How to solve var problem in for loop

When you first learn to use the for loop, you will encounter the following situation.

Problem of repetition

for (var i = 1; i <= 5; i++) {
  setTimeout(function timer() {
    console.log(i)
  }, i * 1000)}Copy the code

Expected results: 1 2 3 4 5

Print result: 6 6 6 6 6

The solution

closure

for (var i = 1; i <= 5; i++) {
    (function (j) {
        setTimeout(function timer() {
            console.log(j)
        }, j * 1000)
    })(i)
}
Copy the code

SetTimeout The third parameter

for (var i = 1; i <= 5; i++) {
   setTimeout(
       function timer(j) {
           console.log(j)
       },
       i * 1000,
       i
   )
}
Copy the code

Define I using let

for (let i = 1; i <= 5; i++) {
    setTimeout(function timer() {
        console.log(i)
    }, i * 1000)}Copy the code