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General Assembly Logo

Control Flow

Control flow is a very important, foundational concept in programming. At it's core, control flow is about responding to data.


  • JavaScript syntax
  • Basic data types (Numbers, Booleans, Strings)
  • Operators (arithmatic, logic, and comparison)
  • Arrays and Objects


By the end of this, developers should be able to:

  • Describe the role of control flow in programming
  • Utilize while loops, for loops, and for/in loops
  • Write conditional statements using if and else if
  • Identify problem types and match to the appropriate control flow statements
  • Iterate over arrays using loops


  1. Fork and clone this repository.
  2. Create a new branch, training, for your work.
  3. Checkout to the training branch.

Introduction (5 min / 0:10)

Control flow is simply applying conditional logic to how your code is run. You may want to run certain blocks of code but not others, or you might want to run certain code blocks over and over again. We probably wouldn't want to show a user account page to a user who isn't logged in, since there would be no data to display!

Types of Control Flow (15 min / 0:25)

We're going to learn about two new forms of control flow; you already know one!

The first form of control flow is: linear.

linear control flow

Source: Eloquent JavaScript, Ch. 02 Program Structure

Linear control flow simply means that our data flows through our code, one line after the next. Execution starts at the first line of code, moves on to the second, then the third, and so on until the end of the file, when execution stops.

The second form of control flow is: conditional.

Conditional control flow

Source: Eloquent JavaScript, Ch. 02 Program Structure

We will often run in to scenarios where we want one block of code to run if, and only if, a particular condition is met. Perhaps we're asking a user for their age and we want to ensure that the value they give us is a number greater than 0. Conditional execution in JavaScript is achieved with the if statement:

if (age >= 16) {
  console.log('You can drive!')

In the above snippet of code, we will only see the message 'you can drive!' if the age variable is 16 or greater. Later, we'll explore else and else if - ways for us to test other conditions or run code specifically when our condition is not met. We'll dive into this pattern more in a few minutes!

The third form of control flow is: looping.

Looping control floow

Looping is extremely powerful and there are two ways to think about it:

  1. Loop until some condition is false
  2. Continue to loop while some condition is true

The distinction may seem minor, but the implications are anything but. In programming (and in JavaScript), we express these with for loops or while loops, respectively.

for loops are great for iterating or counting. Here is an example of a for loops that performs an action 5 times (in this case, prints the string 'hello world'):

for (let i = 0; i < 5; i++) {
  console.log("hello world")

In this snippet of code, the message will continue to print until the condition (i < 5) is not true. We'll dive into this pattern more in a few minutes!

Contrast the above snippet with the following, demonstrating a while loop:

num = 0
while (num <= 5) {
  console.log("hello world")
  num += 1 // short-hand for num = num + 1

This snippet is doing the same thing: printing "hello world" 5 times. The difference is in the condition: while the num variable is less than or equal to 5. We'll dive into this pattern more in a few minutes!

Conditionals (10 min / 0:35)

Conditionals are a common feature of programming languages because it's handy to execute different blocks of code depending on whether a condition(s) is met.

Conditionals follow this pattern:

if (condition) {
  // code here will run if the condition evaluates to true

Review Question: What do 'Truthy' and 'Falsey' mean?

The above is a simple conditional statement, with only a single condition. We can add a negative condition (code that will run if the condition is false) and we can add additional conditions.

If we have code we want to run if our condition is false, we use else:

if (condition) {
  // code here will run if the condition evaluates to true
} else {
  // code here will run if the condition evaluates to false

If we want to test multiple conditions, we can do so by combining else and if:

if (condition) {
  // code here will run if the condition evaluates to true
} else if (anotherCondition) {
  // code here will run if the second condition evaluates to true
} else {
  // code here will run if none of the conditions evaluate to true

We can add as many else/if statements as we want, one for each condition we want to check, but after a while our code becomes hard to read:

let assignmentCompletion = 0

if (assignmentCompletion === 0) {
  console.log("Work on your homework!")
} else if (assignmentCompletion === 1) {
  console.log("Good work, still more to go")
} else if (assignmentCompletion === 2) {
  console.log("Almost there!")
} else {
  console.log("You are done!")

One of the things your instructors will say often is that JavaScript is flexible. We can see an example of that here when discussing conditionls. For instance, if you want to test two conditions without using and else/if you can do so by using the logical operators (&& and ||):

if (condition && condition) {
  // this code will run if both conditions are true

if (condition || condition) {
  // this code will run if one of the conditions is true

This can make your code a little cleaner and easier to read, it can also let you express more complex conditionals. But, use it sparingly! Your logic can quickly become difficult to follow!

