Because you can use them to enforce privacy, closures are useful for encapsulating functionality and managing scope. Closures are quite powerful, but they have one important drawback: because a closure requires the browser to keep memory allocated for the function and its scope, once a closure is no longer needed, the browser sometimes might not return all of the memory to the system. The primary symptom of this would be a memory leak: as the script continues to execute in the browser, the browser consumes more and more memory, eventually consuming all available memory.
Older browses had serious problems with memory leaks due to closures. Modern browsers are much more efficient but can still have problems. One of Those Weak Types, Eh? As we mentioned earlier, JavaScript is weakly typed. This means that if there is a type mismatch problem in an expression, JavaScript will resolve it according to its own rules. Consider as an example the code shown in Listing But JavaScript resolves the type mismatch itself, allowing the program to continue without crashing.
Many people consider weak typing to be a disadvantage, and indeed it is easy for novice JavaScript developers to make plenty of mistakes with weak typing. JavaScript uses these data types as the basis of all of its type management. To determine the type of anything in JavaScript, use the typeof operator, as demonstrated in Listing see Chapter 7 for full details on the typeof operator. Yes, in JavaScript, arrays are objects. In addition, JavaScript has the concept of primitives: non-object simple values.
JavaScript primitives are the building blocks from which the more complex data types are built. Functions, for example, are objects. Consider the example shown in Listing But why? We can do that because, behind the scenes, JavaScript is converting our primitive value to its associated object, thus giving us access to all of the properties and methods of String.
Type Conversion in JavaScript Now that we have all of the basics defined, we can look at how JavaScript actually handles type mismatches. JavaScript has a set of functions it uses to handle conversion from one type to another: toPrimitive , toNumber , and toBoolean. The toPrimitive method takes an input argument, and can also take an optional preferredType argument.
It converts nonprimitives that is to say, Objects to their closest associated primitive type. If the input argument can decompose to more than one primitive type, the preferredType argument can be used to specify which one to choose. Table summarizes the rules that toPrimitive follows, depending on its input argument type: Table Rules for toPrimitive input Argument Type Result Object If valueOf returns a primitive, then return that; else, if toString returns a primitive value, then return that; else, throw an error Everything else No change The toNumber method takes an input argument and tries to convert it to a Number, as shown in Table Table In the case of the conditional, Expression is reduced to a boolean using toBoolean.
In the if statement, we see JavaScript applying toBoolean on "Primitive String", which evaluates to true, so execution moves into the code block. Now that we understand the rules, the results of Listing actually makes perfect sense. Consider the common code pattern shown in Listing , in which we are trying to provide a default for an argument in a function. Listing shows a better way to write this code. The first pattern is a syntax pattern that you will see used all the time in JavaScript, and which you will use many times yourself.
The second is an implementation pattern that is likewise very common, and will help you begin organizing your JavaScript into manageable modules. Is it possible to define a function and then immediately execute it without having to call it separately? So, if we just place a pair of parentheses after a function declaration, will that execute it? For example, would the code in Listing work?
When the JavaScript interpreter sees the keyword function, it assumes what follows is a declaration to be added to the scope and not an expression to be evaluated. You have to explicitly tell the interpreter that your function is an expression to be evaluated, and you do this by putting parentheses around it, as shown in Listing Everything within the expression is private unless you specifically return something to the global scope, which makes it a lot easier to keep your global scope clear of unneeded clutter.
The immediately invoked anonymous function pattern is used throughout JavaScript development, most notably in the module pattern. The Module Pattern Imagine you are working on a large JavaScript application with many other developers. You need a way to encapsulate sections of code so that they can have a private namespace, so that you can avoid conflicts with existing code.
How would you do this? With the module pattern, of course. The module pattern uses an immediately invoked function to create a closure for all of your encapsulated code. You can have private members, and you can even publish public APIs. The basic pattern is shown in Listing Within the module there is a private scope that is self-contained and safe from modification. You can even easily extend the module by reprocessing it through an immediately invoked function.
All you have to do is pass the original module into the new immediately invoked function as an argument, as shown in Listing Creating Submodules Module. Summary In this chapter we tackled head-on the things that people find most difficult about JavaScript. You should also be familiar now with immediately executed function expressions and the module pattern, and how the module pattern employs closure to maintain scope and enforce privacy. With this chapter under your belt, you are ready to dig deeper into the nuts and bolts of JavaScript.
In the next chapter we will cover the details that we glossed over in this chapter, from expressions and statements to objects, and all the way to functions and flow control. We delved quite deep into a few of the concepts that people struggle with when learning the language. We will begin by reviewing some basic matters of formatting JavaScript code, especially as related to the examples in this book.
Then we will cover expressions and statements, the two most basic building blocks of JavaScript from which all JavaScript programs are built. With that groundwork laid, we can then discuss creating more complex statements with operators.
