2.7. Collections

Now that we have looked at the primitive variables and programming constructs of JavaScript it is time to move on to the JavaScript equivalents of lists and dictionaries.

As a preliminary to this section it is important for you to understand that everything in JavaScript is an object. and that all objects are capable of acting like dictionaries. In JavaScript you can add an attribute to any object. myObj.someattribute = somevalue

2.7.1. Lists/Arrays

One of the fundamental data types in Python is the list. Although JavaScript does not have lists, JavaScript does have Arrays that can act a lot like lists.

Some of the fundamental list operations we will look at in this section.

  • append, pop

  • index

  • slice

  • in / not in

  • Creating lists of iterables

Python has many ways to add and remove items from lists. Lets look at the most common ones and their JavaScript equivalents.







Push adds to the end



both pop off the end



JavaScript uses shift to remove from the front



Unshift adds at the start

l.insert(idx, newitem)

l.splice(idx, 0, newitem)

splice inserts at index deleting 0 items first

del l[idx]

l.splice(idx, 1)

splice can also just delete the item at idx

l1 + l2


You cannot add two arrays together using + in JavaScript

The last one definitely requires a bit of explanation. The + operator is not defined for JavaScript arrays but it also won’t give an error if you try it. Instead JavaScript silently (and evilly) converts the two arrays to strings and concatenates the strings! So, [1,2] + [3,4] --> '1, 23, 4' Yikes!

Both Python and JavaScript support indexing, but negative indexing is not supported by JavaScript. You will just get ‘undefined’ as a result.

Slicing is supported in JavaScript but only through the slice method.







both slices start at 2 and end at 3 inclusive



both slice from 2 to the end



both slice from the beginning through 3 inclusive



both slice from 5 from the end to 1 from the end inclusive

Unlike indexing, the slice method will accept negative numbers as offsets from the end of the list as both the starting and ending values. Checking for membership

In Python we often write if something in mylist: In JavaScript we can write this a couple of ways:

Hold on, what is going on with that last example? Using “in” to test for membership is very tempting for Python programmers, and in fact in the first example looks like it works. But that example is misleading! The in operator only works on the keys of an object. The items in an array are not the same as the keys of a JavaScript object. But, since Arrays are objects it does not throw an error, it just works in a confusing way. In the example above it is saying that the object mylist does not have a key called “foo”. What are the keys of an Array?

If you run the example you can see that for an Array, the keys are the index values of the items in the Array. Don’t use in with Arrays.

Finally, lets look at a few convenient ways to make Arrays.

In Python there are a couple of very common patterns for converting parts of strings to lists: splitting a string, and converting all the characters of a string into array elements.

Lets look at another early Python program. We are going to read numbers from a file and produce a histogram that shows the frequency of the various numbers. The data file we will use has one number between 0 and 9 on each line of the file. Here is a simple Python program that creates and prints a histogram.

Now if we run this program on a data file that looks like this:

We will get output that looks like this:

0 occurred 0 times
1 occurred 1 times
2 occurred 1 times
3 occurred 1 times
4 occurred 1 times
5 occurred 3 times
6 occurred 0 times
7 occurred 0 times
8 occurred 1 times
9 occurred 1 times

Lets review what is happening in this little program. In the first line we create a list and initialize the first 10 positions in the list to be 0. Next we open the data file called ‘test.dat’. Third, we have a loop that reads each line of the file. As we read each line we convert it to an integer and increment the counter at the position in the list indicated by the number on the line we just read. Finally we iterate over each element in the list printing out both the position in the list and the total value stored in that position.

To write the JavaScript version of this program we will have to introduce several new JavaScript concepts. First, you will see the JavaScript equivalent of a list, called an ArrayList. Next you will see three different kinds of loops used in JavaScript. Two of the loops we will use are going to be very familiar, the third one is different from what you are used to in Python but is easy when you understand the syntax:


Used with boolean expression for loop exit condition.


Used to iterate over a sequence. This is very similar to for i in xxx where xxx is a list or string or file.


Used to iterate through a sequence of numbers. This is most similar to for i in range(), except the syntax is different.

