freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/declare-a-read-only-variable-with-the-const-keyword.md

---
id: 587d7b87367417b2b2512b41
title: Declare a Read-Only Variable with the const Keyword
challengeType: 1
forumTopicId: 301201
---

## Description
<section id='description'>
The keyword <code>let</code> is not the only new way to declare variables. In ES6, you can also declare variables using the <code>const</code> keyword.
<code>const</code> has all the awesome features that <code>let</code> has, with the added bonus that variables declared using <code>const</code> are read-only. They are a constant value, which means that once a variable is assigned with <code>const</code>, it cannot be reassigned.

```js
const FAV_PET = "Cats";
FAV_PET = "Dogs"; // returns error
```

As you can see, trying to reassign a variable declared with <code>const</code> will throw an error. You should always name variables you don't want to reassign using the <code>const</code> keyword. This helps when you accidentally attempt to reassign a variable that is meant to stay constant. A common practice when naming constants is to use all uppercase letters, with words separated by an underscore.
  
  <strong>Note:</strong> It is common for developers to use uppercase variable identifiers for immutable values and lowercase or camelCase for mutable values (objects and arrays). In a later challenge you will see an example of a lowercase variable identifier being used for an array.
</section>

## Instructions
<section id='instructions'>
Change the code so that all variables are declared using <code>let</code> or <code>const</code>. Use <code>let</code> when you want the variable to change, and <code>const</code> when you want the variable to remain constant. Also, rename variables declared with <code>const</code> to conform to common practices, meaning constants should be in all caps.
</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>var</code> should not exist in your code.
    testString: getUserInput => assert(!getUserInput('index').match(/var/g));
  - text: <code>SENTENCE</code> should be a constant variable declared with <code>const</code>.
    testString: getUserInput => assert(getUserInput('index').match(/(const SENTENCE)/g));
  - text: <code>i</code> should be declared with <code>let</code>.
    testString: getUserInput => assert(getUserInput('index').match(/(let i)/g));
  - text: <code>console.log</code> should be changed to print the <code>SENTENCE</code> variable.
    testString: getUserInput => assert(getUserInput('index').match(/console\.log\(\s*SENTENCE\s*\)\s*;?/g));

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function printManyTimes(str) {

  // Only change code below this line

  var sentence = str + " is cool!";
  for (var i = 0; i < str.length; i+=2) {
    console.log(sentence);
  }

  // Only change code above this line

}
printManyTimes("freeCodeCamp");
```

</div>



</section>

## Solution
<section id='solution'>

```js
function printManyTimes(str) {

  const SENTENCE = str + " is cool!";
  for (let i = 0; i < str.length; i+=2) {
    console.log(SENTENCE);
  }

}
printManyTimes("freeCodeCamp");
```

</section>