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.

Previous: Java md5 encryption, why do you usually use 0-9 a murf encryption after encryption?

Next: Why is vue $on not triggered?

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1ea6aab-47bad.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1ea6aab-47bad.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?