JS: For Loops

Learning Goals

  • Write for loop syntax
  • Understand and explain the components of a for loop and how they can be used in conjunction with arrays


  • Array Used to store a collection of items/multiple values in a single variable
  • Element A single item stored in an array. An element can be of any data type
  • Loops A quick way to do something repeatedly
  • Control Flow The order in which the computer executes statements in a script. The order of execution can change whenever the computer runs across the (extremely frequent) structures that change the control flow, such as conditionals and loops
  • Bracket Notation How we access individual elements of an array. Either to express the element, or assign a new element

Warm Up

Part 1 - Set Up

In a repl.it, declare a variable that stores an array, and console.log it. Your array should contain 3 or more JavaScript objects as its elements. Some ideas:

  • A list of your friends (and their birthdays or hobbies)
  • A list of Hogwarts students (name, House, “cool” status)

Or, if you’re not feeling creative, you can take the example below as a starter and modify it a bit:

var pets = [
    name: 'Tilly',
    type: 'cat',
    favoriteTreat: 'cheese',
    human: 'Leta'
    name: 'Sodie',
    type: 'dog',
    favoriteTreat: 'milkbones',
    human: 'Amy'
    name: 'Pumpernickel',
    type: 'cat',
    favoriteTreat: 'kibble',
    human: 'Eric'

Part 2 - Warm Up, Explore

Once in your breakout room, trade repl.it links with your partner. This way, you are each exposed to a new array that you will work with during activity. You will each need to fork the repl.it.

  • Take a moment to familiarize yourself with the array your partner declared

For each step that follows, make sure to console.log the value(s) you are accessing/have changed!

  • Access the first element in the array
  • Access a value from the second element (an object) in the array
  • Update a value from the third element in the array

If you finish before your partner, add another object to the array. Log it to the console to make sure it is stored properly.

💬 Before moving on to Part 3, check in with each other and make sure both partners were able to accomplish the task and know what their code is doing.

Part 3 - Collaborate

For this part, use the pets array that is at the top of the Warm Up section. The variable that stores the array needs to be in the repl.it file you are working in.
Your task is to write the code that uses data from that array to print out (using console.log):


Then, print out:

// the ordering of the names is not a typo!

You can write the code in the same repl.it or a new one. Be ready to share your code with the whole group!

(Without using a loop) Can you come up with another solution in your code to get the same outcome? If so, be ready to share out. If not, be ready to explain why that was a challenge.


There are times when we want to repeat the same operation multiple times over a set of data. Loops allow us to do just that by running through our data one by one and executing code to accomplish a goal.

For example, for each element in an array, if a conditional returns true, a code block will be run and the condition will be checked again. This pattern will be repeated until the conditional returns false.

Let’s take a look at the structure of the most commonly used type, the for loop:

for ([initialExpression]; [condition]; [incrementExpression]) {

Which looks like this when we implement it in code:

for (var i = 0; i < 10; i++) {

If we break this down, we see that our loop is constructed from the following parts:

  • the keyword for
  • a set of rules, or conditions (var i = 0; i < 10; i++ )
  • opening and closing curly braces which contain our code
  • the code that we want our loop to execute: console.log(i);

Let’s dig into the three statements separated by semicolons that make up or our conditions:

  • We begin with initialization. Where do we want our loop to start? The first statement var i = 0; creates a variable that is assigned the value of 0. This variable is commonly named i, or index, and will act as the counter. It is created the first time the loop is run.
  • The next statement sets the condition that tells the loop when to stop running: i < 10;. In this case, the condition indicates that the loop will stop when i equals 10. The condition may use a variable that is assigned a value.
  • Finally, with the statement i++ we update the value of our counter i. This adds 1 to the value of i. This syntax is using the increment operator ++, which is a way of writing i = i + 1. It is also possible to decrement downwards using the decrement operator --, which is a way of writing i = i - 1.

The statement within the curly braces executes each time the loop runs. In this case, we can see we are logging the value of i to the console.

📚 As we talk through these concepts in class, instructors will screen share code in a repl.it and/or diagram on a tablet. Anything we write/draw during that time will be shared in the cohort Slack channel!

Looping With Arrays

for loops are commonly used to take action on every item in an array. To do this, we can use the initializer and condition to tell the loop to perform the action the same number of times for the number of elements in the array. Typically, we set the initializer at 0, and set the stop condition to the length of the array. The code in the snippet that follows is also available in this repl.it.

var fruits = ['apples', 'oranges', 'bananas'];

for (var i = 0; i < fruits.length; i++) {
  console.log("I have some " + fruits[i]);

The for loop is not magically tied to the fruits array in the example above. It’s important to note that when we want to call a specific fruit, we call the fruits array then use bracket notation with the i variable to access fruits[0], then fruits[1], then fruits[2] - when i holds each of those respective values.

Predict & Discover

First, make a prediction: What would happen if fruits.length was replaced with the Number 8 in the example above?

