var let const区别

var let const的区别

• var && let

  声明一个变量　var name="tony"

  function varTest() {
    var x = 31;
    if (true) {
      var x = 71;  // same variable!
      console.log(x);  // 71
    }
    console.log(x);  // 71
  }
  
  function letTest() {
    let x = 31;
    if (true) {
      let x = 71;  // different variable
      console.log(x);  // 71
    }
    console.log(x);  // 31
  }
  

  let 不允许重复声明一个变量

  <script>
      if (true) {
        let a;
        let a; // Uncaught SyntaxError: Identifier 'a' has already been declared
      }
  
      if(true){
        var d;
        var d;    //不会报错
      }
  
      if (true) {
        var c;
        let c; // Uncaught SyntaxError: Identifier 'c' has already been declared
      }
  
      if (true) {
        let d;
        var d; // Uncaught SyntaxError: Identifier 'd' has already been declared
      }
  </script>
  

  • const

    声明一个常量，并不是这个常量补课班，而是这个常量不能再次被赋值

    <script>
        /*不会报错，因为names1指向的地址不变，改变的只是内部数据*/
        const names1 = [];
              names1[0] = 1;
              names1[1] = 2;
              names1[2] = 3;
              names1[3] = 10;
        console.log(names1);
    
        /*出错，因为变量names2指向的地址不能发生改变，应始终指向[]所在的地址,[1,4]与[6,7]不是同一个地址*/
        const names2=[1,4];
              names2=[6,7];   //报错
    </script>