Data Wrangling in JS: 01 Basics

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A basic dataset

Let’s get some terminology out of the way before we start to work with the real data.

Look at this simple table, call it a spreadsheet if you like:


It tells you something about 3 penguins. You’ll learn more about these penguins and where they live when we dig into the real data set (in the next post). Spoiler alert: they were observed by researchers in the Palmer Archipelago, Antarctica.

In the table above, we only have 3 rows, each with a row number and two more columns that tell us something about the penguins’ species and their home island. This is the typical spreadsheet-like representation that most of us have in mind, esp. when coming from a background of data.frame and tibble in R or pd.DataFrame in Python.

Variables, values, columns, observations

I will refer to a whole column of a table as a variable, such as species. Within each column, there are lots of values, such as Adelie. Values are often also referred to as observations.

But what does the typical JavaScript representation look like?

Here are the lines I wrote to define the data in the table above:

let somePenguins = [
    {row: 1, species: "Adelie", island: "Torgersen"},
    {row: 2, species: "Adelie", island: "Biscoe"},
    {row: 3, species: "Chinstrap", island: "Torgersen"},

Notice how it is basically the same as in the table, but more verbose.

Why do I say verbose?

If you compare the table and the code block, you’ll notice that the row numbers run from 1 to 3 and they’re all visually aligned in a column. The difference is, that in the table, the column name row is not repeated. In the code block however, we have the more verbose version that states row each time. But if you focus on the values (e.g. 1, 2, 3), you’ll see they’re right there and they even look like they’re in a column! Same for the other two variables: if you focus on the data entries highlighted in green in the code block, you see the same as in the familiar spreadsheet-style table.

But let’s dig a little deeper into this simple code block. Technically speaking somePenguins is an array of objects. This is important, because it tells us what kind of functions/methods we can use to work with this data.

If you’re new to JavaScript and you’re not sure what an array and an object is… Well you’ve come to the right place! I’ll point out the most important basics and link to further resources for you to go through. If you know this already, feel free to skim these sections or jump directly to the next post.

Arrays in JavaScript

An array is just a convenient way to store of similar things under the a single variable name. Like this:

let penguinSpecies = ["Adelie", "Adelie", "Chinstrap"];

See, by enclosing the list of items in square brackets [] JavaScript registers this as an array. And the nice thing about that is: each array comes with useful methods out of the box. We’ll use the most common ones below to learn about our penguin data.

The most simple example is getting the length of the array. You simply do:

console.log(penguinSpecies.length) // prints 3
What is 'console.log'?

If this is your first time working with JavaScript: what print is in other languages, here it is console.log. It prints something into the console.

This tells you how many entries your array has: in this case 3. Try logging somePenguins.length. It works the same way even though in that case it’s not just a list of strings, but an array of objects.

Why not .length()?

Technically speaking, .length is a property and not a method. That is to say, it is not a function that you can call, but rather a property that gets updated automagically to keep track of the arrays size.

Objects in JavaScript

To showcase an object, let’s create one. Let’s say we want to document some facts about one of our penguins. Let’s put down where Rita lives and what type of penguin she is:

let factsAboutRita = {species: "Adelie", island: "Dream"}

This time we enclosed our data in curly brackets {} and gave each entry a key: and a value. Pretty neat to store data, right? There are a couple of functions that help us to access e.g. all keys in any given object.

Keys and values
  • keys(): The Object.keys() method returns an array of a given object’s own enumerable string-keyed property names.
  • values(): The Object.values() method returns an array of a given object’s own enumerable string-keyed property values.

You’ll see keys() in action in just a minute.

If we want to access a specific value we can use the objects’ name and the key via the dot notation:

console.log(factsAboutRita.species) // prints "Adelie"
Bracket notation

Bracket notation provides an alternative way to access object properties.

console.log(factsAboutRita["species"]) // prints "Adelie" too

I’ll continue to use the dot notation in the examples below, because it’s easier to type and more common in general.

You can see a comparison of both notations on MDN here.


Great, now let’s circle back to our initial array of objects to recap:

let somePenguins = [
    {row: 1, species: "Adelie", island: "Torgersen"},
    {row: 2, species: "Adelie", island: "Biscoe"},
    {row: 3, species: "Chinstrap", island: "Torgersen"},

The whole of somePenguins is an array [] that has useful methods that JavaScript offers us (more on that below).

Each entry in our array is an object {}. All of our objects can be thought of as the rows of our table. Inside these rows, we have the key and value pairs.

This data representation is more verbose than the spreadsheet-style that you might be used to, but it actually makes reading the data easier by eye and is more precise for coding. What’s more, an array of objects is the standard data representation that you will get if you read in external data with d3 via the d3-fetch or d3-dsv modules. So it is a pretty standard way of representing data, especially in the context data visualizations for the web. In addition, an array is also the kind of structure that Svelte {#each} blocks need, so it’s very useful if you want make great data visualizations with Svelte & D3.

Next, we’re gonna pull in the real dataset and then take a look at the vanilla JavaScript functions that we can leverage to work with the data and answer some basic questions about the data.

Next up

© 2023-2024 Sebastian Lammers

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