JavaScript Tutorials for the C/C++ programmer, part 2 : Iterators

Make sure you’ve read over Part 1 of this tutorial. Currently, this post “Part 2” is definitively a “work in progress.”


  • MDN
  • Wikipedia
  • Dr Axel Rauschmeyer site
  • Eric Elliott’s Medium posts on JavaScript Scene

Code will generally be restricted to Github gists which I’ll link to so that article can focus on the concepts.


In JavaScript, an Iterable is a data structure that implements the Symbol.iterator factory. More on Symbols.

An Iterator is just a pointer for traversal, similar to a database cursor, or a C++ 14 Iterator.

With iterations over collections, we always assume that we are mapping/transforming/producing outputs, and/or that a statement can be executed for each element.

Recall that we don’t have to study much about Iterators in JS before we come across in-depth knowledge of how Symbols work.

String, Array, TypedArray, Map and Set are all built-in iterables, because each of their prototype objects implements an @@iterator method. In order to be iterable, an object must implement the @@iterator method, meaning that the object (or one of the objects up its prototype chain) must have a property with a @@iterator key which is available via constant Symbol.iterator… MDN

We will leave the discussion of Symbols to another future blog post.

We will also assume that by covering Iterators for Arrays and Objects, that the use cases for Sets, Maps, TypedArray will seem intuitive. We will leave WeakMaps and WeakSets for a later discussion as well.

JavaScript Iterators Gists

Sets and Maps

Gist “JavaScript Iterators: Sets and Maps” for more info.



Iterable and Iterator Protocols

Recall that an Object can be iterable, and that an Object “is an iterator” when it implements next and done values and API. We will leave the in-depth discussion of Iterators to another future blog post.

for of iterates over the iterable properties of an Iterable. That’s one example of implementation of Iterable/Iterable protocols.

For now, we will just discuss simple ways to iterate over Arrays.

Array iterators

Use Cases:

  • Iterate over an Array and transform its values into an output / use existing values
  • Filter to filter out undesired elements
  • Map to apply one function to all elements (similar to transform)
  • Reduce to accumulate values

We will begin with the simpler Array iterators and work our way into transducers.

Note that there are many, many Array.Prototype.DoAThing methods, search the MDN site to learn more.



Returns: a boolean value … IF some elements meet requirement.

const above10 = elem => elem > 10

[1,2,3].some(above10) // false

[10,11].some(above10) // true


Returns: a boolean value … IF all elements meet requirement.

Code is pretty much the same as with .some.


Just iterators for collections. Executes a provided function for each Array element.


Filters out undesired elements. There’s a callback function that returns a boolean whether or not the element is included in the new set.

Since filter returns “new data” instead of mutating existing data, we can think of filter as being a safe function to use for immutability.

We will use the indexOf() function, which returns a position within the selected data structure.

Filter example: Return only unique elements (make a Bag into a Set)

If you’re familiar with some aspects of Set theory, you’ll know that a Set has only unique elements, and a Bag (or MultiSet) can have repeated elements. Essentially, this question asks us to create a Set out of a Bag/MultiSet.

(For C++ related examples, recall Standard Library std::set, std::multiset)

Input: “apple”, “pear”, “peach”, “pear”, “apple”, “kiwi”

Expected Output: “apple”, “pear”, “peach”, “kiwi”

const fruit = ["apple", "pear", "peach", "pear", "apple", "kiwi"]

const aSet = fruit.filter((elem, index, fruity) => fruit.indexOf(elem) === index)

fruitySet   //  ["apple", "pear", "peach", "kiwi"]

Note the ES6 arrow function syntax. We could have used ES5:

const aSet =
  fruit.filter(function(elem, index, fruity) {
    fruit.indexOf(elem) === index

Which is logically equivalent to (and more verbose than):

const aSet = fruit.filter((elem, index, fruity) => fruit.indexOf(elem) === index)


Map is used to transform an entire data structure. Please see my Sets and Maps Gist for more info.

“Pure functions are all about mapping. Functions map input arguments to return values, meaning that for each set of inputs, there exists an output. A function will take the inputs and return the corresponding output.” “Mapping” — Eric Elliott

Since JS map returns “new data” instead of mutating existing data, we can think of filter as being a safe function to use for immutability.

(For C++ related examples, recall Standard Library std::transform)

Map example: Double all numbers (transform them!)

Input: 1,2,3,4 Expected Output: 2,4,6,8

const nums = [1,2,3,4]

const doubled = (nums) => nums*2 )

doubled // [2, 4, 6, 8]

Again, we note ES6 arrow function syntax.

Reduce and Transducers

Reduce Iterates over a collection and returns an accumulated value.

Transducers combine operations, hence skipping intermediary computational steps typically created by chaining JavaScript methods together. Please see my Transducers Gist for more info.

Object, Set, Map Iterators

JavaScript Iterators: Sets and Maps

In ES6 (and beyond) we tend to use Maps instead of Objects for iteration, because:

  • the Object prototype chain can be problematic for iteration;
  • Maps are more performant;
  • Maps keys can be any data type; Object keys can only be a String or a Symbol.

Please see my Gist for more info.

Before we go into Object iterators, let’s talk about how JS objects work.

  • Enumerable Properties : does it have its own?
  • Iterator and Iterable Protocols

Enumerable Properties: “On THIS object, not down the prototype chain”

An Object has Enumerable properties when its properties are its own properties, instead of just being properties that belong to another object somewhere in the Prototype Chain. Recall that we covered the Prototype Chain in the first post of this series.

There are a lot of methods for Objects in JS on the MDN site. How can I intuitively tell the difference?

To think simply about whether a method or context refers to the enumerability of an object, think about what it is.

If it’s enumerable, it has its own stuff.

It makes sense now why hasOwnProperty() refers to whether an object’s property is an enumerable one.

See more on MDN.


This method returns an array of object’s Own Enumerable Properties, in key value pairs.


This does the same thing, but only returns/operates on keys, not values/pairs.

Recall the Object keys can only be a String or a Symbol.


Same as above, only values instead of keys.

for in

for in iterates over the enumerable properties of an Object. It returns unordered results. (If you want order, use a for loop or similar).

for in is interesting because it enumerates over properties that are also in the Prototype Chain. MDN-recommended methods .hasOwnProperty() and .propertyIsEnumerable() can be used to. If you read the previous section, these should seem intuitive.

Subjects I will cover later include:

  • Isomorphic JS applications

    Rendering JS on server, and hence being able to use same functions

  • Function Currying

    In functional programming, functions aren’t “called”, they’re “applied.” For partial application of functions – applying only one argument at a time. Makes things more testable, easier to reason about…

  • Spread Operator, ES6 Destructuring, ES6 Shorthand

  • Functional programming : deterministic, predictable code

  • Immutability

    There are two ways to change data: update (mutate) existing data, or create a new/updated copy of the data.

    When we change existing data, side effects (bad things) can occur.

    • Cover freeze() and clone() methods
  • Pure Functions

    In Eric Elliot’s functional programming tutorials on Medium, one of the first things he mentions in his Pure Functions tutorial is how objects are passed into functions in JS by reference (and primitives are passed in by value).

If we mutate that object inside of that function, it will mutate the original object at its original memory location. That causes side effects which are unknown when we (“we” meaning, flow of execution control) are operating within the context of the function that brings that object in as a parameter.

So long for now!


Category: tutorials