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Learn how to write automated loops in JavaScript.

Loops

diagram representing the decision to eat based on whether or not you are hungry—conceptually similar to a conditional

A _loop_ is a programming tool that repeats a set of instructions until a specified condition, called a _stopping condition_ is reached. As a programmer, you'll find that you rely on loops all the time! You’ll hear the generic term _iterate_ when referring to loops; iterate simply means "to repeat". 

When we need to reuse a task in our code, we often bundle that action in a function. Similarly, when we see that a process has to repeat multiple times in a row, we write a loop. Loops allow us to create efficient code that automates processes to make scalable, manageable programs. 

As illustrated in the diagram, loops iterate or repeat an action until a specific condition is met. When the condition is met, the loop stops and the computer moves on to the next part of the program. 

In some cases, you want a piece of code to run at least once and then loop based on a specific condition after its initial run. This is where the `do...while`  statement comes in.

A [`do...while`](https://www.codecademy.com/resources/docs/javascript/loops?page_ref=catalog) statement says to do a task once and then keep doing it until a specified condition is no longer met. The syntax for a `do...while` statement looks like this:
```js
let countString = '';
let i = 0;

do {
  countString = countString + i;
  i++;
} while (i < 5);

console.log(countString);
```

In this example, the code block makes changes to the `countString` variable by appending the string form of the `i` variable to it. First, the code block after the `do` keyword is executed once. Then the condition is evaluated. If the condition evaluates to `true`, the block will execute again. The looping stops when the condition evaluates to `false`.

Note that the `while` and `do...while` loop are different! Unlike the `while` loop, `do...while` will run at least once whether or not the condition evaluates to `true`.
```js
const firstMessage = 'I will print!';
const secondMessage = 'I will not print!'; 

// A do while with a stopping condition that evaluates to false
do {
 console.log(firstMessage)
} while (true === false);

// A while loop with a stopping condition that evaluates to false
while (true === false){
  console.log(secondMessage)
};

```
**Note:** To exit out of an infinite loop in an exercise, **refresh the page** - then fix the code for your loop(s).

Do...While Statements

When we have a loop running inside another loop, we call that a _nested loop_. One use for a nested `for` loop is to compare the elements in two arrays. For each round of the outer `for` loop, the inner `for` loop will run completely.

Let’s look at an example of a nested `for` loop:
```js
const myArray = [6, 19, 20];
const yourArray = [19, 81, 2];
for (let i = 0; i < myArray.length; i++) {
  for (let j = 0; j < yourArray.length; j++) {
    if (myArray[i] === yourArray[j]) {
      console.log('Both arrays have the number: ' + yourArray[j]);
    }
  }
}
```
Let's think about what's happening in the nested loop in our example. For each element in the outer loop array, `myArray`, the inner loop will run in its entirety comparing the current element from the outer array, `myArray[i]`, to each element in the inner array, `yourArray[j]`. When it finds a match, it prints a string to the console. 

Now it's your turn to write a nested loop!

**Note:** To exit out of an infinite loop in an exercise, **refresh the page** - then fix the code for your loop(s).

Nested Loops

Imagine we're looking to adopt a dog. We plan to go to the shelter every day for a year and then give up. But what if we meet our dream dog on day 65? We don't want to keep going to the shelter for the next 300 days just because our original plan was to go for a whole year. In our code, when we want to stop a loop from continuing to execute even though the original stopping condition we wrote for our loop hasn't been met, we can use the keyword `break`. 

 The [`break` keyword](https://www.codecademy.com/resources/docs/javascript/loops?page_ref=catalog) allows programs to “break” out of the loop from within the loop's block.

Let’s check out the syntax of a `break` keyword:
```js
for (let i = 0; i < 99; i++) {
  if (i > 2 ) {
     break;
  }
  console.log('Banana.');
}

console.log('Orange you glad I broke out the loop!');
```

This is the output for the above code:
```
Banana.
Banana.
Banana.
Orange you glad I broke out the loop!
```
`break` statements can be especially helpful when we’re looping through large data structures! With breaks, we can add test conditions besides the stopping condition, and exit the loop when they're met.

The break Keyword

Great job! In this lesson, we learned how to write cleaner code with loops. You now know:
- Loops perform repetitive actions so we don’t have to code that process manually every time. 
- How to write `for` loops with an iterator variable that increments or decrements
- How to use a `for` loop to iterate through an array
- A nested `for` loop is a loop inside another loop
- `while` loops allow for different types of stopping conditions
-  Stopping conditions are crucial for avoiding infinite loops.
- `do...while` loops run code at least once&mdash; only checking the stopping condition after the first execution
- The `break` keyword allows programs to leave a loop during the execution of its block

Review

Before we write our own loops let's take a moment to develop an appreciation for loops. The best way to do that is by showing you how cumbersome it would be if a repeated task required you to type out the same code every single time.

Repeating Tasks Manually

Instead of writing out the same code over and over, loops allow us to tell computers to repeat a given block of code on its own. One way to give computers these instructions is with a [`for` loop](https://www.codecademy.com/resources/docs/javascript/loops?page_ref=catalog). 

The typical `for` loop includes an _iterator variable_ that usually appears in all three expressions. The iterator variable is initialized, checked against the stopping condition, and assigned a new value on each loop iteration. Iterator variables can have any name, but it’s best practice to use a descriptive variable name.

