Assessing the output of a saved function within a loop using a variable declared with let

Question

Assessing the output of a saved function within a loop using a variable declared with let

From what I understand, when the loop variable of a for loop is declared using var, any changes made to that variable are reflected globally. For example:

var printNumTwo;
for (var i = 0; i < 3; i++) {
  if (i === 2) {
    printNumTwo = function() {
      return i;
    };
  }
}
console.log(printNumTwo());

In the code snippet above, the console will display 3. This is because in the final iteration, the variable i will be equal to 3. Therefore, when calling printNumTwo, the updated value of i will be returned. However, this behavior changes when using let:

let printNumTwo;
for (let i = 0; i < 3; i++) {
  if (i === 2) {
    printNumTwo = function() {
      return i;
    };
  }
}
console.log(printNumTwo());

The second code snippet will output 2.

let printNumTwo;
for (let i = 0; i < 3; i++) {
  if (i === 2) {
    printNumTwo = function() {
      return i;
    };
    i = 3;
  }
}
console.log(printNumTwo());

Surprisingly, the third code snippet prints 3. What could be the reason behind this difference?

source: The initial two code blocks can be found here.

javascript ecmascript-6

Answer 1

Answer №1

The initial scenario demonstrates how var confines i within the function, causing i++ to transform i from 2 to 3, ultimately halting the loop. At this point, the sole value of i is 3, and when printNumTwo is called, it returns i which is 3.

In contrast, the following example shows that using let limits the scope of i to the block level, resulting in printNumTwo encapsulating it. During the final iteration of the loop, i stands at 2, and consequently, printNumTwo outputs i as 2. (Although a new i emerges with the value of 3, it remains unused by any function).

Moving on to the third instance, once again utilizing let isolates i within the block. This time, however, the difference lies in the assignment i = 3;, effectively setting all existing i instances to 3. Consequently, the last generated function (while also replacing previous ones) yields i as 3.

Answer 2

The initial scenario demonstrates how var confines i within the function, causing i++ to transform i from 2 to 3, ultimately halting the loop. At this point, the sole value of i is 3, and when printNumTwo is called, it returns i which is 3.

In contrast, the following example shows that using let limits the scope of i to the block level, resulting in printNumTwo encapsulating it. During the final iteration of the loop, i stands at 2, and consequently, printNumTwo outputs i as 2. (Although a new i emerges with the value of 3, it remains unused by any function).

Moving on to the third instance, once again utilizing let isolates i within the block. This time, however, the difference lies in the assignment i = 3;, effectively setting all existing i instances to 3. Consequently, the last generated function (while also replacing previous ones) yields i as 3.

Answer 3

Answer №2

Why does the above code output 3 instead of the expected value?

The reason for this is that you are assigning the value of 3 to the variable i which is being closed over by the function printNumTwo. The timing of the assignment relative to when printNumTwo was created is not important; what matters is that it is the variable i that is being used within printNumTwo.

The distinction between using var and let in for loops lies in the fact that a new variable is created for each iteration of the loop body with let. Since you are updating this variable within the loop body, the function closing over it (printNumTwo) will observe this updated value when invoked later.

To put it simply:

function create(i) {
    // This function captures `i`
    const fn = function() {
        return i;
    };

    // This updates the captured `i` value
    ++i;

    return fn;
}
const printValue1 = create(1);
console.log(printValue1()); // Outputs 2 (1 + 1)

const printValue27 = create(27);
console.log(printValue27()); // Outputs 28 (27 + 1)

In reply to a comment on the initial question:

A similar scenario occurs in the second code block (i++ in the update section of the for loop), but why does it output 2?

Ah! I now understand the confusion. While there is an update to i, it affects a different instance of i compared to the one enclosed by printNumTwo.

