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

Using Express.js to import a SQL file

What is the process for setting up a new router?

What are the differences between a Chrome app and extension? Is there any other way to access the tabs currently open in your

How do you incorporate ScrollTop animations using @angular/animations?

Activate the saturation toggle when a key is pressed in JavaScript

Ways to initiate SVG animations using Angular Component functions?

Im testing the creation of a global style using styled-components

Error occurred while attempting to run 'postMessage' on the 'Window' object within GoogleTagManager

Tips for discovering the exact positions of child divs that are Nth siblings within a parent div

attempting to retrieve the selected values from within an iframe

In order to enhance user experience, I would like the tabs of the dropdown in the below example to be activated by

Steps for incorporating the getElementByClassName() method

Verify the type of email domain (personal or corporate)

What is the best way to pinpoint the origin of the element.style being injected by JavaScript?

What is the process by which browsers manage AJAX requests when they are made across

How can I empty the value of a UI widget (such as an input field, select menu, or date picker) in Webix UI?

Best practices for correctly parsing a date in UTC format using the date-fns library

Encountering a 404 XHR Error when attempting to add a component in Angular 4.1.0 within Plunker

Using Vue to alter data through mutations

Trigger an Ajax request upon user confirmation, which will only be prompted based on the result of a previous Ajax call