ES6的基础知识(一)

github上面的demo

let&const

• let不支持变量提高

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

• let不能重复声明

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

• let支持块级做用域

  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的功能,还不能更改声明后的值,不过能够对声明的对象增长属性和更改属性值

  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名称
  let { name: myname, age: myage } = {
    name: "xiaohong",
    age: 25
  };

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

{
  // 默认赋值,如果给age赋值将覆盖默认值
  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
}

展开运算符

• 函数中使用展开运算符

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

• 数组中函数中使用展开运算符

  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:在这以前字符串拼接用+号来完成,如今用``和S{}便可代替字符串的拼接

  let name = 'xiaohong', age = 25;
   let str = `我叫:${name},今年${age}岁了`;
   console.log(str); // 我叫:xiaohong,今年25岁了   
  
  {
   // 自定义模板字符串的返回值
   let name = "xiaohong",
     age = 25;
   // ...rest做为参数只是放在最后
   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:${name},今年${age}岁了`;
   console.log(str); // 我叫:XIAOHONG,今年25岁了
  }

• 判断字符串以某个字符串开头

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

• 判断字符串以某个字符串结尾

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

• 判断字符创是否包含某个字符串

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

• 将字符串重复生成

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