Finding the Most Frequent String in a JavaScript Array

In the following tutorial, I’ll show you how to write a function that takes an array of strings and returns the most frequent string(s) in that array. First I’ll go through the basic concepts and setup, we’ll test it to make sure it works, then I’ll explore a few ways in which you can handle bad data and errors. We’ll also consider some ways to make the outputted data more user friendly. Let’s get started!

We’ll start with a pure function, one that takes an array of strings as input and returns another array as output (the outputted array will ultimately contain the string(s) that occur most frequently in the input). The setup looks like this:

Next we’ll define an object that we’ll fill with data, using the strings in the array as keys and the number of times those strings occur as values. We’ll also add in a mostFreq variable that will be used to keep track of how many times the most frequent string(s) occur(s). The code now looks like this:

Now we’ll get in to the meat of the function, the processing logic. I’ve chosen to use JavaScript’s native forEach method that is an inherent to all arrays. If for some reason you wanted to use a for or while loop, you could do that as well. Here’s what the forEach approach looks like using ECMAScript 6 syntax (don’t worry, we’ll discuss the logic involved right after looking at the code):

The logic is this: the ea variable represents the current array element on each iteration of forEach. The first if statement asks Does the string represented by ea exist as a key in obj? If the string is not the name of a key in obj (i.e., if obj[ea] === undefined evaluates to true), then we declare and initialize in obj the string represented by ea. We initialize it to a value of 1. The value 1 here represents the number of times we’ve encountered the string in arr. On the other hand, If the string is already the name of a key in obj, then we increment that key’s value by 1 (i.e., we add one more to the count of how many times we’ve seen the string in question in arr).

The second if block there is used to compare values of the keys in obj (i.e., the strings in arr) to the current most frequent count (i.e., the value of mostFreq). mostFreq was initialized equal to 0, so if the value of ea in obj is greater than 0 (as it will be as for any string encountered in arr), then mostFreq is updated to the value of obj[ea]. Additionally, the which array gets added to it the current value of ea (i.e., the current string in our iteration through arr). The else if portion of the second if block accounts for the condition where two or more strings in arr share the current value of mostFreq. If two or more strings in arr share the condition of being most frequent in arr, then the latter string(s) encountered is/are pushed to the which array so that they can equitably share the title.

The final step in our function is to return the newly populated which array. The entirety of the code looks like this:

Here it is live with a few tests so that we know it’s working:

See the Pen NRLWdz by Develop Intelligence (@DevelopIntelligenceBoulder) on CodePen.

But let’s say we want to make it a little more user friendly with respect to the output. We can convert the which array to a string by using Array’s toString method. We can then append a sentence (using ES6 template literals) in order to clarify the output. Like this:

Examples of input and output:

Hmm… that second one doesn’t look quite right. We can write in some logic to handle our output sentence’s grammar when there’s more than one most frequent string. I’m going to use Array’s join method in order to help us out here. Here’s the live version:

See the Pen kkRaBd by Develop Intelligence (@DevelopIntelligenceBoulder) on CodePen.

There we go!

Now… for one final consideration… error handling. What if someone passes in some bad data? Or let’s say someone tries to pass in two arrays instead of just one; or perhaps an array that contains an element that isn’t a string. What someone tries to pass in data that isn’t an array at all? Let’s write some logic to handle these erroneous conditions. Live example:

See the Pen YGOzRA by Chase Allen (@ccallen001) on CodePen.

And there we have it! A function that takes an array of strings as input, checks for and handles potential errors, and ultimately returns a user friendly response indicating the most common string(s) in the array. Please feel free to comment and share. Thank you for reading!

Chase Allen
Chase is a self-taught HTML, CSS, and JavaScript enthusiast. He enjoys creating web pages and web applications. His interests include art, music, movies, food, tech, and just general musings.