[Js]js variable with VAR and non-VAR difference

It has been said to be clear. The following code is a repeating

<script>    //带var和不带var的区别:    // 1.只有带var的才可以预解释,所以在赋值的前操作不会报错.    console.log(num); //undefined    var num = 100;    // 2.不带var的不会进行预解释,赋值前调用会报错,    console.log(num2); //Uncaught ReferenceError: num2 is not defined    num2 = 200;    // 3. 本质区别:    //    - 1)带var相当于给window 增加了一个num3属性,属性值是12    //    - 2)给全局作用域增加了全局变量    console.log(num3); //12 windows.num3    console.log(window.num3);    var num3 = 12;    //    - 不带var 仅仅相当于给window 增加了一个num3属性,属性值是12    num4 = 12;    console.log(num4); //可以不用写window.    console.log(window.num4); //  windows.num3,</script>

[Js]js variable with VAR and non-VAR difference