Professor: Magnificent! I’m now even more excited about next week, when we’re
going to make your math worksheet generator interactive.1
11.2 JavaScript Objects Revisited
Professor: You already k now plenty about objects in JavaScript. You know that an
object is a composite value in the sense that it combines multiple values. It can in-
corporate either primitives or other objects, which act like its properties or members.
If a property is a function object, which you can invoke to do something, then such
a property is often called a method. You access an object’s properties by their names
using the property access operator in the form of either a dot or a pa ir of square brack-
ets. It is im portant that objec ts with the exception of strings are mutable. You also
know that JavaScript objects are manipulated by reference rather than by value, and
that you crea te an obje ct using the new operato r in com bination with an appropriate
constructo r.
That said, you can also create an object by means of an obje ct literal, which is simply a
comma-separated list of name and value pairs e nclosed within a pair of curly bra ckets.
A property name may either be a JavaScript identifier or a string literal, and must be
separated from its value by a colon (:). A value can be any JavaScript expression,
whose value—either primitive or object—becomes the value of the pro perty. Here’s
an example :
//An object with two properties representing a complex number
//1 + 3i:
var complex = {re: 1, im: 3};
//The conjugate of the above number:
var conjugate = {re: complex.re, im: -complex.im};
I created two objects, an object complex with the real part set to one and the im ag-
inary part set to three, and its conjugate. I used more com plicated expressions for
property values to construct the conjugate. You recall complex numbers from high
school algebra, don’t you?
Maria: Vag uely, but yes. Is this the square root of m inus one?
Professor: The square root of minus one is the imaginary unit i. A complex number
is composed of the real and imaginary parts and can be written as a + bi, where a is
the real part and b is the imaginary part. You get the conjugate of a complex number
simply by multiplying its imaginary par t by minus one.
1The math worksheet example continues on page 249.
11.2. JavaScript Objects Revisited 207
You can now access the properties re a nd im of the two above objects as follows:
complex.re //Returns 1
complex.im //Returns 3
conjugate.re //Returns 1
conjugate.im //Returns -3
An object can also be seen as a mapping of strings, or keys to values, which is some-
times called a “hash table,” “associative array,” or “dictionary.” You can in fact con-
struct a dictionary quite easily. Here, for example, is a tiny fraction of an English–
German dictionary:
var englGerm = {
"programming language": "Programmiersprache, die",
"summer": "Sommer, der"
};
Because the first of the dic tionary entries is not a valid JavaScript identifier, the on ly
way it can be accessed is using square brackets:
//Returns a string "Programmiersprache, die"
englGerm["programming language"]
If, however, a string satisfies the rules that apply to JavaScript identifiers, then either
a d ot or a pair of square brackets can be used. That is true regardless of the fact that
the name was originally specified in string form rather than as an id entifier:
englGerm["summer"] //Returns "Sommer, der"
englGerm.summer //Returns "Sommer, der"
Remember, you have alre ady seen something like this in association w ith accessing
the properties of th e Math objec t on page 1 53.
Note that JavaScript obje cts are dyna m ic, which means that you can add properties to
objects on the fly later in the program. For example, you can easily add a new entry to
the englGerm dictionary like this:
var english = prompt("Enter an English word.");
var german = prompt("Enter the German translation.");
englGerm[english] = german; //Adds and sets a new property
Since we’re going to get more and more involved with objects, let me po int out that
JavaScript basically knows three types of objects:
• First, there are native objects, which are those defined by the E CMAScript spec-
ification. For example, arrays, functions, and dates are all native objects.
208 Meeting 11. Building Your Own Objects
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