A `for` loop contains three expressions separated by `;` inside the parentheses: 
1. an _initialization_ starts the loop and can also be used to declare the iterator variable.
2. a _stopping condition_ is the condition that the iterator variable is evaluated against&mdash; if the condition evaluates to `true` the code block will run, and if it evaluates to `false` the code will stop.
3. an _iteration statement_ is used to update the iterator variable on each loop.

The `for` loop syntax looks like this:
```js
for (let counter = 0; counter < 4; counter++) {
  console.log(counter);
}
```

In this example, the output would be the following:
```
0
1
2
3
```

Let’s break down the example:
* The initialization is `let counter = 0`, so the loop will start counting at `0`.
* The stopping condition is `counter < 4`, meaning the loop will run as long as the iterator variable, `counter`, is less than 4.
* The iteration statement is `counter++`. This means after each loop, the value of `counter` will increase by 1. For the first iteration `counter` will equal `0`, for the second iteration `counter` will equal 1, and so on.
* The code block is inside of the curly braces, `console.log(counter)`, will execute until the condition evaluates to `false`. The condition will be false when `counter` is greater than or equal to 4 &mdash; the point that the condition becomes false is sometimes called the _stop condition_.

This `for` loop makes it possible to write `0`, `1`, `2`, and `3` programmatically.

The For Loop

What if we want the `for` loop to log `3`, `2`, `1`, and then `0`? With simple modifications to the expressions, we can make our loop run backward!

To run a [backward `for` loop](https://www.codecademy.com/resources/docs/javascript/loops?page_ref=catalog), we must:
* Set the iterator variable to the highest desired value in the initialization expression.
* Set the stopping condition for when the iterator variable is less than the desired amount.
* The iterator should decrease in intervals after each iteration.

We'll practice by changing the `for` we wrote previously to now go in reverse. When writing/changing loops, there is a chance that our stopping condition isn't met and we get a dreaded _infinite loop_ which essentially stops our programming from running anything else! To exit out of an infinite loop in an exercise, refresh the page - then fix the code for your loop.

Looping in Reverse

`for` loops are very handy for iterating over data structures. For example, we can use a `for` loop to perform the same operation on each element on an array. [Arrays](https://www.codecademy.com/resources/docs/javascript/arrays?page_ref=catalog) hold lists of data, like customer names or product information. Imagine we owned a store and wanted to increase the price of every product in our catalog. That could be a lot of repeating code, but by using a `for` loop to iterate through the array we could accomplish this task easily. 

To loop through each element in an array, a `for` loop should use the array's `.length` property in its condition. 

Check out the example below to see how `for` loops iterate on arrays:
```js
const animals = ['Grizzly Bear', 'Sloth', 'Sea Lion'];
for (let i = 0; i < animals.length; i++){
  console.log(animals[i]);
}
```

This example would give you the following output:
```
Grizzly Bear
Sloth
Sea Lion
```
In the loop above, we've named our iterator variable `i`. This is a variable naming convention you'll see in a lot of loops. When we use `i` to iterate through arrays we can think of it as being short-hand for the word **i**ndex. Notice how our stopping condition checks that `i` is less than `animals.length`. Remember that arrays are zero-indexed, the index of the last element of an array is equivalent to the length of that array minus 1. If we tried to access an element at the index of `animals.length` we will have gone too far!

With `for` loops, it’s easier for us to work with elements in arrays.

Looping through Arrays

You’re doing great! We're going to teach you about a different type of loop: the [`while` loop](https://www.codecademy.com/resources/docs/javascript/loops?page_ref=catalog). To start, let's convert a `for` loop into a `while` loop:
```js
// A for loop that prints 1, 2, and 3
for (let counterOne = 1; counterOne < 4; counterOne++){
  console.log(counterOne);
}

// A while loop that prints 1, 2, and 3
let counterTwo = 1;
while (counterTwo < 4) {
  console.log(counterTwo);
  counterTwo++;
}
```
Let's break down what's happening with our `while` loop syntax: 
- The `counterTwo` variable is declared before the loop. We can access it inside our `while` loop since it's in the global scope. 
- We start our loop with the keyword `while` followed by our stopping condition, or _test condition_. This will be evaluated before each round of the loop. While the condition evaluates to `true`, the block will continue to run. Once it evaluates to `false` the loop will stop. 
- Next, we have our loop's code block which prints `counterTwo` to the console and increments `counterTwo`. 

What would happen if we didn't increment `counterTwo` inside our block? If we didn't include this, `counterTwo` would always have its initial value, `1`. That would mean the testing condition `counterTwo < 4` would always evaluate to `true` and our loop would never stop running! Remember, this is called an _infinite loop_ and it's something we always want to **avoid**. Infinite loops can take up all of your computer's processing power potentially freezing your computer. 

So you may be wondering when to use a `while` loop! The syntax of a `while` loop is ideal when we don’t know in advance how many times the loop should run. Think of eating like a `while` loop: when you start taking bites, you don't know the exact number you'll need to become full. Rather you'll eat `while` you're hungry. In situations when we want a loop to execute an undetermined number of times, `while` loops are the best choice.