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?

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.

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-64f5-1bf41c0-31ec1.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1bf41c0-31ec1.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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