 # Working with flags in JavaScript

Now we will learn how to work with flags. The flag is a special variable that can only take two values: `true` and `false`. Using flags, you can solve problems that check for the absence of something: for example, you can check that there is no element with a certain value in an array. Let's get started.

Let's solve the following problem: given an array of numbers, you need to check if it has an element with the value `3` or not. If there is, we print `'+++'`, if not, we print `'---'`.

First, let's try to print `'+++'`. To do this, let's go through all the elements of our array and with if we ask if the current element is equal to the value `3`. If equal - we'll output `'+++'`:

```let arr = [1, 2, 3, 4, 5]; for (let elem of arr) { if (elem == 3) { console.log('+++'); } }```

But our solution is not very good: after all, if the array contains more than one value `3`, but several, then `'+++'` will be displayed several times. Let's remake our array (make two elements with value 3) and make sure of it:

```let arr = [1, 2, 3, 4, 3, 5]; for (let elem of arr) { if (elem == 3) { console.log('+++'); // outputs several times } }```

Let's fix the problem: end the loop with `break` if the element is already found:

```let arr = [1, 2, 3, 4, 3, 5]; for (let elem of arr) { if (elem == 3) { console.log('+++'); break; // terminates the loop } }```

Let's now try to make it so that if there are no elements with the value `3` in the array at all, `'---'` is displayed. A common mistake is to add `else` to our if - in this case, `'---'` will be displayed for all elements that are not `3`:

```let arr = [1, 2, 3, 4, 5]; for (let elem of arr) { if (elem == 3) { console.log('+++'); // outputs for element 3 } else { console.log('---'); // outputs for elements 1, 2, 4, 5 } }```

So the idea of adding `else` is a bad idea, not a working one. To solve problems of this type (such problems are quite common) they use the so-called flags.

As mentioned above, a flag is a variable that can take two values: `true` or `false`.

So, let's make the variable `flag` with the following value: if it is equal to `true`, then there is an element `3` in the array, and if ```false⁅ /c⁆, then there is no such element. ```

``` Initially, we set the variable flag to the value false - that is, we will assume that there is no element 3 in the array: let arr = [1, 2, 3, 4, 5]; let flag = false; // consider that element 3 is not in the array Then we'll run the loop with if just like we did before. If the loop detects that the array has element 3, then set the variable flag to true and exit the loop with break: let arr = [1, 2, 3, 4, 5]; let flag = false; // consider that element 3 is not in the array for (let elem of arr) { if (elem == 3) { flag = true; // the element exists - redefine the flag variable break; // get out of the loop } } The answer to the question, if the array has 3 or not, we can only give after the loop. And we already have this answer: after the loop, the variable flag could remain false or could change its value to true if the loop found 3 in the array: let arr = [1, 2, 3, 4, 5]; let flag = false; for (let elem of arr) { if (elem == 3) { flag = true; break; } } // here the flag variable is either true or false Now, after the loop, we can make an if that looks at the variable flag and displays '+++' or '---': let arr = [1, 2, 3, 4, 5]; let flag = false; for (let elem of arr) { if (elem == 3) { flag = true; break; } } if (flag === true) { console.log('+++'); } else { console.log('---'); } Given an array: let arr = ['a', 'b', 'c', 'd', 'e']; Check that this array contains the element 'c'. If yes, print '+++', and if not, print '---'. Write code to check if a number is prime or not. A prime number is only divisible by 1 and itself, and is not divisible by other numbers. ```
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