How to pass variable to anonymous function

Question

I want to pass variable setTimeoutfunction and do something with that. When I alert value of i it shows me numbers that i did not expected. What i m doing wrong? I want log values from 1 till 8.

var end=8;
for (var i = 1; i < end; i ++) {
       setTimeout(function (i) {
           console.log(i);   

       }, 800);
   }

Answer 1

The standard way to solve this is to use a factory function:

var end=8;
for (var i = 1; i < end; i ++) {
       setTimeout(makeResponder(i), 800);
   }

function makeResponder(index) {
    return function () {
        console.log(index);   
   };
}

Live example | source

There, we call makeResponder in the loop, and it returns a function that closes over the argument passed into it (index) rather than the i variable. (Which is important. If you just removed the i argument from your anonymous function, your code would partially work, but all of the functions would see the value of i as of when they ran, not when they were initially scheduled; in your example, they'd all see 8.)

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Update From your comments below:

...will it be correct if i call it in that way setTimeout(makeResponder(i),i*800);?

Yes, if your goal is to have each call occur roughly 800ms later than the last one, that will work:

Live example | source

I tried setTimeout(makeResponder(i),setInterval(i));function setInterval(index) { console.log(index*800); return index*800; } but it's not work properly

You don't use setInterval that way, and probably don't want to use it for this at all.

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Further update: You've said below:

I need first iteration print 8 delay 8 sec, second iteration print 7 delay 7 sec ........print 2 delay 2 sec ...print 0 delay 0 sec.

You just apply the principles above again, using a second timeout:

var end=8;
for (var i = 1; i < end; i ++) {
       setTimeout(makeResponder(i), i * 800);
   }

function makeResponder(index) {
    return function () {
        var thisStart = new Date();
        console.log("index = " + index + ", first function triggered");
        setTimeout(function() {
            console.log("index = " +
                        index +
                        ", second function triggered after a further " +
                        (new Date() - thisStart) +
                        "ms delay");
        }, index * 1000);
   };
}

Live example | source

I think you now have all the tools you need to take this forward.

Answer 2

Your problem is that you are referring to the variable i some time later when your setTimeout() function fires and by then, the value of i has changed (it's gone to the end of the for loop. To keep each setTimeout with it's appropriate value of i, you have to capture that value i separately for each setTimeout() callback.

The previous answer using a factory function does that just fine, but I find self executing functions a little easier than factory functions to type and follow, but both can work because both capture the variables you want in a closure so you can reference their static value in the setTimeout callback.

Here's how a self executing function would work to solve this problem:

var end=8;
for (var i = 1; i < end; i ++) {
       (function (index) {
           setTimeout(function() {
               console.log(index);
           }, 800);
       })(i);
   }

To set the timeout delay in proportion to the value of i, you would do this:

var end=8;
for (var i = 1; i < end; i ++) {
    (function (index) {
        setTimeout(function() {
            console.log(index);
        }, index * 800);
    })(i);
}

The self executing function is passed the value of i and the argument inside that function that contains that value is named index so you can refer to index to use the appropriate value.

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Using let in ES6

With the ES6 of Javascript (released in 2015), you can use let in your for loop and it will create a new, separate variable for each iteration of the for loop. This is a more "modern" way to solve a problem like this:

const end = 8;
for (let i = 1; i < end; i++) {            // use "let" in this line
     setTimeout(function() {
         console.log(i);
     }, 800);
 }