A re-introduction to JavaScript

Overview

JavaScript is an object oriented dynamic language; it has types and operators, core objects, and methods. Its syntax comes from the Java and C languages, so many structures from those languages apply to JavaScript as well. One of the key differences is that JavaScript does not have classes; instead, the class functionality is accomplished by object prototypes. The other main difference is that functions are objects, giving functions the capacity to hold executable code and be passed around like any other object.

Let's start off by looking at the building block of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript's types are:

• Numbers
• Strings
• Booleans
• Functions
• Objects

... oh, and Undefined and Null, which are slightly odd. And Arrays, which are a special kind of object. And Dates and Regular Expressions, which are objects that you get for free. And to be technically accurate, functions are just a special type of object. So the type diagram looks more like this:

• Number
• String
• Boolean
• Object
  • Function
  • Array
  • Date
  • RegExp
• Null
• Undefined

And there are some built in Error types as well. Things are a lot easier if we stick with the first diagram, though.

Numbers

Numbers in JavaScript are "double-precision 64-bit format IEEE 754 values", according to the spec. This has some interesting consequences. There's no such thing as an integer in JavaScript, so you have to be a little careful with your arithmetic if you're used to math in C or Java. Watch out for stuff like:

0.1 + 0.2 = 0.30000000000000004

In practice, integer values are treated as 32-bit ints (and are stored that way in some browser implementations), which can be important for bit-wise operations. For details, see The Complete JavaScript Number Reference.

The standard numeric operators are supported, including addition, subtraction, modulus (or remainder) arithmetic and so forth. There's also a built-in object that I forgot to mention earlier called Math to handle more advanced mathematical functions and constants:

You can convert a string to an integer using the built-in parseInt() function. This takes the base for the conversion as an optional second argument, which you should always provide:

If you don't provide the base, you can get surprising results:

That happened because the parseInt function decided to treat the string as octal due to the leading 0.

If you want to convert a binary number to an integer, just change the base:

Similarly, you can parse floating point numbers using the built-in parseFloat() function which uses base 10 always unlike its parseInt() cousin.

You can also use the unary + operator to convert values to numbers:

> + "42"
42 

A special value called NaN (short for "Not a Number") is returned if the string is non-numeric:

NaN is toxic: if you provide it as an input to any mathematical operation the result will also be NaN:

You can test for NaN using the built-in isNaN() function:

JavaScript also has the special values Infinity and -Infinity:

You can test for Infinity, -Infinity and NaN values using the built-in isFinite() function:

Strings

Strings in JavaScript are sequences of characters. More accurately, they're sequences of Unicode characters, with each character represented by a 16-bit number. This should be welcome news to anyone who has had to deal with internationalisation.

If you want to represent a single character, you just use a string of length 1.

To find the length of a string, access its length property:

There's our first brush with JavaScript objects! Did I mention that strings are objects too? They have methods as well:

Other types

JavaScript distinguishes between null, which is an object of type 'object' that indicates a deliberate non-value, and undefined, which is an object of type 'undefined' that indicates an uninitialized value — that is, a value hasn't even been assigned yet. We'll talk about variables later, but in JavaScript it is possible to declare a variable without assigning a value to it. If you do this, the variable's type is undefined.

JavaScript has a boolean type, with possible values true and false (both of which are keywords). Any value can be converted to a boolean according to the following rules:

1. false, 0, the empty string (""), NaN, null, and undefined all become false
2. all other values become true

You can perform this conversion explicitly using the Boolean() function:

However, this is rarely necessary, as JavaScript will silently perform this conversion when it expects a boolean, such as in an if statement (see below). For this reason, we sometimes speak simply of "true values" and "false values," meaning values that become true and false, respectively, when converted to booleans. Alternatively, such values can be called "truthy" and "falsy," respectively.

Boolean operations such as && (logical and), || (logical or), and ! (logical not) are supported; see below.

Variables

New variables in JavaScript are declared using the var keyword:

If you declare a variable without assigning any value to it, its type is undefined. 

An important difference from other languages like Java is that in JavaScript, blocks do not have scope; only functions have scope. So if a variable is defined using var in a compound statement (for example inside an if control structure), it will be visible to the entire function.

