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Benefit from user-defined variables in JavaScript

Programming languages are subsequent to ineffective with out variables. These particular information-holding areas can retailer numbers, textual content strings, objects, and different information sorts. As soon as saved, this info can be utilized later in your program. With a variable, an individual’s identify could possibly be saved and used in some unspecified time in the future within the script.

Variables are non permanent holders of knowledge. They will maintain:

  • Numeric values (“numbers”) — numbers that may be added collectively. Instance: 2+2 ends in 4
  • Character strings — a set of textual content, resembling “JavaScript” or “My identify is Mudd”
  • True/false values — the Boolean true and false
  • Objects — JavaScript or user-defined objects (JavaScript variables can maintain just a few different kinds of information, however these are by far the commonest sorts)

(Notice: As with most trendy programming languages, JavaScript helps array variables along with the essential scalar variables used to carry the above information sorts. We’ll consider single-value variables for this column and commit a separate column to arrays.)

JavaScript variables “belong” to the script doc that created them; the variables are misplaced when the doc is unloaded. As well as, the contents of a variable are erased once you assign a brand new worth to them. Although a variable created in a single doc script isn’t normally seen by one other doc script, JavaScript does present methods to share variables between scripts. You do that by referencing the identify of the doc together with the identify of the variable.

A number of JavaScript directions create and retailer variables, however the fundamental technique to accomplish this manually is with the equals (=) project operator. The fundamental syntax is:

VariableName = worth

The primary argument is the identify of the variable. Variable names will be very lengthy, however you might be restricted within the characters you might use. For extra info on legitimate variable names, see the part on Variable identify limits.

The second argument is the contents of the variable. You’ll be able to put all kinds of stuff right into a variable, together with a quantity, a string, a math expression (resembling 2+2), and varied different issues that we’ll get to in a bit.

Pascal customers could also be tempted to assemble the variable project utilizing :=. Remember that this syntax isn’t supported in JavaScript.

Following is a extra particular rundown of the 4 commonest contents you’ll be able to place in JavaScript variables, together with examples.

Contents positioned in JavaScript variables

Numbers in variables

A quantity is a number of digits saved within the laptop in such a manner that JavaScript can carry out math calculations with them. JavaScript helps each integers and floating-point values. To position a quantity in a variable, simply present the variable identify, the equals signal (the variable project operator), and the worth you wish to use. For instance, the next is what you do to position the quantity 10 in a variable named MyVar:

MyVar = 10;

Strings in variables

A string is a number of textual content characters organized in reminiscence in single file. Strings can include numbers (digits), letters, punctuation, or a mix of those components. Math calculations can’t be carried out on strings. Strings are assigned to JavaScript variables by being enclosed in a set of single or double quotes:

"I'm a string"


'I'm a string'

Notice that double or single quotes are acceptable; not like some languages, resembling Perl, JavaScript makes no distinction between the 2 types of quote marks. This working instance reveals the way to place a string right into a variable:

MyVar = "That is JavaScript";

Boolean values in variables

There are two Boolean values: true and false. Some programming languages haven’t got a separate set of Boolean values, and as a substitute use 0 for false, and 1 or -1 (or some other non-zero worth) for true. JavaScript can use these numbers to signify true and false however, as well as, reserves the phrases “true” and “false” to imply Boolean true and false. You’ll be able to consider the Boolean true and false values as being equal to on/off or sure/no. To assign a Boolean worth to a variable, present simply the phrase true or false, with out quotes. This is an instance:

MyVar = true;

Objects in variables

Variables can include objects, together with JavaScript objects. There are mainly two sorts of object variables:

  • Variables that include built-in browser-related objects — window, doc, navigator, and so forth — really are references to the unique object. They’re like copies, however the copies change if the unique adjustments. In some instances, making a change to the thing within the variable impacts the unique JavaScript object.

  • Variables that include user-defined objects signify the precise object. Make a change to the thing within the variable, and you alter solely that object.

To assign a JavaScript object to a variable, present the identify of the thing, as in:

MyVar = navigator;

To assign a brand new copy of a user-defined object to a variable, use the brand new assertion, and supply the identify of the thing operate:

MyVar = new myObject();

SUBHEAD Variable identify limits

With regards to the names you can provide variables, JavaScript provides a substantial amount of latitude. JavaScript variables will be nearly limitless in size, though for sensible causes you will in all probability hold your variable names underneath 10 or 15 characters. Shorter variable names assist JavaScript execute this system quicker. Preserve the next in thoughts when naming your variables:

  • Variable names ought to include letters solely — with out areas. You should utilize numbers so long as the identify does not begin with a digit. For instance, MyVar1 is appropriate, however 1MyVar isn’t.

  • Do not use punctuation characters in variable names. Exception: the underscore character ( _ ). That’s, the variable My_Var is appropriate, however My*Var isn’t. Variables can start with the underscore character.

  • Variable names are case-sensitive. The variable MyVar is a distinctly completely different variable from myVar, myvar, and different variations.

Understanding JavaScript’s “free” variable information sorts

In contrast to another programming languages, in JavaScript there is no such thing as a must explicitly outline the kind of variable you wish to create. This JavaScript habits is known as “free information typing,” and it differs from C and Java, each of which use strict information typing.

In JavaScript there is no such thing as a must differentiate variable sorts by appending particular characters to the top of the variable identify, resembling MyVar$ for a string variable (or, for that matter, $MyVar for a scalar variable, a la Perl). JavaScript internally decodes the variable kind based mostly on its content material.

