JavaScript array filterWith the Array filter() method, you provide a function that returns an array containing the “filtered” elements. It is within this function that you provide the logic that determines how your original array is filtered.

Sometimes you have an array, but want to reduce the number of elements you work with, based on a certain logic. For example, you may have an array with the days of the week, but you only want to work with the days that start with the letter “T”, which means that you only want the elements “Tuesday” and “Thursday”. Well, JavaScript’s Array filter() method provides a simple and elegant interface by which you can solve this problem. In addition to the straightforward syntax, the Array filter() method eliminates the need for boilerplate code such as a for-loop. To further explain, we’ve demonstrated this in the code examples that follow.

Using a for-loop to filter an Array -Example # 1

See the Pen Array.prototype.filter- Challenge by Kevin Chisholm (@kevinchisholm) on CodePen.

In Example # 1, we use a “while” loop to iterate the word’s array. “While” loops are perfectly valid in JavaScript, but they always make me nervous, because whenever I’ve started to use a “while” loop, I immediately ask myself: “is there a better way to do this?” The main reason is that with a “while” loop, if you’re not careful and your “while” logic is faulty, the loop could theoretically run forever. This is a bit off-topic, but I just like to point out that it’s important to use “while” loops with care.

Back to Example # 1. Inside of the “while” loop, we use the value of “i” to determine if the current word should be pushed to the “shorter” array or the “longer” array. Once this loop has completed, we update the UI to show the number of “shorter” and “longer” words. Again, perfectly valid code, but kind of verbose and less than optimal.

Using the Array filter() method -Example # 2

See the Pen Array.prototype.filter – Solution # 1 by Kevin Chisholm (@kevinchisholm) on CodePen.

Example # 2 is a major improvement over the previous code. We leverage the Array filter() method to determine the number of “shorter” and “longer” words. There are a couple of areas of improvement here.

First, we’ve removed the need to manually push words to the “shorter” and “longer” arrays, because of the simple syntax of the Array filter() method. Notice that as we create the “shorter” and “longer” arrays, we assign the value by executing the Array filter() method. We provide an anonymous function to the Array filter() method, and the logic is simple: the sole argument passed to that anonymous function is the iterated word, and inside of the function we simply determine if the word’s length is greater than or less than five.

Second, we’ve removed the “while” loop. This is a major improvement, given the fact that it reduces the amount of code, but also since there is no longer any possibility that our code will run forever because of the use of a “while” loop.

Passing a function to the Array filter() method – Example # 3

See the Pen Array.prototype.filter – Solution # 2 by Kevin Chisholm (@kevinchisholm) on CodePen.

The winsPrize() function approach of Example # 3 differs a bit. Instead of using an anonymous function when executing the Array filter() method, we pass a function declaration. Also, we’ve chained the Array filter() method off of an array literal (in other words: we create the array and then immediately execute the filter method). This is not necessarily a “more correct” approach, it’s just less verbose. The logic of the winsPrize() function is similar to the code in Example # 2. The main difference is that we pass a function declaration instead of an anonymous function.