Making Sense of the JavaScript Map Array Method

This is the first article of a series about the map, filter, and reduce array methods to clear up the confusion on how these methods work.

Omg my brain--dude just lost me with map and filter

Yeah, I know the feeling. The first time I saw some code using map, filter, and reduce, I had no idea how they worked. Hours of Youtube and about 200 Chrome tabs later, and I finally figured them out. This post aims to save you from the same struggle by diving deep into the map array method.

What you should know first

Before you try to understand the array methods, there are a few concepts you should at least be familiar with first:

  • What is an array
  • What is an arrow function
  • Passing functions to other functions (think setInterval and setTimeout)

If you're not at least familiar with these topics, bookmark this tab and look into those first. I plan to write some posts on them too, but for now you'll have to open up tab 201 and head back to Google.

If Google isn't working and you don't have time to spend all day learning them on your own, you can also book a session with me. If you feel ready, let's get into it!

Array methods make common tasks simpler

You may not realize this, but you can do everything that map does with only a for loop. Everything. In fact, you can replicate almost all of the array methods with just a for loop, .push, and some if-else statements. So why use the array methods at all?

The main value you get from using array methods like map is they allow you to do things you would already do with simpler, cleaner syntax. To demonstrate this, l'll show you an example using for and forEach loops.

Let's say you have an array of ages and you want to calculate if that age would make you middle-aged or not. According to Wikipedia, a person is middle-aged "from about 45 to 65" years old. Let's use some code to represent that.

function isMiddleAged(age) {
return age >= 45 && age <= 65;
}
isMiddleAged(50); // returns true
isMiddleAged(21); // returns false

Here we just created a simple function that returns true if the age is between 45 and 65 and false otherwise. At the bottom we called isMiddleAged just to test it out. So far, so good.

But now, we want to calculate this for an array of ages. If "use a for loop" is screaming at you as a solution, then your intuition is right! Let's use one.

function isMiddleAged(age) {
return age >= 45 && age <= 65;
}
// create some random ages
const ages = [21, 54, 89, 23, 61, 48, 49];
for (let i = 0; i < ages.length; i++) {
// pull out the age from the array
const age = ages[i];
// print out true or false
console.log(isMiddleAged(age));
}

Here we used the standard for loop to print out if each age is considered middle aged or not, but we could have used the forEach array method to accomplish the same goal.

function isMiddleAged(age) {
return age >= 45 && age <= 65;
}
// the same ages as before
const ages = [21, 54, 89, 23, 61, 48, 49];
// printing if each age isMiddleAged
ages.forEach((age) => {
console.log(isMiddleAged(age));
});

If this is confusing, copy the code, open up the console or your code editor, and run both examples. You will see that both the normal for loop and the forEach loop both give the same output.

If you can use both a for loop and a forEach loop to output the values, why should you use one or the other?

There are a few reasons, but in this case 3 stand out:

  • Here we just want to do something "for each" value, so the forEach makes that intent more clear
  • We are only using i as an index to get each value from the array, so it is not necessary
  • You get to write less code

There are multiple other ways to loop like with the for..of or the for..in loops that could have also been used here. I used the forEach method because it keeps the code simple and clean.

Makes sense...but what does this have to do with map and filter?

The map, filter, and reduce functions simplify other common array operations the same way forEach did in this example.

A world without map, filter, and reduce

Let's use a few more examples with normal for loops to really show the value of each function.

For the first example, let's say you have an array of containing the number of years a student has been in college. The goal is to convert them into a "classification" based on the number of years.