Practice Conditionals (15 min / 0:50)

Let's spend some time practicing writing conditionals. Open up this exercise and work through the prompts. Get as far as you can in the time alotted and feel free to come back and review these!

Break (10 min / 1:00)


for Loops (10 min / 1:10)

for loops are extremely powerful, but there are a couple of parts to them that we have to learn before we can see their full power. Let's start with a simple example:

for (let i = 0; i < 5; i++) {

The first thing to notice here is the overall structure. We need the for keywork, followed by the parenthesis and everything inside of them ((let i = 0; i < 10; i++)), and then the block of code (inside the {} braces).

Let's break down what's happening between the parenthesis (()), because there are three parts to this expression:

  1. let i = 0 sets up an iteratee, a variable that we will iterate (increase or decrease by some number) with each iteration of the loop.
  2. i < 5 is the comparison expression - it's actually a conditional! The loop will continue to execute until this expression evaluates to false.
  3. i++ controls how the iteratee should change after each iteration.

These three pieces of the for loop are what make them powerful - we can alter these to iterate in hundreds of different ways. Before we get to that, let's walk through how this loop works, as if we were the JavaScript compiler:

  1. We arrive at the for loop. Create a variable, i, and set it equal to 0.
  2. Check our condition (i < 5). Is it true? Is i less than 5? Yes!
  3. Run the block of code - in this case, print the value of i to the console.
  4. Increment the value of i. In this case, increase it by 1.
  5. Return to step 2 and repeat until our condition is no longer true.

We have a lot of flexibility in how we iterate our loop just based on the three expressions inside our for loop. We can count up or down by changing the third expression, we can also change the iteration by more than just one:

This loop counts up to 100 in increments of 5:

for (let i = 0; i < 100; i += 5) {
  // code goes here...

This loop counts down from 100:

for (let i = 100; i >= 0; i--) {
  // code goes here...

Practice for Loops (15 min / 1:25)

Let's spend some time practicing writing loops. Open up this exercise and work through the prompts. Get as far as you can in the time alotted and feel free to come back and review these!

Iterating over Arrays (10 min / 1:35)

One of the areas where for loops really shine is when working with arrays. We can set the value of i to 0 and use the array's length in the condition to perform some action on every item in the array. That looks like this:

let instructors = ["Don", "Hector", "Jimmy", "Zakk", "Eva"]

for (let i = 0; i < instructors.length; i++) {

Let's unpack what's happening here:

  • We're setting our incrementer to be 0 and our condition is set up to continue looping while i is less than the length of the array (i < instructors.length)
  • Inside our loop, we're using i to retrieve values from the array by their index.

For the first iteration of the loop, i is set to 0 (let i = 0). Since i is 0, we can use it to retrieve the first item in the array, at an index of 0 (instructors[i]). That will continue while i is less than instructors.length (in this case, 5). With each iteration, the value of i will increase by 1, so we'll retrieve the next item in the array!

Practice Iterating over Arrays (15 min / 1:50)

Return to the previous exercise and work through the prompts in Part 2. These prompts will give you a chance to practice iterating over arrays.

Break (10 min / 2:00)

while Loops (10 min / 2:10)

A while loop is like an if statement but it will execute the content of its block repeatedly until the condition becomes false. (i.e. the while loop continues while the condition is true).

num = 0
while (num <= 10) {
  num += 1 // short-hand for num = num + 1

There's a potential gotcha with working with while loops: because the loop continues for as long as the condition is true, you have to break the loop manually. In a for loop, the conditions to break the loop are all encapsulated in the definition of the loops. In a while loop, that logic has to be inside the code block. This makes while loops powerful, a lot of games are built with while loops, for instance; but it also means you have to be careful as you can run into an infinite loop.

Because of this, they're perfect for cases when you need a block of code to run (loop) an unknown number of times:

let passwordGuess = ''
while (passwordGuess !== 'password') {
  passwordGuess = prompt('You have been imprisoned in the code-block of a while loop! What is the magic word to exit?')
alert('Argh! You have escaped! I am so lonely, no one ever wants to stay.')

Practice while Loops (15 min / 2:25)

Return to the previous exercise and work through the prompts in part 3. These prompts will give you the chance to practice writing for loops.

Closing (5 min / 2:30)

You can get really far in programming with the knowledge you have right now: data types, arrays, conditionals, and loops! The biggest challenge beginners have with control flow is expressing their thinking in code and combining the patterns together (like putting a for loop in a while loop or a conditional in a for loop, etc), but don't worry - you'll get lots of practice!

Additional Resources

The best way to really learn control flow is to practice solving problems! Luckily, codewars has a ton of practice problems! Look at their collections on loops and control flow


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