Lesson Wednesday

Array.prototype.forEach() is a relatively new addition to JavaScript. Under the hood, it uses what's known as a for loop.

In this lesson, we'll cover how a for loop works. Then, in the next lesson, we'll discuss when to favor Array.prototype.forEach() and when to stick with a for loop. Finally, we'll add a function to our text analyzer application that uses a for loop.

Let's take a look at a basic example of a for loop. We'll see what it prints to the console and then we'll go over all of the parts of the loop.

Here's a loop that logs the value of index. We'll be talking about what the index is in just a moment.

for (let index = 1; index <= 3; index += 1) {

Let's see what this logs to the console.

The for loop logs 1, 2, then 3 and finally undefined.

As we can see, the index is an incrementing number. We can also see that the return of a for loop is undefined, just like it is with Array.prototype.forEach(). (This isn't surprising - remember, under the hood, Array.prototype.forEach() is using a for loop.)

Here is the syntax of a for loop. This is pseudocode, which means it isn't written like JavaScript. It's written in plain English to explain how it works.

// Pseudocode alert!

for (let index = startingValue; index <= endingValue; index += increment ) {
  // Code that should run each time through the loop!

Now let's break this down further.

  • The for statement takes three parameters: initialization, condition, and final expression.

  • The initialization parameter is the starting value of the loop. In the example above, we called it startingValue. index is a variable we initialize with our starting value. Very often, that starting value is 0 or 1, but depending on your use case, it could be any number.

The initialization parameter is executed before the loop runs. Because the initialization loop is a variable that changes, we need to use let with it. const will not work. If we omit let, we are accidentally scoping the value of index globally - a very bad idea, especially if there are other for loops using index in the codebase.

  • The condition parameter tells the loop when to stop running - or more accurately when to keep running. The loop will stop when the condition is no longer true. In our example, when the index variable is NOT less than or equal to 3, the loop will stop. Until then, it will keep going. The condition is evaluated every time the loop runs. We can use other operators to denote this depending on how our loop works, including <, >, and >=.

  • The final expression parameter usually changes the value of index by incrementing or decrementing it. In our example, the index variable will be increased by 1 after each time the loop is executed.

  • Each time the loop runs, the code within the loop will log the value of index.

  • Note that each parameter has a semicolon as a separator. A for loop will not work without these separators.

In the loop above, we specify that the index will start at 1 and end at 3. Each time through the loop, the index will increment by 1. The loop will run 3 times before it reaches 3 and ends.

Now it's time for a bit of a brainteaser - how many times will the for loop below run and what will the value of index be in the console each time it runs? See if you can figure it out before running the code in the console.

for (let index = 1; index < 7; index += 2) {

Now let's take a look at the code:

Console shows index incrementing 1,3,5 and then undefined.

There are a couple of little gotchas here. First, we are incrementing by 2, not 1. That means the value of index will increase like this... 1, 3, 5... The most common increment and decrement is 1, but it's important to see exactly what's happening with the final expression parameter.

The other gotcha is where it stops. Did you think the loop would run until the index reached 7? If so, you're not alone. In the example above, we changed the operand we use in the condition parameter from <= to <. If we'd used <= instead, the loop would've run one more time and the index would've ended at 7.

So as you can see, for loops can be tricky! We'll discuss that further in the next lesson.

Now that we've dissected a very basic for loop, how can we actually use it with a collection like an array? Unlike Array.prototype.forEach(), a for loop isn't called on an array.

Remember how we doubled all the values of an array with Array.prototype.forEach() and then saved those values in a new array? Here's how we did it:

const array = [0,1,2,3,4,5];
let doubledArray = [];
array.forEach(function(element) {
  doubledArray.push(element * 2);

Now let's do the same with a for loop.

const array = [0,1,2,3,4,5];
let doubledArray = [];
for (let index = 0; index < array.length; index +=1) {
  doubledArray.push(array[index] * 2);

Some of this should look familiar - we don't change the array we loop over. We also need to initialize a new array called doubledArray that we'll push the doubled values into - just like we did when using Array.prototype.forEach().

