: function called 1 times.trackcount(); // outputs: function called 2 times.trackcount(); // outputs: function called 3 times.

const obj = {    firstname: 'john',    lastname: 'doe',    _age: 0, // conventionally use an underscore for the backing property    get fullname() {        return `${this.firstname} ${this.lastname}`;    },    set age(newage) {        if (newage &gt;= 0 &amp;&amp; newage <p>使用 getter 和 setter 来封装对象的内部状态，提供一种受控的方式来访问和修改属性。</p><h3>      19.!!棒棒操作员</h3><p>那个！！ （双重否定）运算符将值转换为其布尔值。</p><h4>      代码示例：</h4><pre class="brush:php;toolbar:false">const value = 'abc';const value1 = 42;const value2 = '';const value3 = null;const value4 = undefined;const value5 = 0;console.log(!!value); // outputs: true (truthy value)console.log(!!value1); // outputs: true (truthy value)console.log(!!value2); // outputs: false (falsy value)console.log(!!value3); // outputs: false (falsy value)console.log(!!value4); // outputs: false (falsy value)console.log(!!value5); // outputs: false (falsy value)

// creating a mapconst mymap = new map();// setting key-value pairsmymap.set('key1', 'value1');mymap.set(1, 'value2');mymap.set({}, 'value3');// getting values from a mapconsole.log(mymap.get('key1')); // outputs: 'value1'console.log(mymap.get(1)); // outputs: 'value2'console.log(mymap.get({})); // outputs: undefined (different object reference)

// Creating a Setconst mySet = new Set();// Adding values to a SetmySet.add('apple');mySet.add('banana');mySet.add('apple'); // Duplicate value, ignored in a Set// Checking size and valuesconsole.log(mySet.size); // Outputs: 2 (only unique values)console.log(mySet.has('banana')); // Outputs: true// Iterating over a SetmySet.forEach((value) => {    console.log(value);});// Outputs:// 'apple'// 'banana'