Two Ways to Check for Palindromes in JavaScript

This article is based on Free Code Camp Basic Algorithm Scripting “Check for Palindromes”.

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. The word “palindrome” was first coined by the English playwright Ben Jonson in the 17th century, from the Greek roots palin (“again”) and dromos (“way, direction”). — src. Wikipedia

In this article, I’m going to explain two approaches, first with built-in functions and second using a for loop.

Algorithm Challenge

Return true if the given string is a palindrome. Otherwise, return false.

A palindrome is a word or sentence that’s spelled the same way both forward and backward, ignoring punctuation, case, and spacing.

Note. You’ll need to remove all non-alphanumeric characters (punctuation, spaces and symbols) and turn everything lower case in order to check for palindromes.

We’ll pass strings with varying formats, such as “racecar”, “RaceCar”, and “race CAR” among others.

Provided test cases

palindrome(“race car”) should return true
palindrome(“not a palindrome”) should return false
palindrome(“A man, a plan, a canal. Panama”) should return true
palindrome(“never odd or even”) should return true
palindrome(“nope”) should return false
palindrome(“almostomla”) should return false
palindrome(“My age is 0, 0 si ega ym.”) should return true
palindrome(“1 eye for of 1 eye.”) should return false
palindrome(“0_0 (: /-\ :) 0–0”) should return true

Which Regular Expression will we need to pass the last test case?

Regular expressions are patterns used to match character combinations in strings.

When the search for a match requires something more than a direct match, the pattern includes special characters.

To pass the last test case, we can use two Regular Expressions:

/[^A-Za-z0–9]/g  or

/[\W_]/g

\W removes all non-alphanumeric characters:

\W matches any non-word character
\W is equivalent to [^A-Za-z0–9_]
\W matches anything that is not enclosed in the brackets

What does that mean?

[^A-Z] matches anything that is not enclosed between A and Z

[^a-z] matches anything that is not enclosed between a and z

[^0-9] matches anything that is not enclosed between 0 and 9

[^_] matches anything that does not enclose _

But in our test case, we need palindrome(“0_0 (: /-\ :) 0–0”) to return true, which means “_(: /-\ :)–” has to be matched.

We will need to add “_” to pass this specific test case.

We now have “\W_”

We will also need to add the g flag for global search.

We finally have “/[\W_]/g”

/[\W_]/g was used for pure demonstrative purpose to show how RegExp works. /[^A-Za-z0–9]/g is the easiest RegExp to choose.

1. Check for Palindromes With Built-In Functions

For this solution, we will use several methods:

The toLowerCase() method to return the calling string value converted to lowercase.
The replace() method to return a new string with some or all matches of a pattern replaced by a replacement. We will use one of the RegExp we just created earlier.
The split() method splits a String object into an array of strings by separating the string into sub strings.
The reverse() method reverses an array in place. The first array element becomes the last and the last becomes the first.
The join() method joins all elements of an array into a string.

Without comments:

2. Check for Palindromes With a FOR loop

Half-indexing (len/2) has benefits when processing large strings. We check the end from each part and divide the number of iterations inside the FOR loop by two.

Without comments:

“How to Solve FCC Algorithms” is a series of articles on the Free Code Camp Algorithm Challenges where I try to propose several solutions and explain step by step what happens behind the hood.

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