ES6 synchronous asynchronous execution and block level scope

  const promise = new Promise((resolve,reject)=>{
            console.log(1);
            resolve();
            console.log(2);
        })
        promise.then(()=>{
            console.log(3);
        })
        console.log(4);

structure => Constructors execute synchronously ,.then() Asynchronous execution .

So the output 1243~

console.log(a);//undefined

var a = 1;

If variables are overridden, use var Will be covered normally , Use let Error will be reported when overwriting

var There is no block-level scope , To be rude is to say that the apricots will come out of the wall . It still works normally when used outside the module .

But instead let Just report a mistake .

const Once the variable value is declared, it cannot be changed again -- Will report a mistake , Assignment must be made. - Otherwise an error

let Once a variable is declared, it can be changed , And become the changed value .

The following code can do without introducing other variables , Do not use addition and subtraction to make a And b The values of are swapped

        let a = 1;
        let b = 2;
        [a,b]=[b,a]
        console.log(a,b);

When outputting again a The value is 2,b The value is 1 了 .

I wish you all a happy Lantern Festival ！！！