Airbnb JavaScript Style Guide() {

const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9

const foo = [1, 2];
const bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;

// bad
var count = 1;
if (true) {
    count += 1;
}

// good, use the let.
let count = 1;
if (true) {
    count += 1;
}

// const 及 let 只存在于他们被定义的区块内。
{
    let a = 1;
    const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

// bad
const item = new Object();

// good
const item = {};

// bad
const superman = {
    default: { clark: 'kent' },
    private: true,
};

// good
const superman = {
    defaults: { clark: 'kent' },
    hidden: true,
};

// bad
const superman = {
    class: 'alien',
};

// bad
const superman = {
    klass: 'alien',
};

// good
const superman = {
    type: 'alien',
};

function getKey(k) {
    return `a key named ${k}`;
}

// bad
const obj = {
    id: 5,
    name: 'San Francisco',
};
obj[getKey('enabled')] = true;

// good
const obj = {
    id: 5,
    name: 'San Francisco',
    [getKey('enabled')]: true,
};

// bad
const atom = {
    value: 1,

    addValue: function (value) {
    return atom.value + value;
    },
};

// good
const atom = {
    value: 1,

    addValue(value) {
    return atom.value + value;
    },
};

const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
    lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
    lukeSkywalker,
};

const anakinSkywalker = 'Anakin Skywalker';
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    lukeSkywalker,
    episodeThree: 3,
    mayTheFourth: 4,
    anakinSkywalker,
};

// good
const obj = {
    lukeSkywalker,
    anakinSkywalker,
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    episodeThree: 3,
    mayTheFourth: 4,
};

// bad
const bad = {
    'foo': 3,
    'bar': 4,
    'data-blah': 5,
};

// good
const good = {
    foo: 3,
    bar: 4,
    'data-blah': 5,
};

// bad
const items = new Array();

// good
const items = [];

const someStack = [];

// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');

// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i++) {
    itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map(x => x + 1);

// bad
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
    const flatten = memo.concat(item);
    flat[index] = memo.concat(item);
});

// good
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
    const flatten = memo.concat(item);
    flat[index] = flatten;
    return flatten;
});

// bad
inbox.filter((msg) => {
    const { subject, author } = msg;
    if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
    } else {
    return false;
    }
});

// good
inbox.filter((msg) => {
    const { subject, author } = msg;
    if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
    }

    return false;
});

// bad
function getFullName(user) {
    const firstName = user.firstName;
    const lastName = user.lastName;

    return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
    const { firstName, lastName } = user;
    return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
    return `${firstName} ${lastName}`;
}

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

// bad
function processInput(input) {
    // 这时神奇的事情出现了
    return [left, right, top, bottom];
}

// 呼叫时必须考虑回传资料的顺序。
const [left, __, top] = processInput(input);

// good
function processInput(input) {
    // 这时神奇的事情出现了
    return { left, right, top, bottom };
}

// 呼叫时只需选择需要的资料
const { left, right } = processInput(input);

// bad
const name = "Capt. Janeway";

// good
const name = 'Capt. Janeway';

// bad
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

// bad
const errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// good
const errorMessage = 'This is a super long error that was thrown because ' +
    'of Batman. When you stop to think about how Batman had anything to do ' +
    'with this, you would get nowhere fast.';

// bad
function sayHi(name) {
    return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
    return ['How are you, ', name, '?'].join();
}

// bad
function sayHi(name) {
    return `How are you, ${ name }?`;
}

// good
function sayHi(name) {
    return `How are you, ${name}?`;
}

// bad
const foo = function () {
};

// good
function foo() {
}

// 立即函式（IIFE）
(function () {
    console.log('Welcome to the Internet. Please follow me.');
}());

// bad
if (currentUser) {
    function test() {
    console.log('Nope.');
    }
}

// good
let test;
if (currentUser) {
    test = () => {
    console.log('Yup.');
    };
}

// bad
function nope(name, options, arguments) {
    // ...stuff...
}

// good
function yup(name, options, args) {
    // ...stuff...
}

// bad
function concatenateAll() {
    const args = Array.prototype.slice.call(arguments);
    return args.join('');
}

