JavaScript Lazy Evaluation: Iterables & Iterators

Iterables and Iterators. The base building blocks of lazy evaluation algorithms. Let’s see what are they made of and how they can be useful.

Lazy evaluation, every functional programmer’s wet dream. Soon enough, we will look at generators. But first, let’s get a grasp of what iterators and iterables are because to me, they’re a prerequisite for generators.

Lazy evaluation

Lazy evaluation means to delay the evaluation of an expression until it’s needed. Lazy evaluation is sometimes referred to as call-by-need.

The opposite of lazy evaluation is an eager evaluation. It’s an evaluation strategy used in most programming languages.

Lazy evaluation makes it possible to:

  • define potentially infinite data structures
  • increase performance by avoiding needless computations
  • customize iteration behavior for data structures that want its elements accessible to the public


Iterators in JavaScript (since ECMAScript 6) are what make it possible to lazy evaluate and create user-defined data sequences.

Iteration is a mechanism for traversing data. Iterators are pointers for traversing elements of data structure, called Iterable. A pointer for producing a sequence of values.

An iterator is an object that can be iterated over. It abstracts a container of data to make it behave like an iterable object.

The iterator does not compute the value of each item when instantiated. The next value is generated only when requested. This is useful, especially for large data sets or sequences of an infinite number of elements.


Iterables are data structures that want their elements accessible to the public.

Many APIs accept iterables, for example:

  • new Map([iterable])
  • new WeakMap([iterable])
  • new Set([iterable])
  • new WeakSet([iterable])
  • Promise.all([iterable])
  • Promise.race([iterable])
  • Array.from([iterable])

There’re also statements and expressions that expect iterables, for example:

  • **for ... of** (loop)
  • **...** (spread operator)
  • const **[a, b, ..]** = iterable; (destructuring assignment)
  • **yield*** (generator delegation)

There’s a number of already built-in iterables in JavaScript:

String , Array , TypedArray , Map , Set.

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JavaScript Lazy Evaluation: Iterables & Iterators
Coy  Roberts

Coy Roberts


Javascript ES6 Iterators: Complete Guide on Iterables and Iterators

Javascript iterators is a new concept of ES6 in which it processes each of the items in the collection, which is a pervasive operation. Iterators are a new way to _loop _over any collection in JavaScript.

They were introduced in ES6 and have become popular since they are widely JavaScript provides several ways of iterating over the collection, from simple for loops to map() and filter() Iterators and  Generators bring the concept of iteration directly into the core language and provide a mechanism for customizing the behavior of loops.

Iterators in Javascript

An iterator is a new concept of ES6. An Iterator is an object that lets us iterate through an Array, an Object, a String, or even custom Objects.

The Iterator allows us to effectively loop over a collection of objects like an arraystringobjects, or other data structures.

The most common Iterator in Javascript is the Array iterator, which returns each value in the associated array in sequence.

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Rahul Jangid


What is JavaScript - Stackfindover - Blog

Who invented JavaScript, how it works, as we have given information about Programming language in our previous article ( What is PHP ), but today we will talk about what is JavaScript, why JavaScript is used The Answers to all such questions and much other information about JavaScript, you are going to get here today. Hope this information will work for you.

Who invented JavaScript?

JavaScript language was invented by Brendan Eich in 1995. JavaScript is inspired by Java Programming Language. The first name of JavaScript was Mocha which was named by Marc Andreessen, Marc Andreessen is the founder of Netscape and in the same year Mocha was renamed LiveScript, and later in December 1995, it was renamed JavaScript which is still in trend.

What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

Today almost all websites are using it as web technology, mind is that there is maximum scope in JavaScript in the coming time, so if you want to become a programmer, then you can be very beneficial to learn JavaScript.

JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */


Advantages and Disadvantages of JavaScript

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The essential JavaScript concepts that you should understand

As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

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Ajay Kapoor


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