学的这个视频，用来预习字节青训营前端题

# 模版字符串

	const username = "john";
	const age = 30;

	console.log("My name is " + username + " and my age is " + age);
	const a = `My name is ${username} and my age is ${age}`;
	console.log(a);

都能打印 My name is john and my age is 30

# 字符串内置方法

	const s = "Hello world!";
	console.log(s.length); //12
	console.log(s.toUpperCase());//HELLO WORLD!
	console.log(s.toLowerCase());//hello world!
	console.log(s.substring(0, 5).toLowerCase());//hello

	const b = "computer, science, it, lmao";
	console.log(b.split(", "));//(4) ['computer', 'science', 'it', 'lmao']

# 数组

	const numbers = new Array(1, 2, 3, 4, 5);
	console.log(numbers);//(5) [1, 2, 3, 4, 5]

好自由的数组，喜欢喵

	const fruits = new Array("apple", "pear", "orange", 1, 2, a, true);
	console.log(fruits);//(7) ['apple', 'pear', 'orange', 1, 2, 'My name is john and my age is 30', true]
	console.log(fruits[1]);//pear
	fruits[3] = "grape";
	fruits[8] = "false"
	console.log(fruits);//(9) ['apple', 'pear', 'orange', 'grape', 2, 'My name is john and my age is 30', true, 空白，'false']
	console.log(typeof fruits[7]);//undefined

后面加入

	numbers.push(6);
	console.log(numbers);//(6) [1, 2, 3, 4, 5, 6]

前面加入

	numbers.unshift(0);
	console.log(numbers);//(7) [0, 1, 2, 3, 4, 5, 6]

前面提出

	numbers.shift();
	console.log(numbers);//(6) [1, 2, 3, 4, 5, 6]

后面提出

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

反转

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

判断是否为数组

	console.log(Array.isArray(fruits));//true
	console.log(Array.isArray(a));//false
	console.log(Array.isArray('fruits'));//false

数组中某元素的位置

	console.log(fruits.indexOf("orange"))//2
	console.log(fruits.indexOf("qwq"))//-1 数组里面没有 qwq 元素

# 对象

	const person = {
	firstName: "john",
	lastName: "Doe",
	age: 30,
	hobbies: ["music", "computer", "gaming"],
	address: {
	streets: "50 main st",
	city: "Newyork",
	state: "NY",
	}
	};

	console.log(person)//{firstName: 'john', lastName: 'Doe', age: 30, hobbies: Array(3), address: {…}}
	console.log(person.firstName, person.lastName)//john Doe
	console.log(person.hobbies[1])//computer
	console.log(person.address.state);//NY

	const {
	firstName, lastName, address: { city },
	} = person;
	console.log(city);//Newyork
	person.email = "[email protected]"
	console.log(person.email)//[email protected]

# 对象数组和 json

	const todos = [
	{
	id: 1,
	text: "Take out trash",
	isCompleted: true,
	},
	{
	id: 2,
	text: "Meeting with boss",
	isCompleted: true,
	},
	{
	id: 3,
	text: "Gaming",
	isCompleted: false,
	},
	]
	console.log(todos[1].text);//Meeting with boss

转换为 JSON

const todoJSON = JSON.stringify(todos);

console.log(todoJSON);//[{"id":1,"text":"Take out trash","isCompleted":true},{"id":2,"text":"Meeting with boss","isCompleted":true},{"id":3,"text":"Gaming","isCompleted":false}]

# if,switch,for,while

if 和 C 一样

	const x = "10";
	if(x==10){
	console.log("x is 10");//this
	}else{
	console.log("x is not 10");
	}

三等号保证数据类型相同

	if(x===10){
	console.log("x is 10");
	}else{
	console.log("x is not 10");//this
	}

三目运算符和 C 一样

	const p=10;
	const color=x>10?"red":"blue";
	console.log(color);//blue

switch 和 C 一样用

	for (let yyy of todos){
	console.log(yyy.id);
	}//123

相当于

	for(let i=0;i<todos.length;++i){
	console.log(todos[i].id);
	}

# 函数

	let c = 1;
	function add(x) {
	c = c + x;
	}
	add(9)
	console.log(c);//10

# 闭包

	function makeAdder(a){
	return function(b){
	return a+b;
	};
	}
	let d = makeAdder(5);
	let sum =d(6);
	console.log(sum);//11

# 总结

诶怎么感觉就学了 let 、const、console.log () 和 function 呢。。。其他有 C++ 和 python 的影子

破案了，原来都是基础语法，不过应该够应付一下算法题 (

前端