// good
function concatenateAll(...args) {
    return args.join('');
}

// really bad
function handleThings(opts) {
    // 不！我们不该变动函式的参数。
    // Double bad: 如果 opt 是 false ，那们它就会被设定为一个物件，
    // 或许你想要这么做，但是这样可能会造成一些 Bug。
    opts = opts || {};
    // ...
}

// still bad
function handleThings(opts) {
    if (opts === void 0) {
    opts = {};
    }
    // ...
}

// good
function handleThings(opts = {}) {
    // ...
}

var b = 1;
// bad
function count(a = b++) {
    console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3

// bad
function handleThings(opts = {}, name) {
    // ...
}

// good
function handleThings(name, opts = {}) {
    // ...
}

// bad
var add = new Function('a', 'b', 'return a + b');

// still bad
var subtract = Function('a', 'b', 'return a - b');

// bad
const f = function(){};
const g = function (){};
const h = function() {};

// good
const x = function () {};
const y = function a() {};

// bad
function f1(obj) {
    obj.key = 1;
};

// good
function f2(obj) {
    const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
};

// bad
[1, 2, 3].map(function (x) {
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});

// bad
[1, 2, 3].map(number => {
    const nextNumber = number + 1;
    `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map(number => `A string containing the ${number}.`);

// good
[1, 2, 3].map((number) => {
    const nextNumber = number + 1;
    return `A string containing the ${nextNumber}.`;
});

// bad
[1, 2, 3].map(number => 'As time went by, the string containing the ' +
    `${number} became much longer. So we needed to break it over multiple ` +
    'lines.'
);

// good
[1, 2, 3].map(number => (
    `As time went by, the string containing the ${number} became much ` +
    'longer. So we needed to break it over multiple lines.'
));

// bad
[1, 2, 3].map((x) => x * x);

// good
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].map(number => (
    `A long string with the ${number}. It’s so long that we’ve broken it ` +
    'over multiple lines!'
));

// bad
[1, 2, 3].map(x => {
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});

// bad
const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = item => { return item.height > 256 ? item.largeSize : item.smallSize; }

// bad
function Queue(contents = []) {
    this._queue = [...contents];
}
Queue.prototype.pop = function () {
    const value = this._queue[0];
    this._queue.splice(0, 1);
    return value;
}


// good
class Queue {
    constructor(contents = []) {
    this._queue = [...contents];
    }
    pop() {
    const value = this._queue[0];
    this._queue.splice(0, 1);
    return value;
    }
}

// bad
const inherits = require('inherits');
function PeekableQueue(contents) {
    Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function () {
    return this._queue[0];
}

// good
class PeekableQueue extends Queue {
    peek() {
    return this._queue[0];
    }
}

// bad
Jedi.prototype.jump = function () {
    this.jumping = true;
    return true;
};

Jedi.prototype.setHeight = function (height) {
    this.height = height;
};

const luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
class Jedi {
    jump() {
    this.jumping = true;
    return this;
    }

    setHeight(height) {
    this.height = height;
    return this;
    }
}

const luke = new Jedi();

luke.jump()
    .setHeight(20);

class Jedi {
    constructor(options = {}) {
    this.name = options.name || 'no name';
    }

    getName() {
    return this.name;
    }

    toString() {
    return `Jedi - ${this.getName()}`;
    }
}

// bad
class Jedi {
    constructor() {}

    getName() {
    return this.name;
    }
}

// bad
class Rey extends Jedi {
    constructor(...args) {
    super(...args);
    }
}

// good
class Rey extends Jedi {
    constructor(...args) {
    super(...args);
    this.name = 'Rey';
    }
}

// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;

// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';

// bad
// filename es6.js
export { es6 as default } from './airbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;

const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
    sum += num;
}

sum === 15;

// good
let sum = 0;
numbers.forEach(num => sum += num);
sum === 15;

// best (使用 javascript 的高阶函式)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

const luke = {
    jedi: true,
    age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;

const luke = {
    jedi: true,
    age: 28,
};

function getProp(prop) {
    return luke[prop];
}

const isJedi = getProp('jedi');

// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();

// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// （比较上述例子找出错误）
const items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';

// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;

