console.log(a); // undefined
var a=2;
############
console.log(a); // ReferenceError: Cannot access 'a' before initialization
let a=2;

console.log(a); // undefined
var a=2;
// 编译器看到 var 的时候
var a;
console.log(a);
a = 2;
// 在后续繁杂项目中会出现不小的问题。ES6 提出了let。let没有变量提升

var a=2;
var a=3;
console.log(a);  // 3
###############
let a=2;
let a=3;
console.log(a);  // SyntaxError: Identifier 'a' has already been declared

const PI = 3.1415;
PI // 3.1415

PI = 3;
// TypeError: Assignment to constant variable.

const PI = 3.1415;
PI // 3.1415

PI = 3;
// TypeError: Assignment to constant variable.

const foo;
// SyntaxError: Missing initializer in const declaration

if (true) {
  const MAX = 5;
}

MAX // Uncaught ReferenceError: MAX is not defined

if (true) {
  console.log(MAX); // ReferenceError
  const MAX = 5;
}

var message = "Hello!";
let age = 25;

// 以下两行都会报错
const message = "Goodbye!";
const age = 30;

const person = {
  name:'jerry'
}
person.name = "jerrywang"  // 可以修改对象内的值
console.log(person.name);  // jerrywang

const person = {
  name:'jerry'
}
// 不可以重新声明对象
person = {              // TypeError: Assignment to constant variable.
    name:"jerrywang"
}
// or
const person = {              // TypeError: Assignment to constant variable.
    name:"jerrywang"
}
console.log(person.name);

// 声明一个数组
var arr = [];
// 循环10次
for (var i = 0; i < 10; i++) {
  // 每次都保存生成一个函数
  arr[i] = function () {
    return i;
  };
}
console.log(arr);  // (10) [ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ]
// arr[5]() 调用执行第6个函数
console.log(arr[5]()); // 结果是 10

// 声明一个数组
var arr = [];
// 循环10次
for (let i = 0; i < 10; i++) {
  // 每次都保存生成一个函数
  arr[i] = function () {
    return i;
  };
}
console.log(arr);  // (10) [ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ, ƒ]
// arr[5]() 调用执行第6个函数
console.log(arr[5]()); // 结果是 5

var RegExp=10;
console.log(RegExp);    // 10
console.log(window.RegExp); // 10

// let声明的变量，不会影响我们挂载到window对象上的属性
let RegExp=10;
console.log(RegExp);    // 10
console.log(window.RegExp); // ƒ RegExp() { [native code] }

let a = 1;
let b = 2;
let c = 3;

let [a, b, c] = [1, 2, 3];

let [foo, [[bar], baz]] = [1, [[2], 3]];
foo // 1
bar // 2
baz // 3

let [ , , third] = ["foo", "bar", "baz"];
third // "baz"

let [x, , y] = [1, 2, 3];
x // 1
y // 3

let [head, ...tail] = [1, 2, 3, 4];
head // 1
tail // [2, 3, 4]

let [x, y, ...z] = ['a'];
x // "a"
y // undefined
z // []

let [foo] = [];
let [bar, foo] = [1];

let [x, y] = [1, 2, 3];
x // 1
y // 2

let [a, [b], d] = [1, [2, 3], 4];
a // 1
b // 2
d // 4

// 报错
let [foo] = 1;
let [foo] = false;
let [foo] = NaN;
let [foo] = undefined;
let [foo] = null;
let [foo] = {};

let [x, y, z] = new Set(['a', 'b', 'c']);
x // "a"

function* fibs() {
  let a = 0;
  let b = 1;
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}

let [first, second, third, fourth, fifth, sixth] = fibs();
sixth // 5

let [foo = true] = [];
foo // true

let [x, y = 'b'] = ['a']; // x='a', y='b'
let [x, y = 'b'] = ['a', undefined]; // x='a', y='b'

// ES5
let node = {
    foo:'aaa',
    bar:'bbb'
}
let foo = node.foo;
let bar = node.bar;
foo // "aaa"
bar // "bbb"

// ES6
let node = {
    foo:'aaa',
    bar:'bbb'
}
let { foo, bar } = node;
foo // "aaa"
bar // "bbb"

let { bar, foo } = { foo: 'aaa', bar: 'bbb' };
foo // "aaa"
bar // "bbb"

let { baz } = { foo: 'aaa', bar: 'bbb' };
baz // undefined

let {foo} = {bar: 'baz'};
foo // undefined

// 例一
let { log, sin, cos } = Math;

