JavaScript Strings

Write a JavaScript program to reverse a string

Description: Reverse the string “JavaScript”.

Hint:

• Split the string into an array of characters, reverse it and join it back.

JavaScript program to reverse a string

const text = "JavaScript";

// 1. split, reverse, join
console.log(text.split("").reverse().join(""));

// 2. loop
let result = "";
for (const ch of text) {
  result = ch + result;
}
console.log(result);

Output:

How it works

• Strings cannot be reversed directly, because they have no reverse() method, but arrays do.
• In the loop each new character is added to the front of the result.

Read JavaScript Strings.

Write a JavaScript program to check palindrome string

Description: Check if the words “madam”, “Level” and “hello” are palindromes.

Hint:

• A palindrome reads the same backwards.
• Change the text to lowercase first.

JavaScript palindrome program

function isPalindrome(text) {
  const clean = text.toLowerCase();
  return clean === clean.split("").reverse().join("");
}

for (const w of ["madam", "Level", "hello"]) {
  console.log(w + (isPalindrome(w) ? " is a palindrome" : " is not a palindrome"));
}

Output:

How it works

• toLowerCase() makes “Level” equal to “level”.
• The text is compared with its reverse.

Write a JavaScript program to count vowels in a string

Description: Count the vowels in “Tutorials Class”.

Hint:

• Vowels are a, e, i, o and u.
• match() with the g flag returns all matches, or null if none.

JavaScript program to count vowels

const text = "Tutorials Class";
const matches = text.toLowerCase().match(/[aeiou]/g);

console.log("Vowels:", matches ? matches.length : 0);

Output:

How it works

• /[aeiou]/g finds every vowel.
• If there is no vowel, match() returns null, so we check it first.

Read Regular Expressions.

Write a JavaScript program to count the frequency of words in a sentence

Description: Count each word of the sentence “to be or not to be”.

Hint:

• Split the sentence into words.
• Use an object where the word is the key and the count is the value.

JavaScript word frequency program

const sentence = "to be or not to be";
const counts = {};

for (const word of sentence.split(" ")) {
  counts[word] = (counts[word] || 0) + 1;
}

console.log(counts);

Output:

How it works

• counts[word] || 0 gives 0 for a new word, and then 1 is added.
• Objects keep the order in which keys were added.