Why do `setTimeout` calls within JavaScript `for` loops often result in failure?

Question

Why do `setTimeout` calls within JavaScript `for` loops often result in failure?

Can you help me understand a concept? I'm not inquiring about fixing the code below. I already know that using the let keyword or an iffe can capture the value of i. What I need clarification on is how the value of i is accessed in this code snippet. I recently came across a blog post discussing why the following code does not work as expected. You can check out the post here: Blog post

for (var i = 1; i <= 5; i++) {
    setTimeout(function() { console.log(i); }, 1000*i);     // 6 6 6 6 6
}

The writer argues that the code fails because it passes the variable i by reference instead of by value. Consequently, when the loop ends and the callbacks are triggered, they all refer to the variable i which contains the final value of 6. Is this explanation correct?

This leads to my own interpretation. I believe that we aren't actually passing anything to the callback functions of setTimeout during the loop execution. Instead, we are setting up asynchronous calls. When these closure callback functions eventually run, they access the variable i based on lexical scoping rules. In essence, the closures look for the variable i within the scope where the callbacks were created, resulting in the value of 6 since it's evaluated after the for loop finishes.

So, which explanation holds true - does the function resolve i to 6 due to passing it as reference each time or is it because of lexical scoping?

javascript settimeout

Answer 1

Answer №1

It is evident that the behavior in question stems from lexical scoping. The timer functions, when executed after the current code finishes running, seek to access the variable i by traversing the scope chain. Due to lexical scoping, i is only defined once in the scope chain (one level above the timer functions), and at that point, i holds the value of 6 because the loop has already ended.

In JavaScript, using the var keyword assigns variables function or global scope based on the declaration's location. In this case, declaring var i places the variable i in the global scope (since the code is not within a function), resulting in each timer function referencing the same globally declared i. As the timer functions run only after the loop completes, they all refer to the final value of i, which is 6 set by the loop.

To resolve this issue, switch from var i to let i to introduce block scope for i.

let establishes block scope for the variable, creating a new scope for i with each iteration of the loop. This ensures that each timer function operates on its own instance of i.

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

Answer 2

It is evident that the behavior in question stems from lexical scoping. The timer functions, when executed after the current code finishes running, seek to access the variable i by traversing the scope chain. Due to lexical scoping, i is only defined once in the scope chain (one level above the timer functions), and at that point, i holds the value of 6 because the loop has already ended.

In JavaScript, using the var keyword assigns variables function or global scope based on the declaration's location. In this case, declaring var i places the variable i in the global scope (since the code is not within a function), resulting in each timer function referencing the same globally declared i. As the timer functions run only after the loop completes, they all refer to the final value of i, which is 6 set by the loop.

To resolve this issue, switch from var i to let i to introduce block scope for i.

let establishes block scope for the variable, creating a new scope for i with each iteration of the loop. This ensures that each timer function operates on its own instance of i.

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

Answer 3

Answer №2

Allow me to clarify using your provided code:

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

When the setTimeout() function is executed, the variable i will have values of 1, 2, 3, 4, and 5 accordingly as anticipated. However, once it reaches a value of 6, it exits the looping process.

   let i = 1;
   setTimeout(function() { console.log(i); }, 1000*1);
   i++;
   setTimeout(function() { console.log(i); }, 1000*2);
   i++;
   setTimeout(function() { console.log(i); }, 1000*3);
   i++;
   setTimeout(function() { console.log(i); }, 1000*4);
   i++;
   setTimeout(function() { console.log(i); }, 1000*5);
   i++;
   // At this point, i equals 6 and the loop halts.

After some time has passed, the timeout callback executes, displaying the current value of i. As mentioned earlier, i had already reached a value of 6.

    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.

The issue stems from the absence of ECMAScript 5's block scope. Using (var i = 1;i <=5 ;i++) creates a variable that persists throughout the entire function and can be altered within local or closure scopes. This necessity led to the inclusion of let in ECMAScript 6.

This problem is easily rectified by simply replacing var with let:

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

Answer 4

Allow me to clarify using your provided code:

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

When the setTimeout() function is executed, the variable i will have values of 1, 2, 3, 4, and 5 accordingly as anticipated. However, once it reaches a value of 6, it exits the looping process.

   let i = 1;
   setTimeout(function() { console.log(i); }, 1000*1);
   i++;
   setTimeout(function() { console.log(i); }, 1000*2);
   i++;
   setTimeout(function() { console.log(i); }, 1000*3);
   i++;
   setTimeout(function() { console.log(i); }, 1000*4);
   i++;
   setTimeout(function() { console.log(i); }, 1000*5);
   i++;
   // At this point, i equals 6 and the loop halts.

After some time has passed, the timeout callback executes, displaying the current value of i. As mentioned earlier, i had already reached a value of 6.

    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.
    console.log(i) // i is already 6.

The issue stems from the absence of ECMAScript 5's block scope. Using (var i = 1;i <=5 ;i++) creates a variable that persists throughout the entire function and can be altered within local or closure scopes. This necessity led to the inclusion of let in ECMAScript 6.

This problem is easily rectified by simply replacing var with let:

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

Why do `setTimeout` calls within JavaScript `for` loops often result in failure?

Answer №1

Answer №2

Similar questions

What is the best way to retrieve all the listed TV and film ratings in descending order? Using Django

Use JavaScript to convert only the initial letter to uppercase

Is it possible to retrieve a physical address using PHP or Javascript?

Transferring data from JavaScript to PHP with the help of AJAX

The display of options in React Bootstrap typeahead is not functioning properly

What could be the reason why my sorting function is not functioning properly?

JavaScript on Ruby on Rails stops functioning in js.erb file

Is there a way for me to link my script for use in a Detail.cshtml file?

Is there a way to instruct npm to compile a module during installation using the dependencies of the parent project?

I am encountering an issue with running my Mocha tests. Can anyone provide assistance on how to solve this problem?

Explore flat data querying using Firebase and AngularFire

"Unlock the power of Passport.js: A guide to leveraging async rendering for email templates

Adding a new row to an HTML table using JavaScript

What is preventing me from creating accurate drawings on canvas?

Allow the button to be clicked only when both options from the 1/3 + 1/3 radio buttons are selected

JavaScript alert box

Navigating through JSON data that has been returned

Troubleshooting: Why isn't setMetadata working in NestJS from authGuards

"Identifying Mouse Inactivity in React: A Guide to Detecting When the Mouse

Issue with iPhone CSS displaying differently on mobile devices