For example, if the number of years is "1", then the classification would be "freshman." If it's "3" than the classification is "junior." Here is a function that captures all that logic:

function yearsToClassification(years) {
if (years === 1) {
return "freshman";
} else if (years === 2) {
return "sophomore";
} else if (years === 3) {
return "junior";
} else if (years === 4) {
return "senior";
} else {
return "no classification";
}
}

This works, but we could make the code a lot cleaner with the switch/case syntax.

function yearsToClassification(years) {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
}
let classification = yearsToClassification(1);
console.log(classification); // prints "freshman"
classification = yearsToClassification(5);
console.log(classification); // prints "no classification"

Both examples do the same thing, so you can decide which function is more readable and use that for this next part. Like before, let's iterate over each value, but this time let's create a new array containing all the classifications.

With a for loop that would look something like:

function yearsToClassification(years) {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
}
const years = [1, 2, 3, 2, 9, 1, 4];
const classifications = [];
for (let i = 0; i < years.length; i++) {
// get the current year
const year = years[i];
// convert the year to a classification
const classification = yearsToClassification(year);
// add the classification to the end of the
// classifications array
classifications.push(classification);
}
// print out a nicely formatted table of all
// the classifications
console.table(classifications);

Job done! It did not take too much code, but what if I told you that you could replace all of the for loop logic with one line of code?

bro...you're lying

Talk is cheap, let's look at the code.

function yearsToClassification(years) {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
}
const years = [1, 2, 3, 2, 9, 1, 4];
const classifications = years.map(yearsToClassification);
console.table(classifications);

Still skeptical? Run it and see for yourself.

Show me your ways, sensei.

Breaking down the Array.map method

So what is map anyway? Similarly to the forEach method, the name tells you what it does. We are "mapping" each value in the array to another value. The function that gets passed to the map method determines what each value will turn into.

In this case each year in the array is mapped to a classification. Once all the mapping is done, a new array is returned containing all the new values. That's why we created a new variable called classifications on the same line as years.map.

This can be very confusing the first time you see it, so let's look at it again a different way.

function yearsToClassification(years) {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
}
const years = [1, 2, 3, 2, 9, 1, 4];
// This line of code...
const classifications1 = years.map(yearsToClassifcation);
// does the same thing as this...
const classifications2 = years.map((year) => yearsToClassifcation(year));
// ...which does the same thing as these...
const classifications3 = years.map((year) => {
return yearsToClassification(year);
});
// ...and these lines too.
const classifications4 = years.map((year) => {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
});
// These all print the same thing
console.table(classifications1);
console.table(classifications2);
console.table(classifications3);
console.table(classifications4);

No matter how you write it, map is just taking the current year and converting it to a classification. When it's done, it returns an array of all the classifications.

Here is a simpler example where we create a new array with all the years doubled.

const years = [1, 2, 3, 2, 9, 1, 4];
const doubledYears = years.map((year) => year * 2);
console.table(doubledYears);

The function that gets run for each value in years is year => year * 2. Here is another way to do the exact same thing:

const double = (num) => num * 2;
const years = [1, 2, 3, 2, 9, 1, 4];
const doubledYears = years.map(double);
console.table(doubledYears);

The only difference in the second case is that we created a function called double and passed it to years.map directly. Each year is still getting mapped to the doubled year like before.

To really dial it in, here's another example where we double the year and then convert it to the classification:

function yearsToClassification(years) {
switch (years) {
case 1:
return "freshman";
case 2:
return "sophomore";
case 3:
return "junior";
case 4:
return "senior";
default:
return "no classification";
}
}
function double(num) {
return num * 2;
}
const years = [1, 2, 3, 2, 9, 1, 4];
const classificationsAfterDoubling = years.map((year) => {
// double the year
const doubledYear = double(year);
// get the classification
const classification = yearsToClassification(doubledYear);
// return it so it can be added to the new array
return classification;
});
console.table(classificationsAfterDoubling);

Wrapping up

There are a few key takeaways from this article:

  • The map method maps each value of an array to a new value and returns a new array
  • There is more than one way to solve a problem
  • Array methods can help make your code simpler and more readable

This article primarily focuses on the map array method, but I will be writing more about the filter and reduce next! If you'd like to get updates when they come up, then be sure to sign up for the newsletter!

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