We will talk about variables and how to manage them in your JavaScript programs. Then we will discuss objects and arrays, which will give you the building blocks for everything else. Then we will have an in-depth discussion of functions: what they are, and how to make them, and we will gain some important insights into the dynamic nature of JavaScript. Finally, we will cover how to control our programs with conditionals and loops. You are encouraged to explore the standard itself at www. Formatting JavaScript Code Formatting code is one of the many subjects that will invariably result in a roomful of angry developers shouting at each other.
I once saw someone nearly throw a chair in the middle of an argument about indenting with spaces vs. Broadly, JavaScript uses C-like formatting. JavaScript also uses two styles of comment delimiters. Anything contained within those delimiters, regardless of new lines, is considered a comment.
To quote Section 7. Similarly, JavaScript imposes no requirements for new lines. JavaScript uses semicolons ; to terminate statements. However, semicolons can be considered optional because JavaScript interpreters practice automatic semicolon insertion ASI , which means they attempt to correct code that would be nonfunctional without semicolons by automatically inserting them as needed. As a result you can choose to write your JavaScript without using many or even any semicolons, and instead rely on ASI.
Traditionally, it has been considered a best practice to explicitly use semicolons to terminate statements. However, with the advent of newer meta-scripting languages like CoffeeScript, many people now prefer to write terse code that employs a minimum of semicolons and instead relies on ASI as much as possible.
From a practical standpoint, either method is acceptable in that either method will help produce consistent, functional code. However, as with anything involving programming style, there have been many heated arguments recently about explicit semicolon use versus relying on ASI. If you would like to write JavaScript without semicolons, you are encouraged to review the standard so you know exactly what you are doing.
But the interpreter does not care if the code results in an error when it is executed. It only cares that the code can be executed. To illustrate this, consider the two lines of JavaScript shown in Listing Instead, the interpreter would assume we meant what you see in Listing In this example, JavaScript is unable to determine what you even mean in the left operand, let alone be able to use the result to assign a value.
This is a contrived example, but it does expose the main consideration when relying on ASI: you have to understand the rules in order to use it effectively, whereas explicitly using semicolons leaves no doubt. And not only do you have to understand the rules, anyone who will be working with you on your code will have to understand them as well.
Consistently written code is much easier to read and understand than code written with multiple bracketing styles, inconsistent indentation rules, and variable naming conventions.
When deciding which styles to use in your projects, you should pick what works best for you, your team, and your situation. Consistency is what is important. Expressions and Statements Expressions and statements are the first things to understand really well about JavaScript because they are the basic building blocks for JavaScript programs.
The distinction between expressions and statements is simple, but subtle. Expressions Conceptually, expressions are like words or phrases in a spoken language. They are the simplest building blocks of a program. In JavaScript, an expression is any section of code that resolves to a value. Since literal expressions evaluate to actual values, JavaScript supports the same broad types of expressions as it does of variables: boolean, number, string, and object. Expressions can be as simple as just stating a value, or they can be mathematical or logical operations, as shown in Listing Listing A compound expression is an expression in which one or more of the items in the expression is another expression.
Compound expressions can be as complex and as nested as needed, as in Listing Listing Those expressions are included in a single logical AND expression, so if both evaluate to true, the conditional code will execute. Finally, though an expression can stand on its own, as shown in Listing , such an expression typically is not very useful. The literal expression on the third line of Listing is perfectly valid, though it is not doing anything useful.
The bottom line about expressions even compound expressions is that they only represent values. If you want to actually do anything with those values, you need to use a statement. Statements In JavaScript, a statement is a collection of one or more expressions that performs a specific action. To return to the spoken language analogy, if expressions are words and phrases, then statements are full sentences.
Conceptually, the simplest type of statement is an expression that has a side effect, such as variable assignment or a simple mathematical operation. See Listing for some examples. However, that term can confuse the subtle distinction between expressions and statements, so in this book we will not use it. Just as JavaScript has compound expressions, it also has compound statements. A compound statement is a collection of statements in a block of code, often enclosed in curly brackets.
Excellent examples of compound statements are if statements and loops, shown in Listing Object literals, for example, are expressions, not statements, despite being multiple expressions enclosed in brackets, as you can see in Listing Operators Operators, perhaps unsurprisingly, perform operations on expressions.
Other examples include mathematical operators and most logical operators. Examples include the logical NOT operator or the mathematical negation operator. In addition, JavaScript has one ternary operator, known as the conditional operator. It takes three operands and performs a conditional test: conditionalExpression?
This category includes the conditional operator and operators such as the void operator and the comma operator. However, there is one important operator concept we want to cover here: precedence. Precedence If you have multiple operators in one statement, how do you determine the order in which to execute them?
