What exactly is the explanation behind this behavior in JavaScript when it comes to pass-by-reference

Question

What exactly is the explanation behind this behavior in JavaScript when it comes to pass-by-reference

As a newcomer to JavaScript, I came across an intriguing problem that made me question what would be displayed on the console. The correct answer turned out to be 4 4 4 4, thanks to pass-by-reference. Still, I'm struggling to grasp why this is the outcome. How does pass-by-reference come into play when the output seems to solely rely on the value of i within a simple for-loop iteration?

const array = [10, 21, 31, 41];

for(i = 0; i < array.length; i++) {
  setTimeout(function print(array){
      console.log(i)}, 3000);
}

javascript

Answer 1

Answer №1

Let's break it down step by step:

const array = [10, 21, 31, 41];

for(i = 0; i < array.length; i++) {
  setTimeout(function print(array) { /* stuff... */ }, 3000);
}

// By the time we reach this point, the value of i is 4.

When running this code snippet, we are looping through the array and setting a timeout for each iteration. At the end of the loop, setTimeout has been called four times, but none of the functions provided have executed yet. They will only execute after another 3 seconds.

Crucially, at this stage, the value of i is 4.

Three seconds later...

Finally, the function print passed to setTimeout will run. What does this function actually look like?:

function print(array) { console.log(i); }

Now consider that i is currently 4. Therefore, it will print 4, repeating four times. This is because the function reads the current value of i when it is executed, not when it was defined initially.

In essence, i holds the value 4 when the function print is triggered, resulting in the output of 4.

Answer 2

Let's break it down step by step:

const array = [10, 21, 31, 41];

for(i = 0; i < array.length; i++) {
  setTimeout(function print(array) { /* stuff... */ }, 3000);
}

// By the time we reach this point, the value of i is 4.

When running this code snippet, we are looping through the array and setting a timeout for each iteration. At the end of the loop, setTimeout has been called four times, but none of the functions provided have executed yet. They will only execute after another 3 seconds.

Crucially, at this stage, the value of i is 4.

Three seconds later...

Finally, the function print passed to setTimeout will run. What does this function actually look like?:

function print(array) { console.log(i); }

Now consider that i is currently 4. Therefore, it will print 4, repeating four times. This is because the function reads the current value of i when it is executed, not when it was defined initially.

In essence, i holds the value 4 when the function print is triggered, resulting in the output of 4.

Answer 3

Answer №2

Take another look at the inner function that is triggered by the timeout.

function display(numbers){
    console.log(i);
}

The array provided is not actually utilized at any point, as demonstrated by the fact that the code below produces the same output (4 4 4 4):

for(i = 0; i < numbers.length; i++) {
  setTimeout(function print(){
      console.log(i)}, 3000);
}

Due to pass by reference behavior, the value of i passed into the print() function is a reference to whatever i holds when console.log(i) is executed. With a 3-second delay, the loop completes before console.log(i) is called, resulting in i equalling 4. Even changing the value of i after the loop affects the outcome:

for(i = 0; i < 4; i++) {
  setTimeout(function display(){
      console.log(i)}, 3000);
}

i = "bar";
// outputs bar bar bar bar

Answer 4

Take another look at the inner function that is triggered by the timeout.

function display(numbers){
    console.log(i);
}

The array provided is not actually utilized at any point, as demonstrated by the fact that the code below produces the same output (4 4 4 4):

for(i = 0; i < numbers.length; i++) {
  setTimeout(function print(){
      console.log(i)}, 3000);
}

Due to pass by reference behavior, the value of i passed into the print() function is a reference to whatever i holds when console.log(i) is executed. With a 3-second delay, the loop completes before console.log(i) is called, resulting in i equalling 4. Even changing the value of i after the loop affects the outcome:

for(i = 0; i < 4; i++) {
  setTimeout(function display(){
      console.log(i)}, 3000);
}

i = "bar";
// outputs bar bar bar bar

Answer 5

Answer №3

Utilizing setTimeout in your code introduces asynchronicity - after the 3000 millisecond timeout, the JavaScript loop may end prematurely resulting in the last value of the index being taken, such as the number 4. Explore 'js Eventloop' to gain a deeper understanding of how browsers operate at a fundamental level.

Answer 6

Utilizing setTimeout in your code introduces asynchronicity - after the 3000 millisecond timeout, the JavaScript loop may end prematurely resulting in the last value of the index being taken, such as the number 4. Explore 'js Eventloop' to gain a deeper understanding of how browsers operate at a fundamental level.

Answer 7

Answer №4

Only one value is assigned to i. The original value is not stored when setTimeout is called.

The value of i will be 4 when the print function is executed after the for-loop completes, as that is the final value at the end of the loop.

Answer 8

Only one value is assigned to i. The original value is not stored when setTimeout is called.

The value of i will be 4 when the print function is executed after the for-loop completes, as that is the final value at the end of the loop.

Answer 9

Answer №5

In order to access the 'i' value, one must understand the concept of closure. As the function is executed, the 'i' value changes to 4 and is then printed.

Answer 10

In order to access the 'i' value, one must understand the concept of closure. As the function is executed, the 'i' value changes to 4 and is then printed.

What exactly is the explanation behind this behavior in JavaScript when it comes to pass-by-reference

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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