var let const的區別

• var && let

  宣告一個變數　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 不允許重複宣告一個變數

  <script>
      if (true) {
        let a;
        let a; // Uncaught SyntaxError: Identifier 'a' has already been declared
      }
  
      if(true){
        var d;
        var d;    //不會報錯
      }
  
      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

    宣告一個常量，並不是這個常量補課班，而是這個常量不能再次被賦值

    <script>
        /*不會報錯，因為names1指向的地址不變，改變的只是內部資料*/
        const names1 = [];
              names1[0] = 1;
              names1[1] = 2;
              names1[2] = 3;
              names1[3] = 10;
        console.log(names1);
    
        /*出錯，因為變數names2指向的地址不能發生改變，應始終指向[]所在的地址,[1,4]與[6,7]不是同一個地址*/
        const names2=[1,4];
              names2=[6,7];   //報錯
    </script>