Do you evaluate them strictly left to right? Are there other rules? Consider the example in Listing involving mathematical operators. If you evaluate the statement in Listing from left to right, performing each operation as you come to it, you end up with However, the example actually alerts 14, because some operators are evaluated before others according to a set of rules known as precedence.
In this example, multiplication has a higher precedence than addition or subtraction, so that statement is actually evaluated as shown in Listing , which uses parentheses to indicate precedence explicitly by grouping together the operations as they are actually evaluated. Listing provides another example, involving just addition and subtraction, two operators that are of the same precedence. It depends on the order in which you execute the operations. When you have multiple operators of the same precedence together, they will evaluate according to their associativity: either left to right, or right to left.
In the case of mathematical operators, they are all evaluated left to right, so the value of myVar is Because JavaScript has more than just mathematical operators, it has slightly more complex precedence rules than those that come with mathematics, as you can see in Table Even so, many JavaScript best practices and style guides recommend that, for complex statements with multiple operators, you explicitly state with parentheses the precedence you are intending.
Generally, that makes for more readable code and easier maintenance, though if you have an extremely complex statement, you can end up with lots of parentheses. In that case, it might be worthwhile to break up the single statement into one or more statements, to be fully explicit and reduce the overall number of parentheses.
Variables Broadly speaking, a variable is a named storage location with an associated value. You access the value associated with the storage location by using the name. Each language has its own implementation of variables: how to declare them, what their scope is, and how they are managed. You can use the var keyword for each variable, or you can use the var keyword once and separate the variable declarations with commas.
It also helps JavaScript code compressors, which will take the list of variables and run search and replace on each item to change variable names to single- or double-letter names, thus further reducing the size of the file.
Understanding Variable Scope in JavaScript Just as each language has rules for creating variables, each language has rules that govern where variables can be accessed. This is known as variable scope. As mentioned in Chapter 1, JavaScript has functional scope: when you formally declare a variable using the var keyword, it is limited in scope to the current functional scope and all of the functional scopes contained within the current functional scope.
In other words, if you declare a variable within a given scope, you will be able to access it within a subscope but not within any containing scope. Listing provides an example to illustrate this concept. When you declare a variable in a particular scope, that scope is often referred to as the local scope for that variable.
As you nest functions within one another, you create nested functional scopes that are often referred to as scope chains. This is often referred to as a scope chain lookup, or sometimes just scope lookup. The topmost scope of any JavaScript program is called the global scope.
Any variable declared in the global scope will be available to all scopes in the program, as demonstrated in Listing This essentially creates a new variable limited in scope to that functional scope; this is often referred to as local scope precedence. To demonstrate local scope precedence, see Listing If you attempt to access JavaScript variables before their initialization, you will get an undefined value, but they will be there, and the script will not throw an error.
This can be quite unexpected behavior, especially in the case of overriding variables that have been declared in a higher scope as illustrated in Listing This makes its new value available everywhere within that scope, effectively erasing the value of the variable from the higher scope everywhere within that function. In other words, when a new scope is created, JavaScript immediately declares all of the local variables before doing anything else, including assignments and function calls.
As a result, Listing is parsed as if it were written as show in Listing Because of variable hoisting, many JavaScript best practices and style guides recommend defining all variables at the beginning of their scope before they are accessed, thus explicitly stating what hoisting does invisibly. If you access a variable without declaring it using the var keyword, JavaScript will still perform a scope chain lookup.
If it reaches the global scope and still has not found the variable declaration, it will assume the variable is meant to be global in scope and will add it there.
This is known as implied global scope, an example of which is shown in Listing Managing Variables in JavaScript JavaScript tries to make variable management as easy as possible for the programmer. The interpreter will manage the variables itself, deallocating their memory when all references and any closures are completely finished.
As mentioned in Chapter 1, JavaScript is a weakly typed language, which means it will manage variable type mismatches in expressions according to a specific set of rules. In addition, JavaScript employs the concept of primitives: non-object, simple variables, which themselves can be booleans, numbers, or strings.
Anything that is not a primitive is an object—though JavaScript will transparently change primitives to their associated object types and back again as needed. When it comes to copying variables, JavaScript handles primitives and objects differently. Primitives are passed from one variable instance to another directly. Objects, on the other hand, are passed by reference: setting a new variable equal to an existing object does not copy that object wholesale into the new variable; rather, it only makes the new variable a pointer to the original object.
See Listing for an example. Objects In just about every object-oriented programming language, an object is a collection of properties, and JavaScript is no different. Properties can be either primitives or other objects, including functions. JavaScript objects can be arbitrarily deep, meaning you can have objects that have properties that are objects, which in turn have properties that are objects, and so on, as deeply as you wish.