The update in the for loop's update section happens on a fresh variable representing the next iteration at the beginning of that iteration, rather than on the variable from the previous iteration at the end of that iteration. Here's how it unfolds:

Create a new version of the iteration variable (let's call it iNew)
Copy the value of the existing iteration variable (let's call it iOld) into the new variable (iNew = iOld)
Execute the update logic using iNew (iNew++)

This explains why the original i remains at 2, while the new one assumes the value of 3.

Answer 4

Why does the above code output 3 instead of the expected value?

The reason for this is that you are assigning the value of 3 to the variable i which is being closed over by the function printNumTwo. The timing of the assignment relative to when printNumTwo was created is not important; what matters is that it is the variable i that is being used within printNumTwo.

The distinction between using var and let in for loops lies in the fact that a new variable is created for each iteration of the loop body with let. Since you are updating this variable within the loop body, the function closing over it (printNumTwo) will observe this updated value when invoked later.

To put it simply:

function create(i) {
    // This function captures `i`
    const fn = function() {
        return i;
    };

    // This updates the captured `i` value
    ++i;

    return fn;
}
const printValue1 = create(1);
console.log(printValue1()); // Outputs 2 (1 + 1)

const printValue27 = create(27);
console.log(printValue27()); // Outputs 28 (27 + 1)

In reply to a comment on the initial question:

A similar scenario occurs in the second code block (i++ in the update section of the for loop), but why does it output 2?

Ah! I now understand the confusion. While there is an update to i, it affects a different instance of i compared to the one enclosed by printNumTwo.

The update in the for loop's update section happens on a fresh variable representing the next iteration at the beginning of that iteration, rather than on the variable from the previous iteration at the end of that iteration. Here's how it unfolds:

Create a new version of the iteration variable (let's call it iNew)
Copy the value of the existing iteration variable (let's call it iOld) into the new variable (iNew = iOld)
Execute the update logic using iNew (iNew++)

This explains why the original i remains at 2, while the new one assumes the value of 3.

Answer 5

Answer №3

The scoping of var is limited to the function, whereas the scope of let is restricted to the block.

When a value is assigned to the printNumTwo function, the value of i remains 2 regardless of any changes made to i afterwards.

This example illustrates the distinction between the behaviors of var and let.

for(var i=0; i<3; i++){
    setTimeout(() => console.log('var', i))
}

for(let i=0; i < 3; i++){
    setTimeout(() => console.log('let',i))
}

Answer 6

The scoping of var is limited to the function, whereas the scope of let is restricted to the block.

When a value is assigned to the printNumTwo function, the value of i remains 2 regardless of any changes made to i afterwards.

This example illustrates the distinction between the behaviors of var and let.

for(var i=0; i<3; i++){
    setTimeout(() => console.log('var', i))
}

for(let i=0; i < 3; i++){
    setTimeout(() => console.log('let',i))
}

Assessing the output of a saved function within a loop using a variable declared with let

Answer №1

Answer №2

Answer №3

Similar questions

Transform my Angular implementation to TypeScript programming

Understanding when the @Input parameter has been modified

Hide the element, update its content, and smoothly transition back

Initiate jison states using JSON structure

Is sendFile causing an error due to an invalid JSON format?

Cleanse the array in jQuery by removing any unusual characters

Adjusting the Size of a Static Banner

Extract dynamic JSON objects found within a JSON response

How to efficiently use nested $.each() in DataTables with jQuery

Converting ampersands to HTML special characters in AJAX JSON transformations

Is it possible to assign a class to the initial word within a designated class?

What is the best way to maintain a search input history while navigating search results and returning using the browser button, similar to Google Search in Angular2?

What is the best way to position my content next to my sticky sidebar for optimal visibility?

Modifying specific attributes of an object within the $scope: A step-by-step guide

Tips for creating a jquery modal form that can be reused

What is the reason behind every single request being treated as an ajax request?

Exploring Three.js on Cordova with WebGL

Webpack encounters issues when compiling ES6 syntax from external packages

Steps for making a linked list through the drag-and-drop method

A series of divs continuously scrolling in succession with a stationary title/header