// 例二
const { log } = console;
log('hello') // hello

let { foo: baz } = { foo: 'aaa', bar: 'bbb' };
baz // "aaa"

let obj = { first: 'hello', last: 'world' };
let { first: f, last: l } = obj;
f // 'hello'
l // 'world'

let { foo: foo, bar: bar } = { foo: 'aaa', bar: 'bbb' };

let { foo: baz } = { foo: 'aaa', bar: 'bbb' };
baz // "aaa"
foo // error: foo is not defined

let obj = {
  p: [
    'Hello',
    { y: 'World' }
  ]
};

let { p: [x, { y }] } = obj;
x // "Hello"
y // "World"

let obj = {
  p: [
    'Hello',
    { y: 'World' }
  ]
};

let { p, p: [x, { y }] } = obj;
x // "Hello"
y // "World"
p // ["Hello", {y: "World"}]

const node = {
  loc: {
    start: {
      line: 1,
      column: 5
    }
  }
};

let { loc, loc: { start }, loc: { start: { line }} } = node;
line // 1
loc  // Object {start: Object}
start // Object {line: 1, column: 5}

let obj = {};
let arr = [];

({ foo: obj.prop, bar: arr[0] } = { foo: 123, bar: true });

obj // {prop:123}
arr // [true]

// 报错
let {foo: {bar}} = {baz: 'baz'};

const obj1 = {};
const obj2 = { foo: 'bar' };
Object.setPrototypeOf(obj1, obj2);

const { foo } = obj1;
foo // "bar"

var {x = 3} = {};
x // 3

var {x, y = 5} = {x: 1};
x // 1
y // 5

var {x: y = 3} = {};
y // 3

var {x: y = 3} = {x: 5};
y // 5

var { message: msg = 'Something went wrong' } = {};
msg // "Something went wrong"

var {x = 3} = {x: undefined};
x // 3

var {x = 3} = {x: null};
x // null

const [a, b, c, d, e] = 'hello';
a // "h"
b // "e"
c // "l"
d // "l"
e // "o"

let {length : len} = 'hello';
len // 5

let {toString: s} = 123;
s === Number.prototype.toString // true

let {toString: s} = true;
s === Boolean.prototype.toString // true

let { prop: x } = undefined; // TypeError
let { prop: y } = null; // TypeError

function add([x, y]){
  return x + y;
}

add([1, 2]); // 3

[[1, 2], [3, 4]].map(([a, b]) => a + b);
// [ 3, 7 ]

function move({x = 0, y = 0} = {}) {
  return [x, y];
}

move({x: 3, y: 8}); // [3, 8]
move({x: 3}); // [3, 0]
move({}); // [0, 0]
move(); // [0, 0]

function move({x, y} = { x: 0, y: 0 }) {
  return [x, y];
}

move({x: 3, y: 8}); // [3, 8]
move({x: 3}); // [3, undefined]
move({}); // [undefined, undefined]
move(); // [0, 0]

[1, undefined, 3].map((x = 'yes') => x);
// [ 1, 'yes', 3 ]

let x = 1;
let y = 2;

[x, y] = [y, x];

// 返回一个数组

function example() {
  return [1, 2, 3];
}
let [a, b, c] = example();

// 返回一个对象

function example() {
  return {
    foo: 1,
    bar: 2
  };
}
let { foo, bar } = example();

// 参数是一组有次序的值
function f([x, y, z]) { ... }
f([1, 2, 3]);

// 参数是一组无次序的值
function f({x, y, z}) { ... }
f({z: 3, y: 2, x: 1});

let jsonData = {
  id: 42,
  status: "OK",
  data: [867, 5309]
};

let { id, status, data: number } = jsonData;

console.log(id, status, number);
// 42, "OK", [867, 5309]

jQuery.ajax = function (url, {
  async = true,
  beforeSend = function () {},
  cache = true,
  complete = function () {},
  crossDomain = false,
  global = true,
  // ... more config
} = {}) {
  // ... do stuff
};

const map = new Map();
map.set('first', 'hello');
map.set('second', 'world');

for (let [key, value] of map) {
  console.log(key + " is " + value);
}
// first is hello
// second is world

