jMar"s Blog DevSmash Developer Portal

Friday, March 14, 2008

Understanding Loose Typing in JavaScript

For many front end developers, JavaScript was their first taste of a scripting and/or interpretive language. To these developers, the concept and implications of loosely typed variables may be second nature. However, the explosive growth in the demand for Web 2.0-ish applications has resulted in a growing number of back end developers that have had to dip their feet into pool of client side technologies. Many of these developers are coming from a background in strongly typed languages, such as C# and Java, and are unfamiliar with both the freedom and the potential pitfalls involved in working with loosely typed variables.

Since the concept of loose typing is so fundamental to scripting in JavaScript, an understanding of it is essential. This article is a top level discussion of loose typing in JavaScript. Since there may be subtle differences in loose typing from language to language, let me constrain this discussion to the context of JavaScript. OK, let's dig in...

What is Loose Typing?

Well, this seems like a good place to start. It is important to understand both what loose typing is, and what loose typing is not. Loose typing means that variables are declared without a type. This is in contrast to strongly typed languages that require typed declarations. Consider the following examples:

/* JavaScript Example (loose typing) */
var a = 13; // Number declaration
var b = "thirteen"; // String declaration

/* Java Example (strong typing) */
int a = 13; // int declaration
String b = "thirteen"; // String declaration

Notice that in the JavaScript example, both a and b are declared as type var. Please note, however, that this does not mean that they do not have a type, or even that they are of type "var". Variables in JavaScript are typed, but that type is determined internally. In the above example, var a will be type Number and var b will be type String. These are two out of the three primitives in JavaScript, the third being Boolean.

JavaScript also has other types beyond primitives. The type diagram for JavaScript is as follows (as per Mozilla):

Ya really - Null and Undefined too.

Note, however, that this distinction between primitives and objects will be dismissed in JavaScript 2.0. You can read more about that here.

Type Coercion

Type coercion is a topic that is closely associated with loose typing. Since data types are managed internally, types are often converted internally as well. Understanding the rules of type coercion is extremely important. Consider the following expressions, and make sure you understand them:

7 + 7 + 7; // = 21
7 + 7 + "7"; // = 147
"7" + 7 + 7; // = 777

In the examples above, arithmetic is carried out as normal (left to right) until a String is encountered. From that point forward, all entities are converted to a String and then concatenated.

Type coercion also occurs when doing comparisons. You can, however, forbid type coercion by using the === operator. Consider these examples:

1 == true; // = true
1 === true; // = false

7 == "7"; // = true
7 === "7"; // = false;

There are methods to explicitly convert a variable's type as well, such as parseInt and parseFloat (both of which convert a String to a Number).

Double negation (!!) can also be used to cast a Number or String to a Boolean. Consider the following example:

true == !"0"; // = false
true == !!"0"; // = true

This obviously is not a definitive reference to loose typing in JavaScript (or type coercion for that matter). I do hope, however, that this will be a useful resource to those who are not familiar with these topics, and a good refresher for those who already are. I have tried to insure that the above is accurate, but if you notice anything incorrect, please let me know! And as always, thanks for reading!


Anonymous said...

simple and elegant.

Anonymous said...

Found you via Google...

"Ensure" that you do not forget to use a spell-checker before posting.. Irony alert!! ;)

"I have tried to *insure* that the above is accurate, but *if you notice anything incorrect, please let me know!* And as always, thanks for reading!"

akropp said...

You are mixing loose (aka weak) and dynamic typing here. Loose typing is the "7" + 7 example, mixing types to produce a new value. In a strong typed language this would give you a compilation or runtime error.

In contrast a dynamically typed language is one where the types are determined at runtime. Statically typed languages are languages where the types are determined at compile time.

Anonymous said...

I just don't get the purpose of loosely-typed variables. All it seems to do is make a language appear less threatening to beginners while providing nothing but trouble to everyone else.

One of the nice things about specifying your data type is the inherent safety it provides: for example it often produces compiler errors if you mix up your arguments when calling a function with multiple params.

The main problem is why would you not know what your data will represent? If a variable can be named then it can be given a type too: for example, if you've named a variable "lastName" when why would you ever put an integer into it?

If you don't know what your variable is intending to represent then you shouldn't be using it. These kinds of languages seem obscenely bug-prone and then have these needlessly complicated rules on how the variables are used.

gngl said...

"Loose typing means that variables are declared without a type."

Whoa, stop right there, cowboy. That's hardly what I'd call "loose typing". Typing with untyped variables can be pretty strong, if the values used in the computation carry their type and you have proper operators to manipulate them. Javascript does have the former, but it doesn't have the latter - the horrible coercion rules are what's messy about JS types, not the dynamic typing nature. (Common Lisp, Scheme, Python etc. deal with this just fine, but they don't implicitly convert stuff as in 1+"2" -> "12", that's why they're more sensible.)

"Notice that in the JavaScript example, both a and b are declared as type var."

They're not declared as "type var". In fact, they're not declared at all! They're simply values bound to names. var does not declare variables, it establishes variable bindings.

"JavaScript also has other types beyond primitives."

