What It Does

Javascript’s array map method returns a transformed array. You pass it a function that will transform each value in the array. Map will call your function with each item in the array, passing three values: the array item, the index of that item, and the entire array. Whatever your function returns will become the corresponding item in the new array.

An Example

The best way to understand what map does is through an example. Here’s a function that will transform a title with a leading article (“a,” “an,” or “the”). It returns the same title with the article moved to the end. If you have a list of titles and want to alphabetize them, you want to make sure those leading articles don’t influence the order. You wouldn’t want all the titles starting with “The” sorted with the ‘t’s.

    function formatTitle(title) {
      const articles = ['a', 'an', 'the'];
      let titleWords = title.split(' ');
      let firstWord = titleWords.shift();
      if (articles.includes(firstWord.toLowerCase())) {
        let formattedTitle = `${titleWords.join(' ')}, ${firstWord}`;
        return formattedTitle;
      } else {
        return title;
      }
    }

Breaking Down the Callback Function

Let’s break this function down. The function takes one argument: a title. I know map will pass two more arguments besides the title, but I won’t need those. I only need to name parameters I’m actually going to use in my function. The others will still be passed but won’t have parameter names associated with them.

On line 2, I’ve created an array of the articles that I’ll look for. Later, I’m going to take the first word of the title and see if it appears in this array. That tells me if this title needs to be formatted or not.

Line 3 takes the title and calls the split method passing in a space. split is a string method that breaks a string into an array. The string you pass to split is the string the original string gets split on. This one will get split apart at each space character into an array of the words in the string.

Line 4 uses shift which removes the first value from the array and returns it. That first word is now stored in the firstWord variable, and the titleWords array no longer contains the first word.

On Line 5, I test to see if the articles array contains a lower-case version of firstWord by passing that to the includes method. Every title should start with an uppercase letter, but, by lower-casing the word before comparing, I don’t have to worry if the user of this application is actually doing the right thing and capitalizing their title. The function will just work regardless.

If the first word was found in the articles array, I’m going to build the formatted string. I’m doing this with a template string that starts by re-joining the remaining words in the title using the array’s join method. Passing a string with nothing but a space means that’s what will be placed between the strings in the array when they’re re-joined. Remember, this array doesn’t contain the first word, which we’ve now determined is an article.

I place a comma after that followed by the first word. Then, I drop in the leading article (firstWord) at the end and return that value. If the first word wasn’t an article (the else case), I just return the original title instead.

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Calling Map

To use that callback, I’ll call map on the original array and pass in the formatTitle function. This will return a transformed array with formatted titles instead of the originals. Since it’s returning a new array instead of changing the original array, I’ll probably want to capture it with a variable assignment or pass it straight into another function or method. Here’s what everything looks like assembled into a working application.

A Working Demo

This demo uses the exact callback function shown above. It’s a simple Vue app that displays the top 10 movies on IMDB, sorted alphabetically. In the first list, they’re sorted by title without any modification. In the second list, I’m sorting a new array generated by passing formatTitle into map.

When to Use Map

Maybe you need to format a string in a particular way like the demo above or maybe you need to perform a calculation on each value in an array of numbers. In either case, map is the best tool for the job. Use map any time you need a new array in which each value is a transformation of a value in an existing array.

If you like this article, you might also enjoy my articles on learning to use the Javascript ternary operator and the array’s reduce method.