Now let's look at the for loop. Note that the index starts at 0, not 1. If we are looping through an entire array, the index starts at 0 because the index of the first element in an array is 0.

Next, we specify that the condition parameter is index < array.length;. We could just as easily have written this as index <= array.length - 1. Remember, because the index of the first element in an array is 0 instead of 1, the index of the final element in an array will always be one less than the array's length.

Finally, we increment the index by 1 each time an iteration through the loop is finished. So why do we want to increment by 1 each time through? Let's look at the code in the loop itself to see why:

doubledArray.push(array[index] * 2);

To access a value from our original array, we need to use bracket notation. The first value of array is array[0], the second value is array[1], and so on. Well, that matches our index perfectly! That's exactly why we start with an index of 0 when we use for loops to work with arrays and then increment by 1 until we reach one less than the length of the array.

We are going to look at more examples in a moment to solidify for loops further. But first, an aside to talk about the risk of dreaded OBOEs (off-by-one errors).

OBOEs in for Loops

In general, whenever we are iterating over every element in an array, we should use Array.prototype.forEach() instead of for. It's not just that it's easier to use. The risk of having an OBOE in a for loop is much greater. Let's demonstrate two potential OBOEs based on our doubling example. First, let's imagine that our array doubling example is in a real world application where we figure out the total cost of a certain kind of very important widget. We need to double each element in the array and then add them together in order to figure out the final cost of that widget. (And yes, a more complex algorithm would probably be used in the real world to determine that cost, but bear with us - the exact same errors happen in real world applications.)

Here's the first OBOE. Try it out in the console:

const array = [0,1,2,3,4,5];
let doubledArray = [];
for (let index = 0; index <= array.length; index +=1) {
  doubledArray.push(array[index] * 2);

Here's the result:

[0, 2, 4, 6, 8, 10, NaN]

Why do we get NaN (not a number) as the final result? Well, the final iteration of the loop looks for the value of array[6]. There is no element at that index, which returns undefined. What happens when you multiply undefined by a number? You get NaN - because you can't make a number out of it. And what happens when our widget sellers try to calculate the cost of widgets by adding the numbers together? Our formula will return NaN - because when you add a real number to NaN, it is still NaN.

Now let's take a look at an OBOE in the other direction.

const array = [0,1,2,3,4,5];
let doubledArray = [];
for (let index = 0; index < array.length - 1; index +=1) {
  doubledArray.push(array[index] * 2);

The value of doubledArray in the console is:

[0, 2, 4, 6, 8]

As we can see, the final number got lopped off. What changed here is that we did the following index < array.length - 1. We are saying the final index should be two less than the length of the array, not one. This error is more insidious - we can't sell widgets that cost NaN - and that error will probably get caught quickly. However, this returns an actual number - and if we add all the numbers together, we might be selling widgets at an unsustainable discount. That wouldn't be good!

This is a major reason we should always use Array.prototype.forEach() when we need to iterate through every element in an array. That being said, you should still take some time to practice using for loops to do the same thing - even though in the long term, it won't be the best practice. It's very important to understand a for loop inside and out regardless of how much you'll end up using them!

Other Examples of for Loops

Let's look at a few other examples of for loops. In fact, we will just translate all of the loops we made with Array.prototype.forEach() into for loops. That way, you'll have enough of a foundation to practice creating for loops on your own.

We'll start by using a for loop to add all the numbers in an array together.

let total = 0;
const summands = [1, 2, 3, 4, 5];
for (let index = 0; index < summands.length; index += 1) {
  total += summands[index];

Try this out in the console. You'll see it gives the same result as if we'd done it with Array.prototype.forEach().

Here's a corresponding chart that shows what happens each iteration through the loop:

for loop summands charts

Note that when index is 0 here, summands[index] is 1 because the first element of the array is 1. index is just being used here to refer to a specific element of the array by its index position. It's a bit confusing in this case because the elements of the array are also numbers.