// 获取键名
for (let [key] of map) {
  // ...
}

// 获取键值
for (let [,value] of map) {
  // ...
}

const { SourceMapConsumer, SourceNode } = require("source-map");

// es5写法
function add(a, b) {
  a = a || 10;
  b = b || 20;
  return a + b;
}
console.log(add()); // 30

// es6写法
function add(a = 10, b = 20) {
  return a + b;
}
console.log(add()); //30
// --------------------------------------
function add(a = 10, b = 20) {
    return a + b;
}
console.log(add(12)); //32

// 2. a接受传参=10，b调用执行getVal(5)
function add(a, b = getVal(5)) {	// b的默认值getVal(5)
    // 5. return 10 + 10
    return a + b;
}
// 3. getVal(val=5) 
function getVal(val) {
    // 4. return 5 + 5
    return val + 5;
}
// 1. 调用执行 add 函数
console.log(add(10));   // 20

// ES5 写法
function pick(obj) {
    // 创建一个新的空对象，并赋值给变量 result
    let result = Object.create(null);
    // index = 1 排除0 即排除book对象
    for (let index = 1; index < arguments.length; index++) {
        // result[author] = obj[author]
        // result[author] = "墨颜"
        result[arguments[index]] = obj[arguments[index]]
    }
    return result;
}
// 声明一个对象 book
let book = {
    title: 'es6教程',
    author: '墨颜',
    year: '2019'
}
let bookDate = pick(book, 'author', 'year')
console.log(bookDate)   // {author: '墨颜', year: '2019'}
// ------------------------------------------
function pick(obj) {
    // 创建一个新的空对象，并赋值给变量 result
    let result = Object.create(null);
    for (let index = 1; index < arguments.length; index++) {
        // result[author] = obj[author]
        // result[author] = "墨颜"
        result[arguments[index]] = obj[arguments[index]]
    }
    return result;
}
// 声明一个对象 book
let book = {
    title: 'es6教程',
    author: '墨颜',
    year: '2019'
}
let bookDate = pick(book, 'author', 'title','year')
console.log(bookDate)   // {author: '墨颜', title: 'es6教程', year: '2019'}

// keys = ["author","title","year"]
function pick(obj,...keys) {
    // 创建一个新的空对象，并赋值给变量 result
    let result = Object.create(null);
    for (let index = 0; index < keys.length; index++) {
        result[keys[index]] = obj[keys[index]]
    }
    return result;
}
// 声明一个对象 book
let book = {
    title: 'es6教程',
    author: '墨颜',
    year: '2019'
}
let bookDate = pick(book, 'author', 'title','year')
console.log(bookDate)   // {author: '墨颜', title: 'es6教程', year: '2019'}

function checkArgs(...args){
    console.log(args);          // 真数组
    console.log(arguments);     // 伪数组
    console.log(Array.from(arguments).sort());     // 真数组
}
checkArgs('a','b','c')

Object.create(proto, [propertiesObject])

const maxNum = Math.max(20,30,11,60)
console.log(maxNum);    // 60
// ES5中处理数组中的最大值
const arr = [10,20,30,50,60,100,11,15]
console.log(Math.max.apply(null,arr));  // 100
// ES6中处理数组中的最大值
console.log(Math.max(...arr));  //  100

var f = v => v;

// 等同于
var f = function (v) {
  return v;
};

var f = () => 5;
// 等同于
var f = function () { return 5 };

var sum = (num1, num2) => num1 + num2;
// 等同于
var sum = function(num1, num2) {
  return num1 + num2;
};

var sum = (num1, num2) => { return num1 + num2; }

// 报错
let getTempItem = id => { id: id, name: "Temp" };

// 不报错
let getTempItem = id => ({ id: id, name: "Temp" });

let foo = () => { a: 1 };
console.log(foo()); // undefined

let fn = () => void doesNotReturn();

const full = ({ first, last }) => first + ' ' + last;

// 等同于
function full(person) {
  return person.first + ' ' + person.last;
}

const isEven = n => n % 2 === 0;
const square = n => n * n;

// 普通函数写法
[1,2,3].map(function (x) {
  return x * x;
});

// 箭头函数写法
[1,2,3].map(x => x * x);

// 普通函数写法
var result = values.sort(function (a, b) {
  return a - b;
});