Utilizing the var assertion to assign a variable

JavaScript helps a var assertion that can be utilized to explicitly outline a variable. The syntax is merely the assertion var, an area, and the identical variable project expression detailed above. For example:

var MyVar = "It is a variable";

It’s also possible to use the var assertion with the variable identify to declare the variable however not outline a price for it:

var MyVar;

On this case, you’ve got outlined MyVar in reminiscence however have but to assign a price to it. This method is usually used when organising world variables — variables that may be freely shared anyplace in your script. For extra details about world variables, see the part “Understanding the scope of variables”, under.

String size limitations

JavaScript imposes a restrict of 254 characters for every string variable project in your program. In case you go over the 254-character restrict, JavaScript responds with a “Unterminated string literal” error message. (Notice: That is basically a restrict of JavaScript in Netscape 2.0x; it is a good suggestion to look at it since not all customers have adopted Netscape 3.0.)

You’ll be able to create longer strings by “piecing” them collectively — so long as every bit is 254 characters or much less. After assigning a string to every variable, you mix them utilizing the + character. That is known as “concatenation.” The next instance reveals how concatenation works:

MyVar = "That is the beginning " + of the way you " + " can construct strings";

Every particular person string phase — outlined by textual content inside the quotes — will be as much as 254 characters. To make a string longer than 254 characters, merely add extra segments. One other strategy is to construct strings utilizing the += project operator, like this:

MyVar = "That is the beginning "
MyVar += "of the way you "
MyVar + = can construct strings "

You’ll be able to proceed to concatenate strings this manner so long as your laptop has the reminiscence for it. However, whereas JavaScript can maintain strings bigger than that doable in lots of different programming languages (like Primary’s typical 64K), doing so can severely degrade the efficiency of the system. Clearly, you will not create lots of large string variables. It is simply good to know that, if wanted, a JavaScript variable can accommodate a lot textual content.

The “scope of a variable” has nothing to do with optics or mouthwash, however slightly the extent to which a variable is seen to different elements of a JavaScript program. Until you present express directions to inform JavaScript in any other case, the scope of its variables is managed as follows:

  • Variables outlined outdoors a operate can be found to any operate inside the script, so long as all of the variables are definined within the script of the identical HTML doc. These are known as world variables.

  • Variables outlined inside a operate are additionally world, assuming the var assertion isn’t used when first declaring that variable. That’s, MyVar = “hi there.”

  • Variables outlined inside a operate with the var assertion are “native” to that operate solely. These are known as native variables.

  • World variables stay in reminiscence even after a script has stopped execution. The variable stays in reminiscence till the doc is unloaded.

Native variables are handled as if they do not exist outdoors the operate the place they’re outlined. That manner, you should use the identical variable identify inside a operate, and that variable will not intrude with the same-named variable elsewhere within the script.

Following is an instance that demonstrates this. While you click on the button, the script shows three alert containers. The next particulars what occurs once you click on the button:

  • JavaScript calls firstFunction, which assigns a price of 1 to a neighborhood variable named MyVar. The contents of MyVar is displayed.

  • JavaScript calls secondFunction, which assigns a price of two to a neighborhood variable, additionally known as MyVar. The contents of MyVar are displayed.

  • JavaScript returns to firstFunction, the place the contents of MyVar are once more displayed. The result’s 1, which is the worth of MyVar native to firstFunction.

Referencing variables in different loaded paperwork

When utilizing frames, it’s typically essential to share variables throughout paperwork. A number of frames may have a variable contained in one other body. By their nature, variables (even world ones) should not seen outdoors the doc that created them. So, once you wish to reference a variable in one other doc — and assuming that doc is loaded into the browser — it’s essential explicitly reference that variable by including the window identify in entrance of the variable identify. Right here is syntax:


the place winname is the identify of the doc, and varnameis the identify of the variable. Extra about doc names in a bit.

You’ll be able to assign and reference variables utilizing the next approach. For instance, this units the MyVar variable within the mydoc window to 1:

mydoc.MyVar = 1;

The code under assigns the worth of a neighborhood MyVar variable within the mydoc window.

VarInThisDoc = mydoc.MyVar;

The place do the names for the home windows come from? All of it depends upon how the home windows are used.

To make use of a variable in the principle browser window from a window that you simply created, first present a “hyperlink” to the mum or dad window object, utilizing this technique:

newwindow = window.open ("","NewWindow");  //repeat this for Mac/X Netscape 2.0
newwindow.creator = self;

Then, on this new window you’ll be able to consult with any variable in the principle window utilizing the syntax:


To make use of a variable in a window you’ve got created, consult with it utilizing the thing identify you’ve got supplied once you created the window. For example, you’d use newwindow for a window created with the next:

newwindow = window.open ("","NewWindow");

Now consult with that variable utilizing the syntax:


To make use of a variable outlined within the frameset — that’s, the doc containing

tag — consult with it as mum or dad. This is an instance:

mum or dad.MyVar;

To make use of a variable in one other body doc, consult with it utilizing the body identify you’ve got supplied within the

Gordon McCombis an creator, guide, and lecturer. He has
written 50 books and over a thousand journal articles throughout his
20 years as an expert author. Greater than 1,000,000 copies of
his books are in print. Gordon additionally writes a weekly syndicated
newspaper column on computer systems, reaching a number of million readers
worldwide. Gordon’s newest ebook is The JavaScript
Sourcebook, obtainable from Wiley Laptop Publishing.

This story, “Benefit from user-defined variables in JavaScript” was initially printed by


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