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

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

Description

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

"use strict";
const FAV_PET = "Cats";
FAV_PET = "Dogs"; // returns error

As you can see, trying to reassign a variable declared with const will throw an error. You should always name variables you don't want to reassign using the const 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.

Note: 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.

Instructions

Change the code so that all variables are declared using let or const. Use let when you want the variable to change, and const when you want the variable to remain constant. Also, rename variables declared with const to conform to common practices, meaning constants should be in all caps.

Tests

tests:
  - text: <code>var</code> does 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));

Challenge Seed

function printManyTimes(str) {
  "use strict";

  // change code below this line

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

  // change code above this line

}
printManyTimes("freeCodeCamp");

Solution

function printManyTimes(str) {
  "use strict";

  // change code below this line

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

  // change code above this line

}
printManyTimes("freeCodeCamp");