// 箭头函数写法
var result = values.sort((a, b) => a - b);

const numbers = (...nums) => nums;

numbers(1, 2, 3, 4, 5)
// [1,2,3,4,5]

const headAndTail = (head, ...tail) => [head, tail];

headAndTail(1, 2, 3, 4, 5)
// [1,[2,3,4,5]]

// 普通函数写法
let fn = (function(){
    return function(){
        console.log("hello ES6");
    }
})();
fn()    // hello ES6

// 箭头函数写法
let fn = (()=>{
    return ()=>{
        console.log("hello ES6");
    }
})();
fn()    // hello ES6

let PageHandle = {
    id: 123,
    init: function () {
        document.addEventListener('click', function (event) {
            // 这时候的this已经发生改变，这时候的this指向是定义函数的上下文即document
            // 因为添加的事件是给document添加的，所以this的指向发生了改变，document.doSomeThings 找不到了
            this.doSomeThings(event.type)	// 当你点击文档报错TypeError: this.doSomeThings is not a function
        },false)
    },
    doSomeThings: function (type) {
        console.log(`事件类型：${type},当前id：${this.id}`);
    }
}
PageHandle.init()

// 如何解决上述问题
let PageHandle = {
    id: 123,
    init: function () {
        document.addEventListener('click', function (event) {
            // 这时候的this已经发生改变，这时候的this指向是定义函数的上下文即document
            // 因为添加的事件是给document添加的，所以this的指向发生了改变，document.doSomeThings 找不到了
            this.doSomeThings(event.type)
        }.bind(this),false) // 更改this指向，这里的this指向的是PageHandle
    },
    doSomeThings: function (type) {
        console.log(`事件类型：${type},当前id：${this.id}`);
    }
}
PageHandle.init()

let PageHandle = {
    id: 123,
    init: function () {
        document.addEventListener('click', (event) => {
            // 这时候的this已经发生改变，这时候的this指向是定义函数的上下文即document
            // 因为添加的事件是给document添加的，所以this的指向发生了改变，document.doSomeThings 找不到了
            this.doSomeThings(event.type)	
        // .bind(this)并不能改变函数运行时的this，它只能改变函数被创建时的this
        },false) // 更改this指向，这里的this指向的是PageHandle
    },
    doSomeThings: function (type) {
        console.log(`事件类型：${type},当前id：${this.id}`);
    }
}
PageHandle.init()

let PageHandle = {
    id: 123,
    init: () => {
        document.addEventListener('click', (event) => {
            this.doSomeThings(event.type)	// 当你点击文档报错TypeError: this.doSomeThings is not a function
        },false) 
    },
    doSomeThings: function (type) {
        console.log(`事件类型：${type},当前id：${this.id}`);
    }
}
PageHandle.init()

Element.addEventListener(event, callback, [useCapture])

function foo() {
  setTimeout(() => {
    console.log('id:', this.id);
  }, 100);
}

var id = 21;

foo.call({ id: 42 });
// id: 42

let Person = () => {

};
// function函数也是一个对象，但是箭头函数不是一个对象，是一个语法表达式
let p = new Person();   // TypeError: Person is not a constructor

const foo = 'bar';
const baz = {foo};
baz // {foo: "bar"}

// 等同于
const baz = {foo: foo};

// ES5
const name = 'jerrywang', age = 18;
const person = {
    name: name,
    age: age,
    sayName: function () {
        console.log(this.name);
    }
}
person.sayName();   // jerrywang

// ES6
const name = 'jerrywang', age = 18;
const person = {
    name,
    age,
    sayName() {		// 除了属性简写，方法也可以简写。
        console.log(this.name);
    }
}
person.sayName();   // jerrywang

let cart = {
    wheel:4,
    set(newVal){
        if(newVal < this.wheel){
            throw new Error("轮子数太少了")
        }else{
            return "轮子ok";
        }
        this.wheel=newVal
    },
    get(){
        return this.wheel
    }            
}
cart.get(); // 4
cart.set(6);   // 轮子ok
cart.set(2);   // qq.html:16  Uncaught Error: 轮子数太少了

let user = {
  name: 'test'
};

let foo = {
  bar: 'baz'
};