// bad - unnecessary function call
function checkName(hasName) {
    const name = getName();

    if (hasName === 'test') {
    return false;
    }

    if (name === 'test') {
    this.setName('');
    return false;
    }

    return name;
}

// good
function checkName(hasName) {
    if (hasName === 'test') {
    return false;
    }

    const name = getName();

    if (name === 'test') {
    this.setName('');
    return false;
    }

    return name;
}

// 我们知道这样是行不通的
// （假设没有名为 notDefined 的全域变数）
function example() {
    console.log(notDefined); // => throws a ReferenceError
}

// 由于变数提升的关係，
// 你在引用变数后再宣告变数是行得通的。
// 注：赋予给变数的 `true` 并不会被提升。
function example() {
    console.log(declaredButNotAssigned); // => undefined
    var declaredButNotAssigned = true;
}

// 直译器会将宣告的变数提升至作用域的最顶层，
// 表示我们可以将这个例子改写成以下：
function example() {
    let declaredButNotAssigned;
    console.log(declaredButNotAssigned); // => undefined
    declaredButNotAssigned = true;
}

// 使用 const 及 let
function example() {
    console.log(declaredButNotAssigned); // => throws a ReferenceError
    console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
    const declaredButNotAssigned = true;
}

function example() {
    console.log(anonymous); // => undefined

    anonymous(); // => TypeError anonymous is not a function

    var anonymous = function () {
    console.log('anonymous function expression');
    };
}

function example() {
    console.log(named); // => undefined

    named(); // => TypeError named is not a function

    superPower(); // => ReferenceError superPower is not defined

    var named = function superPower() {
    console.log('Flying');
    };
}

// 当函式名称和变数名称相同时也是如此。
function example() {
    console.log(named); // => undefined

    named(); // => TypeError named is not a function

    var named = function named() {
    console.log('named');
    }
}

function example() {
    superPower(); // => Flying

    function superPower() {
    console.log('Flying');
    }
}

if ([0] && []) {
    // true
    // 阵列（即使为空）为一个物件，所以转换为 true
}

// bad
if (name !== '') {
    // ...stuff...
}

// good
if (name) {
    // ...stuff...
}

// bad
if (collection.length > 0) {
    // ...stuff...
}

// good
if (collection.length) {
    // ...stuff...
}

// bad
switch (foo) {
    case 1:
    let x = 1;
    break;
    case 2:
    const y = 2;
    break;
    case 3:
    function f() {}
    break;
    default:
    class C {}
}

// good
switch (foo) {
    case 1: {
    let x = 1;
    break;
    }
    case 2: {
    const y = 2;
    break;
    }
    case 3: {
    function f() {}
    break;
    }
    case 4:
    bar();
    break;
    default: {
    class C {}
    }
}

// bad
const foo = maybe1 > maybe2
    ? "bar"
    : value1 > value2 ? "baz" : null;

// better
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2
    ? 'bar'
    : maybeNull;

// best
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;

// bad
if (test)
    return false;

// good
if (test) return false;

// good
if (test) {
    return false;
}

// bad
function foo() { return false; }

// good
function bar() {
    return false;
}

// bad
if (test) {
    thing1();
    thing2();
}
else {
    thing3();
}

// good
if (test) {
    thing1();
    thing2();
} else {
    thing3();
}

// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
    thing1();
}

// bad
if (foo === 123 &&
    bar === 'abc') {
    thing1();
}

// bad
if (foo === 123
    && bar === 'abc') {
    thing1();
}

// bad
if (
    foo === 123 &&
    bar === 'abc'
) {
    thing1();
}

// good
if (
    foo === 123
    && bar === 'abc'
) {
    thing1();
}

// good
if (
    (foo === 123 || bar === 'abc')
    && doesItLookGoodWhenItBecomesThatLong()
    && isThisReallyHappening()
) {
    thing1();
}

// good
if (foo === 123 && bar === 'abc') {
    thing1();
}

// bad
!isRunning && startRunning();

// good
if (!isRunning) {
    startRunning();
}

// bad
// make() 根据传入的 tag 名称回传一个新的元件
//
// @param {String} tag
// @return {Element} element
function make(tag) {

    // ...stuff...

    return element;
}

// good
/**
 * make() 根据传入的 tag 名称回传一个新的元件
    *
    * @param {String} tag
    * @return {Element} element
    */
function make(tag) {

    // ...stuff...