Note that we could've done the same thing using a for loop without creating an array. After all, the array variable above just holds an array of sequential numbers.

Here's a version of our sum code that uses a for loop but doesn't require an array to loop through. Try it in the console:

let total = 0;
for (let currentNumber = 1; currentNumber <= 5; currentNumber += 1) {
  total += currentNumber;

When the loop begins we initialize our loop variable currentNumber to 1. Note that we could have used index as the variable name here but currentNumber is a bit clearer since we'll be making use of the variable for something other than iterating through a loop. Specifically, we'll be adding it to total each time through the loop in addition to using it as an iterator. We then execute our loop as long as currentNumber is less than or equal to 5, iterating currentNumber after each run through the loop. Here's a chart showing the steps:

for loop sum chart

The first time through the loop, currentNumber is 1, which is <= 5, so we go ahead and execute the statement in the code block: total += currentNumber. total is now 1 after this first time through the loop. At the end of each time through the loop we're incrementing currentNumber, so currentNumber is now 2.

The second time through the loop, currentNumber is 2, which is still <= 5, so again we execute total += currentNumber, setting total to 3. We again increment currentNumber.

The third time through, currentNumber is 3, so total becomes 6. And we increment currentNumber.

The fourth time through, currentNumber is 4, so total becomes 10 and then we increment currentNumber.

The fifth time through, currentNumber is 5, so total becomes 15 and we increment currentNumber.

The sixth time we try to start going through the loop, currentNumber is 6, which is not <= 5, so we're done with the for loop. We then show the user an alert that includes the total.

Let's do one more example. This time, we'll recreate the example of appending likable things to a string. For practice, you may want to see if you can recreate the example yourself before looking at the answer. Here's the original using Array.prototype.forEach().

let thingsILike = "I like...";
const arrayOfThingsILike = ["bubble baths", "kittens", "good books", "clean code"];
arrayOfThingsILike.forEach(function(thing) {
  thingsILike = thingsILike.concat(" " + thing + "!");

Don't look at the translation to a for loop just yet... try to write it yourself and test it in the console.

Here's the version that uses a for loop:

let thingsILike = "I like...";
const arrayOfThingsILike = ["bubble baths", "kittens", "good books", "clean code"];
for (let i = 0; i < arrayOfThingsILike.length; i+=1) {
  thingsILike = thingsILike.concat(" " + arrayOfThingsILike[i] + "!");

Note that we use i instead of index for our variable name in our example. When you see for loops in the wild, you'll most commonly see the variable named i. This is short for index - and reflects a desire for maximum conciseness. It's totally fine if you use i as well - though it doesn't hurt to use more descriptive names in the short term, especially if it helps you understand what's going on in a loop.

At this point, you're ready to start writing more for loops on your own. Once again, be patient with yourself if it takes time to absorb these new concepts. Don't be hard on yourself if you have OBOEs or other errors in your code - they happen to experienced developers, too, and they are an important part of every developer's learning experience.

Finally, you may be wondering why we'd ever favor a for loop over an Array.prototype.forEach() loop. That's exactly what we'll discuss in the next lesson.


  • For loop: The "traditional" way to write loops in JavaScript. Creates a loop without needing to call a method on an array.

  • Initialization parameter: Initializes a variable with the denoted value. This only happens once, when the loop is first triggered. It needs to be initialized with let.

  • Condition parameter: Tells the loop under what conditions to continue running the loop. As long as the condition is true, the loop will continue running.

  • Final expression parameter: Usually changes the initial value in some way, often by incrementing or decrementing it.


Example for loop:

for (let index = 1; index <= 3; index += 1) {

In the code above...

  • let index = 1; is the initialization parameter.
  • index <=3; is the condition.
  • index += 1 is the final expression.

Example of using a for loop with an array:

const languages = ['HTML', 'CSS', 'Javascript'];
for (let index = 0; index < languages.length; index += 1) {
  alert('I love ' + languages[index] + '!');

Lesson 28 of 42
Last updated May 26, 2022