Now, type out the function above (yes, actually type it! This is a great opportunity for attention to detail and catching your small errors). Replace 8 for fruits.length. Run the code and reflect on your prediction.

Iterator Methods

for loops are not the only way to loop over an array. When pairing with a mentor you might see some of the following methods:

.forEach(), .map(), .find(), .filter(), .reduce()

These are known as iterator methods and they each have their own uses and behaviors. They were all built into the JavaScript language with a for loop!

During your time in Mod 1, we’d like you to only use for loops when you need to iterate over an array. This is not to needlessly make your life harder. The main reason for this request is so that the logic that is applied during each step of the loop must be written explicitly by you, instead of letting the architects of those methods do that work for you. Also, it’s not unlikely that in a different language you work in on the job, you won’t have such handy methods available. Everything in Mod 1 can be completed with your pal for loop! You will get lessons and then extensive practice and experience with the iterator methods in Mods 2-4.

Solo Practice

Join the breakout room associated with the problem you are currently working on. This is meant to mainly be done independently (no one should be screen sharing or “leading” anything), but we want you to have a source of support/place to ask questions if needed.

Mild Heat ⚡️: Annoying Zoo Kid 1

  1. Declare a variable that stores an array of four animals called animals.
  2. Write a for loop that logs "Mommy, I want to see [insert animal name here]! Waaah!"

Medium Heat 🔥: Annoying Zoo Kid 2

  1. Declare a variable that stores an array of four animals called animals.
  2. Declare a function called nameAnimals.
  3. Within your function, create a for loop that logs "Mommy, I want to see [insert animal name here]! Waaah!"
  4. Invoke your function to ensure it is working correctly. Nothing should log to the console if you don’t invoke the function.

Spicy 🔥🔥: User Sentences

Use the following array:

var users = [
    id: 127238,
    firstName: "Matthew",
    handle: "@matthewph",
    lastLoggedIn: 1611937890083,
    followerCount: 723,
    recentPost: "This is the first time I've checked the internet since last March."
  }, {
    id: 728912,
    firstName: "Tanisha",
    handle: "@tdavey",
    lastLoggedIn: 1611937937749,
    followerCount: 2319,
    recentPost: "Wear a mask people, it's not over."
  }, {
    id: 409126,
    firstName: "Megan",
    handle: "@meg36",
    lastLoggedIn: 1611937973534,
    followerCount: 212,
    recentPost: "OMG Gametime AHHHH!!!!!!"
  1. Declare a function called showUserInfo
  2. Within your function, write a for loop
  3. Inside the for loop, console.log a sentence using at least 2 of the values in the objects

Extra Spicy 🔥🔥🔥: Some User Info

Use the array of users above

  1. Declare a function called showImportantPosts that takes one parameter: a number
  2. Check if each user has a follower count above the number that was passed in. If a user has a high enough follower count, console.log their recent post. If a user doesn’t have a high enough follower count, don’t log their recent post
  3. Invoke the function multiple times, with various numbers for the parameter to ensure it’s working as expected

Solo Practice Solutions

  • Solution: Mild Heat ⚡️: Annoying Zoo Kid 1
  • Solution: Medium Heat 🔥: Annoying Zoo Kid 2
  • Solution: Spicy 🔥🔥: User Sentences
  • Solution: Extra Spicy 🔥🔥🔥: Some User Info

Loops and Performance Issues

It’s important to be aware of the potential performance problems that loops can cause. When a browser hits JavaScript, it stops executing anything else on the page until it has processed that script. Since loops can be run on arrays or containers of unknown – and potentially enormous – size, it’s possible for our loop to make a page much, much slower to load.

Additionally, if the condition of your loop never returns false, you will get stuck in what’s known as an infinite loop. This means that your loop will never stop running. Eventually your browser will run out of memory and your script will break.

Here’s an example of an infinite loop. Open a new tab in your browser and run this in your console. Observe what happens.

for (var i = 0; i > -1; i++) {

We can see that this condition will never return false and we’ll be stuck in this loop forever (or at least until our page crashes)! Be mindful of the possibility that you could create infinite loops when leveraging loops in your code. They can happen to the best of us, and knowing what they are is the first step to avoiding and correcting them.

Final Practice

You’ll complete this section with a partner!

Step 1: Pick one of the 4 options linked below as a code challenge. Write a solution and make sure you and your partner both have a full understanding of the solution before moving on.

Step 2: Your next task is to create an educational resource - we can say it’s for the next cohort of students - to understand the concepts behind the code, using the code challenge you just solved as an example. If this is a stretch for you, consider how you best learn and what type of resources are helpful for you!

Some ideas, but don’t feel limited to these:

  • Video
  • Write-up (think blog post)
  • Series of images/diagrams

You will submit your solution to the code challenge and educational resource as your exit ticket in a private Google Form. We’ll ask for volunteers to share out their educational resources when we come back together!

Additional Practice

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