github上面的demo

let&const

• let不支撑变量提拔

   console.log(a); // 此处报错
   let a = "a";
  

• let不能反复声明

   let a = "a";
   let a = "abc"; // 报错
  

• let支撑块级作用域

   if (true) {
     let a = "a";
   }
   console.log(a) // 报错
  
   for (let i = 0; i < 5; i++) {
     setTimeout(() => {
       console.log(i); // 0 1 2 3 4
     });
   }
  

• const除了let的功用,还不能变动声明后的值,不过可以对声明的对象增添属性和变动属性值

   const PI = 3.14;
   PI = 3.141; // 报错
  
   const obj = {
     name: '小张'
   };
   obj.name = '小红';
   obj.age = 25;
  

解构

{
  // 数组解构赋值
  let [a, b, c] = [123, "abc", { name: "xiaohong" }];
  console.log(a, b, c); // 123 'abc' { name: 'xiaohong' }
}

{
  // 对象解构赋值
  let { name, age } = {
    name: "xiaohong",
    age: 25
  };

  console.log(name, age); // xiaohong 25
}

{
  // 对解构赋值的值自定义称号
  let { name: myname, age: myage } = {
    name: "xiaohong",
    age: 25
  };

  console.log(myname, myage); // xiaohong 25
}

{
  // 默许赋值,如果给age赋值将掩盖默许值
  let { name, age = 19 } = {
    name: "xiaohong"
  };

  console.log(name, age); // xiaohong 19
}

{
  // 省略赋值
  let [, , a] = [1, 2, 3];
  console.log(a); // 3
}

睁开运算符

• 函数中运用睁开运算符

   function test(a, b, c) {}
   let arr = [1, 2, 3];
   test(...arr);

• 数组中函数中运用睁开运算符

   let [a, b, ...c] = [1, 2, 3, 4, 5];
   console.log(a, b, c); // 1 2 [ 3, 4, 5 ]
  
   let arr1 = [1, 2, 3];
   let arr2 = [...arr1, 4, 5];
   console.log(arr2); // [ 1, 2, 3, 4, 5 ]

• 类数组变量转成数组

  function test(a, b, c) {
     console.log([...arguments]);
   }
   test(1, 2, 4); // [1 2 4]
  

字符串

• 模板字符串:在这之前字符串拼接用+号来完成,现在用“和S{}即可替代字符串的拼接

   let name = 'xiaohong', age = 25;
   let str = `我叫:${name},本年${age}岁了`;
   console.log(str); // 我叫:xiaohong,本年25岁了   
  
  {
   // 自定义模板字符串的返回值
   let name = "xiaohong",
     age = 25;
   // ...rest作为参数只是放在末了
   function desc(string, ...rest) {
     let str = "";
     for (let i = 0, len = rest.length; i < len; i++) {
       str += string[i] + rest[i];
     }
     str += string[string.length - 1];
     return str.toLocaleUpperCase();
   }
   let str = desc`我叫:${name},本年${age}岁了`;
   console.log(str); // 我叫:XIAOHONG,本年25岁了
  }

• 推断字符串以某个字符串开首

   let str = "hello world!";
   console.log(str.startsWith('h')); // true

• 推断字符串以某个字符串末端

   let str = "hello world!";
   console.log(str.endsWith('!')); // true

• 推断字符创是不是包括某个字符串

   let str = "hello world!";
   console.log(str.includes('hello')); // true

• 将字符串反复天生

   let str = "hello world!";
   console.log(str.repeat(3)); // hello world!hello world!hello world!