JavaScript closure loop issue in set time out

问题

I've found some example in a tutorial (said it was the canonical example)

        for (var i=1; i<=5 ; i++) {
        setTimeout(function() {
            console.log("i: " + i);
        }, i*1000);
    }

Now, I understand that, closure passes in the current scope in to the function, and I assume that it should output 1,2,3,4,5. But instead, it prints number 6 five times.
I ran it in the chrome debugger, and first it goes through the loop without going in to the function while doing the increment of the i value and only after that, it goes in to the inner function and execute it 5 times.
I'm not sure why its doing that, I know, the current scoped is passed in to the function because of closure, but why does it not execute each time the loop iterate?

回答1:

By the time the timeout runs, the for loop has finished, and i is 6, that's why you're getting the output you see. You need to capture i during the loop:

for (var i=1; i<=5 ; i++) {
    (function(innerI) {
        setTimeout(function() {
            console.log("i: " + innerI);
        }, innerI*1000);
    })(i);
}

This creates an inner function with it's own parameter (innerI), that gets invoked immediately and so captures the value of i for use within the timeout.

回答2:

If you didn't want the complex-looking IIFE as explained in James' answer, you can also separate out the function using bind:

function count(i) {
    console.log("i: " + i);
}

for (var i = 1; i <= 5; i++) {
    setTimeout(count.bind(this, i), i * 1000);
}

回答3:

Thank you for you help, I found out another solution and it was a minor change.

On the top of the page I turned on the strict mode and also in the for loop, Instead of var, I used the "let" keyword.

来源：https://stackoverflow.com/questions/29795666/javascript-closure-loop-issue-in-set-time-out

标签

javascript

closures