The following statement (ES6) prints out different values of "I" and "timeCount" on the console. What is the principle?

the code is as follows: 

let timeCount = 0;
for (let i = 0; i < 5; iPP) {
  setTimeout(() => {
    console.log("i..", i);
    console.log("timeCount..", timeCount);
  }, 1000 * timeCount);
  timeCount PP;
}

I is printed from 0 to 4. TimeCount has always been 5. What is the principle?

Es6 javascript
Mar.07,2021

is the so-called closure
I is fine because let is equivalent to

. 
let timeCount = 0;
for (let i = 0; i < 5; iPP) {
    {
        let j = i;// i{}

        setTimeout(() => {
            console.log("i..", j);
            console.log("timeCount..", timeCount);
        }, 1000 * timeCount);
        timeCountPP;
    }

}

and if you put I into the global scope, it will all be 5

. 
let timeCount = 0;
let i = 0;
for (; i < 5; iPP) {
  setTimeout(() => {
    console.log("i..", i);
    console.log("timeCount..", timeCount);
  }, 1000 * timeCount);
  timeCount PP;
}
let timeCount = 0;
for (let i = 0; i < 5; iPP) {
  setTimeout(() => {
**alert(2)**
    console.log("i..", i);
    console.log("timeCount..", timeCount);
  }, 1000 * timeCount);
**alert(1)**
  timeCount PP;
}

setTimeout0alertsetTimoutsetTimeout,timeCount5

Ah. The Nikkei problem.

Please search by yourself: "Block scope"

it's very simple.
because the for loop executes first, and then executes the asynchronous setTimeout .

for the timeCount is 5, and then the printing in setTimeout is 5!

and I is because the block-level variable let

is used in the loop. 
for (let i = 0; i < 5; iPP) {
  setTimeout(() => {
    console.log("i..", i);    // 1234
  }, 100);
}

1, let binding scope
2, you have registered 5 setTimeout, until the execution time timeCount has changed

js [all asynchronous] callback functions are always executed after the normal code has been executed, even the settimeout delay of 0 milliseconds does not change the nature of the callback function.

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