console.log(user, foo)
// {name: "test"} {bar: "baz"}
console.log({user, foo})
// {user: {name: "test"}, foo: {bar: "baz"}}

const obj = {
  f() {
    this.foo = 'bar';
  }
};

new obj.f() // 报错

Object.is('foo', 'foo')
// true
Object.is({}, {})
// false

+0 === -0 //true
NaN === NaN // false

Object.is(+0, -0) // false
Object.is(NaN, NaN) // true

Object.defineProperty(Object, 'is', {
  value: function(x, y) {
    if (x === y) {
      // 针对+0 不等于 -0的情况
      return x !== 0 || 1 / x === 1 / y;
    }
    // 针对NaN的情况
    return x !== x && y !== y;
  },
  configurable: true,
  enumerable: false,
  writable: true
});

const target = { a: 1 };

const source1 = { b: 2 };
const source2 = { c: 3 };

Object.assign(target, source1, source2);
target // {a:1, b:2, c:3}

const target = { a: 1, b: 1 };

const source1 = { b: 2, c: 2 };
const source2 = { c: 3 };

Object.assign(target, source1, source2);
target // {a:1, b:2, c:3}

const obj = {a: 1};
Object.assign(obj) === obj // true

typeof Object.assign(2) // "object"

Object.assign(undefined) // 报错
Object.assign(null) // 报错

let obj = {a: 1};
Object.assign(obj, undefined) === obj // true
Object.assign(obj, null) === obj // true

const v1 = 'abc';
const v2 = true;
const v3 = 10;

const obj = Object.assign({}, v1, v2, v3);
console.log(obj); // { "0": "a", "1": "b", "2": "c" }

Object(true) // {[[PrimitiveValue]]: true}
Object(10)  //  {[[PrimitiveValue]]: 10}
Object('abc') // {0: "a", 1: "b", 2: "c", length: 3, [[PrimitiveValue]]: "abc"}

Object.assign({b: 'c'},
  Object.defineProperty({}, 'invisible', {
    enumerable: false,
    value: 'hello'
  })
)
// { b: 'c' }

Object.assign({ a: 'b' }, { [Symbol('c')]: 'd' })
// { a: 'b', Symbol(c): 'd' }

class Point {
  constructor(x, y) {
    Object.assign(this, {x, y});
  }
}

Object.assign(SomeClass.prototype, {
  someMethod(arg1, arg2) {
    ···
  },
  anotherMethod() {
    ···
  }
});

// 等同于下面的写法
SomeClass.prototype.someMethod = function (arg1, arg2) {
  ···
};
SomeClass.prototype.anotherMethod = function () {
  ···
};

function clone(origin) {
  return Object.assign({}, origin);
}

function clone(origin) {
  let originProto = Object.getPrototypeOf(origin);
  return Object.assign(Object.create(originProto), origin);
}

const merge =
  (target, ...sources) => Object.assign(target, ...sources);

const merge =
  (...sources) => Object.assign({}, ...sources);

const DEFAULTS = {
  logLevel: 0,
  outputFormat: 'html'
};

function processContent(options) {
  options = Object.assign({}, DEFAULTS, options);
  console.log(options);
  // ...
}

const DEFAULTS = {
  url: {
    host: 'example.com',
    port: 7070
  },
};

processContent({ url: {port: 8000} })
// {
//   url: {port: 8000}
// }

console.log(...[1, 2, 3])
// 1 2 3

console.log(1, ...[2, 3, 4], 5)
// 1 2 3 4 5

[...document.querySelectorAll('div')]
// [<div>, <div>, <div>]

function push(array, ...items) {
  array.push(...items);
}

function add(x, y) {
  return x + y;
}

const numbers = [4, 38];
add(...numbers) // 42

function f(v, w, x, y, z) { }
const args = [0, 1];
f(-1, ...args, 2, ...[3]);

const arr = [
  ...(x > 0 ? ['a'] : []),
  'b',
];

[...[], 1]
// [1]

(...[1, 2])
// Uncaught SyntaxError: Unexpected number

console.log((...[1, 2]))
// Uncaught SyntaxError: Unexpected number

console.log(...[1, 2])
// 1 2

const a1 = [1, 2];
const a2 = a1;

a2[0] = 2;
a1 // [2, 2]

const a1 = [1, 2];
const a2 = a1.concat();

a2[0] = 2;
a1 // [1, 2]

const a1 = [1, 2];
// 写法一
const a2 = [...a1];
// 写法二
const [...a2] = a1;

const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e'];

