ES6 adds (I) let and constant

let And var The difference between

1. let Have a scope , If let stay {} Inside curly brackets , Then his scope is limited to curly braces , An error will be reported when output outside .

2. let Cannot repeat declaration ,var The repeated declaration of will overwrite the previous var, however let Can't , When you declare a variable twice , The system will also give you an error .

3. let No variable Promotion , Variable promotion is similar to

var i = 10; Equivalent to var i;i=10; And it will var i; Automatically promote to the front 

however let No, , If

consol.log(i);
let i = 10;

The system will also give you an error , If you use var, The system will not give you an error ,

console.log(i);var i = 10;

At this time, the output value of the console is undefined, This is because the code at this time is equivalent to

var i;console.log(i);i = 10

4.let There is a temporary deadband

 let man = "123456";
        {
            console.log(man);// In a dilemma , So wrong reporting 
            let man = "654321"
        }
        console.log(man);

The rule of constants :

Constants have and let Similar rules , namely , Constants have their own scope 、 Cannot repeat declaration 、 No variable Promotion 、 Temporary deadband , besides , It is better than let Another rule is that you can't change your value , as follows

const i = 10;
i = i+10;
console.log(i);// Report errors , Because constants generally cannot be modified 

The system will directly report an error .

Next, do some exercises ：

1. Use the cycle output button , When you click each button , He will pop up the button of which output he is
2. Because the constant is ES6 newly added , that ES6 How to make a variable constant before

1.

 for(let i = 1;i <= 10; i++){
            let btn = document.createElement("button");
            btn.innerText = i;
            btn.onclick = function(){
                alert(i)
                console.log(i)
            }
            document.body.appendChild(btn);
        }

without let Writing will be troublesome , The output is 10, however let It will make the whole process simple .

2.

Object.defineProperty(Object,'freezeObj',{
            value:function(obj){
                for (var i in obj) {
                    if (obj.hasOwnProperty(i)) {
                        Object.defineProperty(obj,i,{
                            writable:false
                        })
                    }
                }
            Object.seal(obj);
            }
        });
        var xiaoming = {
            age:18,
            name:" Zhang San ",
            function(){
                console.log("123456")
            }
        }
        Object.freezeObj(xiaoming)