JavaScript doesn't actually have any primitives. Everything is an object in JS. Unless, by "primitives", you mean "atomic values" (as opposed to compound/aggregate data structures), but that is a different issue obviously.

@Anonymous: "I just don't get the purpose of loosely-typed variables. All it seems to do is make a language appear less threatening to beginners while providing nothing but trouble to everyone else."

If you really think this, you're a very undemanding person, with respect to your tools!

The purpose of static type systems is to admit correct, sensible programs, AND to reject nonsensical programs. Real-world static type systems, however, have one of the two following problems:

1) They either reject some correct programs (as in, they're overly restrictive), or
2) they accept some incorrect programs (can't catch some typing errors at compile time).

Some languages, such as Java, delight in doing both at the same time: on one hand, you can't specify covariance or contravariance of method parameter types when subclassing, thus being restricted to invariant parameters, but at the same time, Java arrays are covariant by default, thus making it possible for them to cause run-time type errors in some scenarios.

It doesn't seem to be possible to deal with 1) and 2) at the same time. If you try to do that, you'll find yourself designing increasingly complex type systems involving such things as phantom types, existential types, dependent types and what not; and at one moment, the typing of certain programs may even become undecidable, thus making your compilers stuck in a loop or halting with an "I don't know how to check this" kind of message.

Dynamically typed languages simply adopt the "we don't want to reject any program that might make sense, let's just check it at runtime" approach (and with JIT, they at least try to remove as many checks as possible to have it run at a decent speed, and again, this approach seems to be quite successful). Surprisingly, for many people, it works just fine! Let me conclude with a quote by Alan Kay, one of the inventors of Smalltalk, which summarizes the whole issue nicely: "I'm not against types, but I don't know of any type systems that aren't a complete pain, so I still like dynamic typing."

Jeremiah Deasey said...
This comment has been removed by the author.
Jeremiah Deasey said...

Thanks for the examples and walk through.

Also, in your closing statement, the correct word would be "ensure".

make certain that (something) shall occur or be the case.
"the client must ensure that accurate records be kept"
synonyms: make sure, make certain, see to it;

Anonymous said...

7 + 7 + 7; // = 21
7 + 7 + "7"; // = 147
"7" + 7 + 7; // = 777

In the examples above, arithmetic is carried out as normal (left to right) until a String is encountered. From that point forward, all entities are converted to a String and then concatenated.

Might be useful to mention that the arithmetic is carried out until a string is encountered only if you're using the + operator.

"37" + 7 //results in "377"
"37" - 7 //results in 30

Makayla Charleston said...

This is what I have been searching in many websites and I finally found it here. Amazing article. I am so impressed. Could never think of such a thing is possible with it...I think you have a great knowledge especially while dealings with such subjects.
clipping path

FEDojo said...

Thanks for this tutorial! Lot of JS tips which can be used into own code.

electronic signature in word said...

Thank you so much for taking the time to share this information. A great read. I’ll certainly be back.

aliya seen said...

We can see that paraphrase text online is now available for students so they can easily write their papers and project work.

James Kateron said...

Tips for writing effective product or service demonstration and training scripts written by a professional script writer. Several techniques and suggestions for entertaining presentations are demonstrated. See more science manuscript editing

Deny Games said...

Everything is fine, am happy of your blog. Thanks admin for sharing the unique content, you have done a great job I appreciate your effort and I hope you will get more positive comments from the web users.
unblocked games free 4 all | geometry dash online | happy wheels demo | 1 on 1 soccer heads | juegos | online games

Romilda Gareth said...

So you want to be a work from home typist? It's a great job, and you can make great money typing online as long as you know where to start. If you can type, have a computer and internet access then there's no reason why you can't be very successful. See more typing services online

Bogdan Yanov said...

I recently came across your blog
and have been reading along. I thought I would leave my first comment. I don’t know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.I’ll use this information for my essays.

Alex John said...

I for one such as your post; you have shared great bits of knowledge and encounters. Keep it up.
Humanitarian Assistance/Disaster Relief: What Can We Learn from Commercial Supply Chains? Case Solution

Kristofer Kihn said...

Our website is No. 1 in Programming Assignment Help. You can also hire us for your Programming Assignment Help - Programming Homework Help - Programming Project Help.
Online Programming Project Help

Suraj Baba said...

Corporate Packers and Movers in thrive to meet all the needs, needs and specifications of their potential customers to win their greatest believe in and fulfillment.

Packers and Movers Hyderabad
Packers and Movers Bangalore
Packers and Movers Pune

Suraj Baba said...

Packers and Movers Mumbai
Packers and Movers Gurgaon
Packers and Movers Delhi

Suraj Baba said...

Packers and Movers Chennai
Packers and Movers Noida
Packers and Movers Navi Mumbai
Packers and Movers Thane
Packers and Movers Ghaziabad
Packers and Movers Faridabad

Suraj Baba said...

Packers and Movers in Hyderabad
Packers and Movers in Bangalore
Packers and Movers in Pune
Packers and Movers in Mumbai
Packers and Movers in Gurgaon
Packers and Movers in Delhi

kanna said...

This application works based on your phone book list number,so you can chat easily with your phone book list numbers.
download allo apk

kanna said...

