How to use setInterval function within for loop

问题

I\'m trying to run multiple timers given a variable list of items. The code looks something like this:

var list = Array(...);

for(var x in list){
    setInterval(function(){
        list[x] += 10;
        console.log(x + \"=>\" + list[x] + \"\\n\");
    }, 5 * 1000);
}

The problem with the above code is that the only value being updated is the item at the end of the list, multiplied by the number of items in the list.

Can anyone offer a solution and some explanation so I know why it\'s behaving this way?

回答1:

So, a few things:

Most importantly, the callback function you've passed to setInterval() maintains a reference to x rather than the snapshot value of x as it existed during each particular iteration. So, as x is changed in the loop, it's updated within each of the callback functions as well.
Additionally, for...in is used to enumerate object properties and can behave unexpectedly when used on arrays.
What's more, I suspect you really want setTimeout() rather than setInterval().

You can pass arguments to your callback function by supplying additional arguments to setTimout():

var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]);

Numbers will be passed by value rather than reference. Here's an example:

var list = [1,2,3,4];

for (var x = 0, ln = list.length; x < ln; x++) {
  setTimeout(function(y) {    
    console.log("%d => %d", y, list[y] += 10);
  }, x * 500, x); // we're passing x
}

回答2:

var list = [1, 2, 3, 4, 5];

for (var i = 0, len = list.length; i < len; i += 1) {
    (function(i) {
        setInterval(function() {
            list[i] += 10;
            console.log(i + "=>" + list[i] + "\n");
        }, 5000)
    })(i);
}

Here is the working code:

var list = [1, 2, 3, 4, 5];

for (var i = 0, len = list.length; i < len; i += 1) {
    (function(i) {
        setInterval(function() {
            list[i] += 10;
            console.log(i + "=>" + list[i] + "\n");
        }, 5000)
    })(i);
}

Here the index i is stored in an anonymous function, so that it is not overwritten by consecutive loops. setInterval function in your code keeps the reference only to the last value of i.

回答3:

You don't have to use a for cycle with the setInterval statement. Try this:

var list = Array(...);
var x = 0;

setInterval(function() {

    if (x < list.length;) {
        list[x] += 10;
        console.log(x+"=>"+list[x]);
    }

    else return;

    x++;
}, 5000);

回答4:

I don't know how to do this with a for loop but this code here will print out each element in an array at timed intervals:

function displayText(str) {
   $('.demo').append($('<div>').text(str));
}
var i = 0;

var a = [12, 3, 45, 6, 7, 10];

function timedLoop() {
setTimeout(function () {
    displayText(a[i]);
    i++;
    if(i < a.length) {
        timedLoop();
    }
}, 2000)
}

timedLoop();

Using a bit of jquery to show it in the browser.

回答5:

If you have JSON array and jQuery included, you can use:

$.each(jsonArray, function(i, obj) {
    setInterval( function() {
        console.log(i+' '+obj);
    }, 10);
});

回答6:

You can combine forEach and setTimeout to loop over the array with the interval.

let modes = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let interval = 1000; //one second
modes.forEach((mode, index) => {

  setTimeout(() => {
    console.log(mode)
  }, index * interval)
})

来源：https://stackoverflow.com/questions/7749090/how-to-use-setinterval-function-within-for-loop

标签

javascript

for-loop

closures

setinterval