Here is the JavaScript code needed to write the exact same program:

Now, lets look at what is happening in the JavaScript source. First we declare a variable to hold the counts – we are making the assumption that all of the numbers we want to count are between 0 and 9, so we can give our array an initial size, and initialize it with 0 values using the fill method.

Next lines 6 – 9 iterate over the values in the array (for...of) created by the split method. As with Python splitting the string results in an array of strings. So to update our count we need to convert the string to an integer. We use the parseInt function for this. Declaring idx as a const` inside the ``for loop ensures that any attempt to change idx will cause an error, as well as limiting the scope of idx to the loop.

Finally lines 11 – 13 print the results using by iterating over the index values of the array (for...in) and printing out the *count8 value for each.

Advanced Topic

Note, if you know you want to convert the elements of the list to integers and you may use the list several times, a common JavaScript idiom for this would be to use the map function as follows:

data = data.split(',').map(function(t) { return parseInt(t) })

This one liner splits the string, and then applies the parseInt function to every element of the array, returning an array of transformed objects. The map function is a very powerful functional programming tool and worth getting to know in detail. In data science the map reduce programming paradigm is widely used on very large datasets. For example, suppose your task was to add all of the numbers represented by the data string in the example above. You can do it very simply as follows:

2.7.2. Dictionary/Object

Just as Python provides the dictionary when we want to have easy access to key-value pairs, JavaScript also provides us a similar mechanism. In fact all objects in JavaScript

One important difference between Python dictionaries and JavaScript is that you can use either the dotted notation or the index operator to add and retrieve values stored for a key. Of course if your key has a space or dash in the middle of it you are limited to the index notation, but for many keys using the dotted notation is very convenient and very readable.

Some common operations from Python that you will want to know about include:

  • Get all the keys In Python you do this as myDict.keys() in JavaScript it requires a bit more effort:

  • Get all of the values In Python this is myDict.values(). But in JavaScript it takes a lot more work. Using a for loop you can do it like this:

Here is a much more functional approach to the problem that works in one line, but only for browsers that support ECMAScript 6:

This is pretty interesting as it introduces a new syntax: key => myDict[key] this is a bit like a Python lambda expression. In that it does essentially create a very simple function expression. These are called arrow functions or sometimes fat arrow functions in JavaScript. These are pretty new additions to the JavaScript language, so you might also see an equivalent one liner that looks like this:


const vals = Object.keys(myDict).map(function(key) {return myDict[key]})

The arrow is much cleaner and simpler to read once you have seen and understand them.

  • Get all items there really is no use for this in JavaScript as the most common use case for myDict.items() in Python is as a way to iterate over the key-value pairs of a dictionary. Also as a side note, JavaScript does not have a tuple data type. But it is just as easy to do the same iteration in JavaScript.

  • Get a value for a key if it exists, otherwise get a default. In Python we commonly would use the pattern myDict.get(key,"default") where get returns the value for key from myDict8 or “default” if the *key does not exist. In JavaScript the idiom for this is:

myDict['nobodyhome'] || 'default'

This makes use of the fact that if the key ‘nobodyhome’ is not found in myDict it evaluates to undefined which is falsey which causes the next thing in the boolean or statement to be evaluated, causing the full expression to evaluate to ‘default’.

Lets put all of this to work in a full blown example. We will stay with a simple frequency counting example, only this time we will count the frequency of words in a document. A simple Python program for this job could look like this:

Data file: alice30.txt
Down, down, down.  Would the fall NEVER come to an end!  'I
wonder how many miles I've fallen by this time?' she said aloud.
'I must be getting somewhere near the centre of the earth.  Let
me see:  that would be four thousand miles down, I think--' (for,
you see, Alice had learnt several things of this sort in her
lessons in the schoolroom, and though this was not a VERY good
opportunity for showing off her knowledge, as there was no one to
listen to her, still it was good practice to say it over) '--yes,
that's about the right distance--but then I wonder what Latitude
or Longitude I've got to?'  (Alice had no idea what Latitude was,
or Longitude either, but thought they were nice grand words to

Now lets look at how to do it in modern JavaScript.

Other than different syntax, this example is very close to the Python example. We will get into the details of line 6 in a later chapter. but for now suffice to say that it allows us to get all the text from the first paragraph of alice. The same thing that we used as a mock file in the Python example.

Improve the program above to remove the punctuation.

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