It is a best Android app which enables you to remove advertisements from Android apps and games and modify permissions of different apps.
lucky patcher apk download


Kite projects Is providing construction, engineering and facilities management to provide edge protection,BESPOKE ACCESS SOLUTIONS, vertical access and hand railing solutions to Industries in United Kingdom!

Candy Sim said...

this is one of the cult game now, a lot of people enjoy playing them . Also you can refer to the game :
age of war | red ball | earn to die 5 | Tank trouble | happy wheels | earn to die 6
The game controls are shown just under . Movement mechanisms primarily include acceleration and tilting controls.
tank trouble unblocked | wings io | strike force heroes | age of war | hotmail login

Bryan Lasater said...
This comment has been removed by the author.
Bryan Lasater said...

I appreciate this work amazing post for us I like it.
Harvard Business Review HBR Cases Solutions and Analysis

Aimee Elvis said...

sneakers all there - Award proves which people are now in Italian Republic the particular community's tasks are productive. "Burden Italy preferred commercial enterprise pinnacle in Romance client basic safety footings, tsinghua college school related to interpersonal sciences, Romance new institution examine ligue associations such as the adult or otherwise support, 24/7 bits all associations, helpful firms so company bookmarking burden related to marketing researchers assessment. They've been dependent on collective administration along with ethical economic value, jobs together with coverage of this protections also concerns on workforce, ecological shelter then electric resource efficiency but also discharges cutback, such as for example several indications selected champs. Jordan 18 low Italian Republic nowadays, collaborative interpersonal responsibleness beneficial purchase jordans continent target the far back-time period balanced growth of court users, salespeople furthermore positively take part in varied techniques, selfless strength of mind the talent then encounter.

Candy Sim said...

The blog or and best that is extremely useful to keep I can share the ideas
of the future as this is really what I was looking for, I am very comfortable and pleased to come here. Thank you very much.
gold miner|stick war 3| pokemongo
| stick man|animal jam login

Akis Kumar said...

Aptoide is an alternative of Play store and is a huge market place which comes with tons of amazing applications and games
aptoide download

Lalit Sharma said...

deepavali whatsapp status
diwali wishes
happy deepavali

Kapil said...

Allo Download
Allo for MAC
click here

Anonymous said...

We're the best online games website
Single player and multiplayer game

Candy Sim said...

This is one of the cult game now, a lot of people enjoy playing them . Also you can refer to the game :
gold mine strike | pokemon go 2
The game controls are shown just under . Movement mechanisms primarily include acceleration and tilting controls.
stickman games | stick war 2 | animal jam 2

Presidential Suite said...

באתר פרזדינט אסקורט תוכלו להסתכל על תמונות אמיתיות של נערות ליווי סקסיות וחרמניות.

כמו כן באתרינו תוכלו להזמין נערות ליווי בת ים, חולון והסביבה עד לביתכם או לחדר המלון הפרטי והדיסקרטי שלכם.

במכון שלנו תקבלו שירות נשיאותי ברמה מקצועית ואיכותית ביותר - הסכנות שלנו מספקת פנטזיות פנאי ומין ולמבוגרים מעל גיל 18 ובעיקר מתמחה בסיפוק של נערות לווי דיסקרטיות ואקסקלוסיביות להזמנה.

PresidentEscort said...

פרזידנט אסקורט מציג נערות לווי וי.אי.פי ברמה הכי גבוה ומקצועית שיש בארץ ישראל והסביבה של הים התיכנון להציעה בתחום הפנאי לבמוגרים, סקס וליווי.

Leo Herbalis said...

Thank you for sharing in this article
I can learn a lot and could also be a reference
I hope to read the next your article updates
Jual Hammer Of Thor Obat Kuat Sex
Vimax Asli Canada
Vimax Asli Canada Obat Pembesar Penis
Hammer Of Thor Obat Kuat Sex
Jual Vimax Asli Canada Izon Original di Medan
Jual Hammer Of Thor Obat Kuat Sex di Medan

Thomas pedro said...

At present, individuals are getting wage from different sources and in such circumstance they can't demonstrate a specific wellspring of salary. Since keeping record of aggregate month to month wage is extreme for such individuals, they can confront issues in getting advance. check cashing corona

bettycyoung said...

good than other social networking applications. IMO Download play imo on pc utilizing a mouse or your nice.

Daniel Nagata said...

Email is a gateway to enter into cyberspace or internet, but unfortunately a lot of people who still can not make their own email account, the following guidelines:
cara membuat email baru gmail
cara membuat email yahoo baru
Once you have an email account on google or yahoo, you can now register at sosmed or send e-mails, pictures, links to specific files. If you want to register at sosmed, this guide:
cara membuat akun baru facebook
cara membuat akun twitter
May you be helped.

hima sri said...

nice article I am there and mainly many of us filmywap 2017 are downloading the app because in this

cathrynzbrown said...

good However going to theater every time a new playview apk download presented inning accordance with their date of released nice.

Ethel Graff said...

good broadcast messages, group talks, as well as very easy sharing of pictures, WhatsApp Download Android Phone for a very long time, including Innjoo nice.