    return element;
}

// bad
const active = true;  // 当目前分页

// good
// is current tab
const active = true;

// bad
function getType() {
    console.log('fetching type...');
    // 设定预设的类型为 'no type'
    const type = this._type || 'no type';

    return type;
}

// good
function getType() {
    console.log('fetching type...');

    // 设定预设的类型为 'no type'
    const type = this._type || 'no type';

    return type;
}

// also good
function getType() {
    // set the default type to 'no type'
    const type = this._type || 'no type';

    return type;
}

class Calculator extends Abacus {
    constructor() {
    super();

    // FIXME: 不该在这使用全域变数
    total = 0;
    }
}

class Calculator extends Abacus {
    constructor() {
    super();

    // TODO: total 应该可被传入的参数所修改
    this.total = 0;
    }
}

// bad
function foo() {
∙∙∙∙const name;
}

// bad
function bar() {
∙const name;
}

// good
function baz() {
∙∙const name;
}

// bad
function test(){
    console.log('test');
}

// good
function test() {
    console.log('test');
}

// bad
dog.set('attr',{
    age: '1 year',
    breed: 'Bernese Mountain Dog',
});

// good
dog.set('attr', {
    age: '1 year',
    breed: 'Bernese Mountain Dog',
});

// bad
if(isJedi) {
    fight ();
}

// good
if (isJedi) {
    fight();
}

// bad
function fight () {
    console.log ('Swooosh!');
}

// good
function fight() {
    console.log('Swooosh!');
}

// bad
const x=y+5;

// good
const x = y + 5;

// bad
(function (global) {
    // ...stuff...
})(this);

// bad
(function (global) {
    // ...stuff...
})(this);↵
↵

// good
(function (global) {
    // ...stuff...
})(this);↵

// bad
$('#items').find('.selected').highlight().end().find('.open').updateCount();

// bad
$('#items').
    find('.selected').
    highlight().
    end().
    find('.open').
    updateCount();

// good
$('#items')
    .find('.selected')
    .highlight()
    .end()
    .find('.open')
    .updateCount();

// bad
const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
    .attr('width', (radius + margin) * 2).append('svg:g')
    .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
    .call(tron.led);

// good
const leds = stage.selectAll('.led')
    .data(data)
    .enter().append('svg:svg')
    .classed('led', true)
    .attr('width', (radius + margin) * 2)
    .append('svg:g')
    .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
    .call(tron.led);

// good
const leds = stage.selectAll('.led').data(data);

// bad
if (foo) {
    return bar;
}
return baz;

// good
if (foo) {
    return bar;
}

return baz;

// bad
const obj = {
    foo() {
    },
    bar() {
    },
};
return obj;

// good
const obj = {
    foo() {
    },

    bar() {
    },
};

return obj;

// bad
const arr = [
    function foo() {
    },
    function bar() {
    },
];
return arr;

// good
const arr = [
    function foo() {
    },

    function bar() {
    },
];

return arr;

// bad
function bar() {

    console.log(foo);

}

// also bad
if (baz) {

    console.log(qux);
} else {
    console.log(foo);

}

// good
function bar() {
    console.log(foo);
}

// good
if (baz) {
    console.log(qux);
} else {
    console.log(foo);
}

// bad
function bar( foo ) {
    return foo;
}

// good
function bar(foo) {
    return foo;
}

// bad
if ( foo ) {
    console.log(foo);
}

// good
if (foo) {
    console.log(foo);
}

// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);

// bad
const foo = {clark: 'kent'};

// good
const foo = { clark: 'kent' };

// bad
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';

// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

// good
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
    'Whatever wizard constrains a helpful ally. The counterpart ascends!';

// good
$.ajax({
    method: 'POST',
    url: 'https://airbnb.com/',
    data: { name: 'John' },
})
    .done(() => console.log('Congratulations!'))
    .fail(() => console.log('You have failed this city.'));

// bad
const story = [
    once
    , upon
    , aTime
];

// good
const story = [
    once,
    upon,
    aTime,
];

// bad
const hero = {
    firstName: 'Ada'
    , lastName: 'Lovelace'
    , birthYear: 1815
    , superPower: 'computers'
};

// good
const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace',
    birthYear: 1815,
    superPower: 'computers',
};