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Following the chart, you can see that the variables a and b are assigned to anonymous functions created within the scope of another function called foo(); When this happens, the variables in global scope will now actually be live references to the memory location inside foo(); even after the function has finished executing. Calling a(); and b(); will execute the contents of the anonymous functions assigned to them within foo(); Remember that by simply defining foo(); these associates are not yet made. The function foo() must be executed at least to make these connections. Now this is where it gets more interesting. The functions a(); and b(); are assigned within foo() and can now be used outside of foo(). But what do these functions do? They manipulate a variable defined inside the function scope of the function foo() itself. This means that when we call a(); or b(); in the global scope, we are still working with the references to the variable x, which itself resides within the scope of the function foo(). A good metaphor for this would be that a(); and b(); are sort of like tweezers that you use to reach inside the scope of foo(); from the global scope. These functions can manipulate memory (in this case the variable called x) defined inside foo(). This functionality is amazingly useful to make our objects persist. Objects such as variables and functions can be stored within another “container” function such as foo(); in this example. But wait, isn’t foo() also an object, as we discussed earlier? It is. So you can see now, how objects can contain other objects inside them. This cascading property of objects is very important. Storing objects inside other objects is an incredibly useful concept used to design computer software in any language that supports objects (aka, Object-oriented programming languages.) JavaScript is not fully an Object-Oriented Programming (OOP) language, but it does support this “matryoshka”-like quality of objects, where they can be stored within one another. Without this principle, function closures would not make much sense. 23
What Exactly is a JavaScript Variable? Learning isn’t always about explaining new complex subjects. Sometimes it’s about bringing to your attention the deeper details of the already known. In-depth understanding makes programming more interesting. It’s easy to think that you know everything about variables. But there are many details that often slip from the viewfinder of an untrained eye. While working on Revision Two of this book, I have added more chapters to the JavaScript Bootcamp to talk more about the basics of JavaScript programming. One of the subjects deals with JavaScript variables. One of the goals with my writing is to give a deeper insight into simple concepts. I believe that by reaching down to the very reason for why something works a certain way the reader is given an opportunity to become a better JavaScript programmer. When teaching a computer language such as JavaScript numerous tutorials talk about using variables in terms of showing how they work. But little is said about what they are. We are often told that we can define variables using the var keyword. But understanding what they are and where they come from gives us a deeper insight into JavaScript programming. In fact, many JavaScript concepts, such as objects, are much easier to understand if we can compare them with variables. Are They Variables or Objects? * * * Because everything in JavaScript is an object this sometimes creates confusion among the beginners who are seeking to understand JavaScript data types. If everything is an object, then what is a variable? And if a variable is indeed an object, then why are we using the keyword named var to define it? Wouldn’t something like obj be more appropriate? The answers to such questions are often determined by creators of the language specification itself. The fact that someone decided to create a specification does not guarante that this specification will be ideal. Regardless, when it comes to any data in any language, it is always tied to the same concept: all data is stored in computer memory on the microprocessor. Keep in mind that all variables are physically stored in computer memory at random addresses. When we access a variable, we access the part of computer memory it is stored in. A variable is stored at a unique and random memory address chosen by the operating system. We’ll get to how variables are physically stored in computer memory in just a moment. 24
Page 1 and 2: “ Second Revision. July 2012 Unde
Page 3 and 4: Prototype, JavaScript Objects and J
Page 5 and 6: Thought transcends matter. When we
Page 7 and 8: Preface The purpose of this book is
Page 9 and 10: A Lesson in Creativity The web deve
Page 11 and 12: I love jQuery because it allows me
Page 13 and 14: Improving Page Load Performance The
Page 15 and 16: Determining the Placement of Intern
Page 17 and 18: JavaScript 1. The only language peo
Page 19 and 20: What is JavaScript? Web designers u
Page 21 and 22: As you can see, variables can be as
Page 23 and 24: The example above will first execut
Page 25 and 26: Only variables and events should be
Page 27: Notice that variables a and b are c
Page 31 and 32: A variable allocates enough data st
Page 33 and 34: Do you want to really understand pr
Page 35 and 36: What is a Variable? Variables can b
Page 37 and 38: In order to understand bitwise logi
Page 39 and 40: At first this might seem confusing.
Page 41 and 42: Let’s display the variable “a
Page 43 and 44: What’s important is the return va
Page 45 and 46: Inserting Form Values into MySQL Da
Page 47 and 48: I like code that you can “read”
Page 49 and 50: An object is created from a class.
Page 51 and 52: Dynamic Prototype-based Objects in
Page 53 and 54: Without knowing anything at all abo
Page 55 and 56: More About JSON JSON is a data inte
Page 57 and 58: In some programming languages the t
Page 59 and 60: JavaScript from scratch. If you are
Page 61 and 62: property access operator - the . (d
Page 63 and 64: Why does jQuery do exactly the same
Page 65 and 66: Learning jQuery The greatest thing
Page 67 and 68: If this sounds like you, then you w
Page 69 and 70: Web App Initialization To initializ
Page 71 and 72: In JavaScript, we can define variab
Page 73 and 74: As mentioned above, to an experienc
Page 75 and 76: Including jQuery In Your Source Cod
Page 77 and 78: Including jQuery as an External CDN
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Including jQuery UI In the Starting
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What Kind of Elements Can You Selec
Page 83:
To traverse the DOM and make sure t
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Here is a basic inline CSS example
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Because the DIV element has ID of
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After selecting all of these input
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Figure 1 Mentality of a beginner jQ
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Whose Children Are These? If we are
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Next, let’s explore the possibili
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jQuery Events Because jQuery is so
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Of course, the effects of binding a
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Using toggle() Events Toggling betw
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Speed Up Your jQuery Code jQuery is
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Let’s now look at the jQuery way
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Practical jQuery Lessons Instead of
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There is a solution to this problem
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jQuery provides a secondary way of
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Lesson 3: Callback Functions All jQ
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For example, the following statemen
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As you may know an object can conta
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Notice that when we do it this way,
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Alternatively, we can create the pa
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if (“light”.match(/darkness/) =
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jQuery and Regular Expressions In p
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This is probably as simple as it ge
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The following example will demonstr
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Making Good CSS Buttons This tutori
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Sometimes it makes sense to make th
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1. Specify javascript: return false
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Our goal is to match the button wit
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So far, we have briefly discussed t
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Note from the Author Looks like you
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