Inheritance As covered in detail in Chapter 1, JavaScript uses prototypal inheritance rather than classes. Each object has a special prototype property that serves as a pointer to the object from which it was created. When you attempt to access a property on the object, the interpreter checks to see if the desired property exists within the current object.
If the property does not exist, the interpreter checks the prototype. See Chapter 1 for details and examples of prototypal inheritance. Accessing Properties and Enumeration JavaScript provides two ways of accessing properties on objects, as demonstrated in Listing IdentifierName CallExpression. ECMA Edition 5. As an example, consider the need to enumerate all the properties of an object.
Instead, you just query the object for each of its properties and access their values using brackets, as shown in Listing We build a string containing each property and its associated value, one per line, and then alert the string.
This will only include the noninherited properties of an object. See Chapter 5 for details on these two methods and how they differ. Now we can enumerate an object as shown in Listing Using Constructor Functions The traditional method for creating new JavaScript objects is to create a constructor function and use it to make new objects as desired.
To make a constructor function, you simply create a function as you ordinarily would, and add properties to it as needed, as shown in Listing To create a new instance from the constructor, use the new operator, as shown in Listing It sets that new object as the execution scope of the operand so within the operand, the this keyword refers to the new empty object , 3.
It invokes the operand, so the operand can then modify the new object as needed, 4. It returns the value that the operand returns, or if the operand does not return anything, it automatically returns the new object it created in step 1 and that the operand modified in step 3. You can continue down this road and fully emulate classes in JavaScript using this method in combination with others.
Literal notation is a way for you to provide values literally for an object during creation. Literal notation is thus the best way to create singletons in JavaScript. Objects created this way can still be extended later by adding properties and methods as desired, as shown in Listing Extending an Object myObjectLiteral. As shown in Listing , the method takes an object as its parameter and returns a new object with the parameter object as its prototype.
Using Object. Which method to use is largely a matter of taste, though sometimes the choice will be dictated by convention or situation. Some JavaScript libraries, for example, make heavy use of Object. Or, if you are doing extensive work with JSON, you might find it makes more sense to use literals to manage your singletons. And if you really feel you need classlike behavior in your JavaScript programs, then constructor functions are the easiest way to get there.
Arrays In JavaScript, as in most languages, arrays are essentially indexed data structures, with a value associated with each index. This means that you can add and remove items from arrays, and the length of the arrays will change as needed. Thus, you cannot generate boundary errors when you are adding or removing items from arrays. And if you attempt to access an element that does not exist, the interpreter will return undefined instead of throwing an error.
A common mistake in JavaScript is to attempt to retrieve something from an array that does not exist, resulting in undefined, and then attempting to do something with that value without first checking to see if it is undefined.
Depending on what you attempt to do with the value, the interpreter might throw an error at that point, but it will not throw an error at the point when the array was accessed out of bounds. JavaScript arrays have a length property, which contains a number that indicates the length of the array. As elements are added to and removed from the array, this number increases or decreases as needed. The length property can also be set directly, as shown in Listing ; doing so will either remove existing items from or add undefined items to the end of the array, as appropriate.
Data types can be mixed within arrays, too, meaning you can have an array made up of an object, a boolean, a number, and a string. Array values are accessed using the square bracket notation described for objects: the name of the array, followed by a set of square brackets that contain the index of the desired value; for example, in Listing , myArray[2] will access the third item in the array myArray.
You can add properties to arrays just like you can any other object, as demonstrated in Listing Otherwise, it will use the supplied value as the key for a new property on the array object itself.
If you add something to an array with a non-numeric index, as shown in Listing , you are simply adding it as a property on the array object itself, not adding elements to the array. Array Elements vs. Creating arrays using literal notation is similar to creating objects with literal notation. Listing demonstrates that you can have multiple data types within a single array. You can even have objects as your array values, just as you can have arrays as properties of objects.
JavaScript also supports multidimensional arrays, as arrays of arrays, as shown in Listing The most common way to iterate over an array is in a for loop, as shown in Listing Each time through the loop, JavaScript will alert the value stored at that index. It may takes up to minutes before you received it.
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Joe Lewis , Meitar Moscovitz. Packt Publishing. Complete contents include: JavaScript quick start: Familiar with object-oriented programming? This part helps you learn JavaScript quickly and properly.
Pick a topic and jump in. Tips, tools, and libraries: Survey existing style guides, best practices, advanced techniques, module systems, package managers, build tools, and learning resources. Author : Jason D.
This book begins with an accelerated introduction to the newest features of JavaScript so that experienced Web developers can quickly understand the concepts of JavaScript and begin developing their own JavaScript solutions immediately. Beyond the brief descriptions and short syntax snippets found in most references, this book also provides real-life, well-commented JavaScript examples for each documented object, property, method, and event handler.
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