// ES5 的合并数组
arr1.concat(arr2, arr3);
// [ 'a', 'b', 'c', 'd', 'e' ]

// ES6 的合并数组
[...arr1, ...arr2, ...arr3]
// [ 'a', 'b', 'c', 'd', 'e' ]

const a1 = [{ foo: 1 }];
const a2 = [{ bar: 2 }];

const a3 = a1.concat(a2);
const a4 = [...a1, ...a2];

a3[0] === a1[0] // true
a4[0] === a1[0] // true

// ES5
a = list[0], rest = list.slice(1)

// ES6
[a, ...rest] = list

const [first, ...rest] = [1, 2, 3, 4, 5];
first // 1
rest  // [2, 3, 4, 5]

const [first, ...rest] = [];
first // undefined
rest  // []

const [first, ...rest] = ["foo"];
first  // "foo"
rest   // []

const [...butLast, last] = [1, 2, 3, 4, 5];
// 报错

const [first, ...middle, last] = [1, 2, 3, 4, 5];
// 报错

[...'hello']
// [ "h", "e", "l", "l", "o" ]

'x\uD83D\uDE80y'.length // 4
[...'x\uD83D\uDE80y'].length // 3

function length(str) {
  return [...str].length;
}

length('x\uD83D\uDE80y') // 3

let str = 'x\uD83D\uDE80y';

str.split('').reverse().join('')
// 'y\uDE80\uD83Dx'

[...str].reverse().join('')
// 'y\uD83D\uDE80x'

let nodeList = document.querySelectorAll('div');
let array = [...nodeList];

Number.prototype[Symbol.iterator] = function*() {
  let i = 0;
  let num = this.valueOf();
  while (i < num) {
    yield i++;
  }
}

console.log([...5]) // [0, 1, 2, 3, 4]

let arrayLike = {
  '0': 'a',
  '1': 'b',
  '2': 'c',
  length: 3
};

// TypeError: Cannot spread non-iterable object.
let arr = [...arrayLike];

let map = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
]);

let arr = [...map.keys()]; // [1, 2, 3]

const go = function*(){
  yield 1;
  yield 2;
  yield 3;
};

[...go()] // [1, 2, 3]

const obj = {a: 1, b: 2};
let arr = [...obj]; // TypeError: Cannot spread non-iterable object

let arrayLike = {
    '0': 'a',
    '1': 'b',
    '2': 'c',
    length: 3
};

// ES5 的写法
var arr1 = [].slice.call(arrayLike); // ['a', 'b', 'c']

// ES6 的写法
let arr2 = Array.from(arrayLike); // ['a', 'b', 'c']

// NodeList 对象
let ps = document.querySelectorAll('p');
Array.from(ps).filter(p => {
  return p.textContent.length > 100;
});

// arguments 对象
function foo() {
  var args = Array.from(arguments);
  // ...
}

// arguments对象
function foo() {
  const args = [...arguments];
}

// NodeList对象
[...document.querySelectorAll('div')]

Array.from({ length: 3 });
// [ undefined, undefined, undefined ]

Array.of(3, 11, 8) // [3,11,8]
Array.of(3) // [3]
Array.of(3).length // 1

Array() // []
Array(3) // [, , ,]
Array(3, 11, 8) // [3, 11, 8]

Array.of() // []
Array.of(undefined) // [undefined]
Array.of(1) // [1]
Array.of(1, 2) // [1, 2]

function ArrayOf(){
  return [].slice.call(arguments);
}

Array.prototype.copyWithin(target, start = 0, end = this.length)

[1, 2, 3, 4, 5].copyWithin(0, 3)
// [4, 5, 3, 4, 5]

let num = [1,4,-5,10,-11].find((n) => {
    return n <0;
})
console.log(num);   // -5
// 简写
let num = [1,4,-5,10,-11].find(n => n < 0)
console.log(num);   // -5

[1, 5, 10, 15].find(function(value, index, arr) {
  return value > 9;
}) // 10

let num = [1,4,-5,10,-11].findIndex(n => n < 0)
console.log(num);   // 2

let num = [1,4,-5,10,-11].findIndex(n => n > 100)
console.log(num);   // -1