Different types of loops in JavaScript are forEach, for, for...of, and for...in

Question

Different types of loops in JavaScript are forEach, for, for...of, and for...in

Can someone please help me understand the nuances between various types of loops? I am looking for resources that can explain these loops in terms of efficiency, speed, usability, and other factors.

How can I grasp the distinctions presented in the following code snippet?

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (let i = 0; i < digits.length; i++) {
  console.log(digits[i]);
}

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (const index in digits) {
  console.log(digits[index]);
}

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const index of digits) {
  console.log(digits[index]);
}

javascript loops typescript syntax

Answer 1

Answer №1

Looping with for : One common method of looping through an array is by using the i variable as an index to access the elements in the letters array. For more information, refer to MDN Docs

var letters = ["a","b","c"];
for (let i = 0; i < letters.length; i++)
{
    console.log("Index : "+i, "Value : "+letters[i]);
}

The for...in Loop : This loop always iterates over the indices of an array. The variable i will hold the index value of each element during traversal. It can be useful when accessing element indices during iteration. For more details, visit MDN Docs

var letters = ["a","b","c"];
for (var i in letters)
{
    console.log("Index : "+i, "Value : "+letters[i]);
}

The for...of Loop : This loop directly accesses the values of an array during iteration. The variable i will hold the actual element value while traversing through the array. It's suitable when only the values of elements are needed. Learn more from MDN Docs

var letters = ["a","b","c"];
for (var i of letters)
{
    console.log(i);
}

Using forEach Loop : This loop executes a callback function on each element of an array. The callback receives parameters like currentValue, index of the currentValue, and the array itself. For more details, check out MDN Docs

var letters = ["a","b","c"];
letters.forEach(function(value, index, arr){
    console.log("Value : "+value, "Index : "+index, "Complete array : "+arr)
});

An Issue in Your Scenario : The code behaves correctly based on its logic. However, if your array elements match their indices, it can lead to confusion during interpretation.

In the for...in loop: index=0 corresponds to digits[0]=0, index=1 corresponds to digits[1]=1, and so forth.

In the for...of loop: index=0 (where variable index holds the value of digits[0]), the compiler interprets that value at index 0 is accessed in the d array. Thus, it returns the value at digits[0], and the same pattern continues for all elements.

Answer 2

Looping with for : One common method of looping through an array is by using the i variable as an index to access the elements in the letters array. For more information, refer to MDN Docs

var letters = ["a","b","c"];
for (let i = 0; i < letters.length; i++)
{
    console.log("Index : "+i, "Value : "+letters[i]);
}

The for...in Loop : This loop always iterates over the indices of an array. The variable i will hold the index value of each element during traversal. It can be useful when accessing element indices during iteration. For more details, visit MDN Docs

var letters = ["a","b","c"];
for (var i in letters)
{
    console.log("Index : "+i, "Value : "+letters[i]);
}

The for...of Loop : This loop directly accesses the values of an array during iteration. The variable i will hold the actual element value while traversing through the array. It's suitable when only the values of elements are needed. Learn more from MDN Docs

var letters = ["a","b","c"];
for (var i of letters)
{
    console.log(i);
}

Using forEach Loop : This loop executes a callback function on each element of an array. The callback receives parameters like currentValue, index of the currentValue, and the array itself. For more details, check out MDN Docs

var letters = ["a","b","c"];
letters.forEach(function(value, index, arr){
    console.log("Value : "+value, "Index : "+index, "Complete array : "+arr)
});

An Issue in Your Scenario : The code behaves correctly based on its logic. However, if your array elements match their indices, it can lead to confusion during interpretation.

In the for...in loop: index=0 corresponds to digits[0]=0, index=1 corresponds to digits[1]=1, and so forth.

In the for...of loop: index=0 (where variable index holds the value of digits[0]), the compiler interprets that value at index 0 is accessed in the d array. Thus, it returns the value at digits[0], and the same pattern continues for all elements.

Answer 3

Answer №2

The traditional approach:

for (let i = 0; i < digits.length; i++) {
  console.log(digits[i]);
}

This method solely depends on the condition involving i within the loop. If the condition i < digits.length is not met, the loop will terminate.

On the other hand, both alternative versions (using of and in) serve as iterators, meaning they will traverse through all the elements of the array without specifying the total number of items to iterate over.

However, there are distinctions between them:

The of keyword will iterate over the values in the array
The in keyword will iterate over the indices.

Typically, you would use them like this (example modified to highlight the difference):

const chars = ['A','B','C'];

for (const index in chars) {
  // index will be 0,1,2
  console.log(chars[index]);
}

It's preferable if you don't need the index explicitly:

const chars = ['A','B','C'];
for (const value of chars) {
  // value will be 'A','B','C'
  console.log(value);
}

Answer 4

The traditional approach:

for (let i = 0; i < digits.length; i++) {
  console.log(digits[i]);
}

This method solely depends on the condition involving i within the loop. If the condition i < digits.length is not met, the loop will terminate.

On the other hand, both alternative versions (using of and in) serve as iterators, meaning they will traverse through all the elements of the array without specifying the total number of items to iterate over.

However, there are distinctions between them:

The of keyword will iterate over the values in the array
The in keyword will iterate over the indices.

Typically, you would use them like this (example modified to highlight the difference):

const chars = ['A','B','C'];

for (const index in chars) {
  // index will be 0,1,2
  console.log(chars[index]);
}

It's preferable if you don't need the index explicitly:

const chars = ['A','B','C'];
for (const value of chars) {
  // value will be 'A','B','C'
  console.log(value);
}

Different types of loops in JavaScript are forEach, for, for...of, and for...in

Answer №1

Answer №2

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