Operators

JavaScript's numeric operators are +, -, *, / and % - which is the remainder operator. Values are assigned using =, and there are also compound assignment statements such as += and -=. These extend out to x = x operator y.

You can use ++ and -- to increment and decrement respectively. These can be used as prefix or postfix operators.

The + operator also does string concatenation:

If you add a string to a number (or other value) everything is converted in to a string first. This might catch you out:

Adding an empty string to something is a useful way of converting it.

Comparisons in JavaScript can be made using <, >, <= and >=. These work for both strings and numbers. Equality is a little less straightforward. The double-equals operator performs type coercion if you give it different types, with sometimes interesting results:

To avoid type coercion, use the triple-equals operator:

There are also != and !== operators.

JavaScript also has bitwise operations. If you want to use them, they're there.

Control structures

JavaScript has a similar set of control structures to other languages in the C family. Conditional statements are supported by if and else; you can chain them together if you like:

JavaScript has while loops and do-while loops. The first is good for basic looping; the second for loops where you wish to ensure that the body of the loop is executed at least once:

JavaScript's for loop is the same as that in C and Java: it lets you provide the control information for your loop on a single line.

The && and || operators use short-circuit logic, which means whether they will execute their second operand is dependent on the first. This is useful for checking for null objects before accessing their attributes:

Or for setting default values:

JavaScript has a ternary operator for conditional expressions:

The switch statement can be used for multiple branches based on a number or string:

If you don't add a break statement, execution will "fall through" to the next level. This is very rarely what you want — in fact it's worth specifically labelling deliberate fallthrough with a comment if you really meant it to aid debugging:

The default clause is optional. You can have expressions in both the switch part and the cases if you like; comparisons take place between the two using the === operator:

Objects

JavaScript objects are simply collections of name-value pairs. As such, they are similar to:

• Dictionaries in Python
• Hashes in Perl and Ruby
• Hash tables in C and C++
• HashMaps in Java
• Associative arrays in PHP

The fact that this data structure is so widely used is a testament to its versatility. Since everything (bar core types) in JavaScript is an object, any JavaScript program naturally involves a great deal of hash table lookups. It's a good thing they're so fast!

The "name" part is a JavaScript string, while the value can be any JavaScript value — including more objects. This allows you to build data structures of arbitrary complexity.

There are two basic ways to create an empty object:

And:

These are semantically equivalent; the second is called object literal syntax, and is more convenient. This syntax is also the core of JSON format and should be preferred at all times.

Once created, an object's properties can again be accessed in one of two ways:

And...

These are also semantically equivalent. The second method has the advantage that the name of the property is provided as a string, which means it can be calculated at run-time though using this method prevents some JavaScript engine and minifier optimizations being applied. It can also be used to set and get properties with names that are reserved words:

Object literal syntax can be used to initialise an object in its entirety:

Attribute access can be chained together:

Arrays

Arrays in JavaScript are actually a special type of object. They work very much like regular objects (numerical properties can naturally be accessed only using [] syntax) but they have one magic property called 'length'. This is always one more than the highest index in the array.

The old way of creating arrays is as follows:

A more convenient notation is to use an array literal:

Leaving a trailing comma at the end of an array literal is inconsistent across browsers, so don't do it.

Note that array.length isn't necessarily the number of items in the array. Consider the following:

Remember — the length of the array is one more than the highest index.

If you query a non-existent array index, you get undefined:

If you take the above into account, you can iterate over an array using the following:

This is slightly inefficient as you are looking up the length property once every loop. An improvement is this:

An even nicer idiom is:

Here we are setting up two variables. The assignment in the middle part of the for loop is also tested for truthfulness — if it succeeds, the loop continues. Since i is incremented each time, items from the array will be assigned to item in sequential order. The loop stops when a "falsy" item is found (such as undefined).

Note that this trick should only be used for arrays which you know do not contain "falsy" values (arrays of objects or DOM nodes for example). If you are iterating over numeric data that might include a 0 or string data that might include the empty string you should use the i, len idiom instead.

Another way to iterate is to use the for...in loop. Note that if someone added new properties to Array.prototype, they will also be iterated over by this loop:

If you want to append an item to an array, the safest way to do it is like this:

Since a.length is one more than the highest index, you can be assured that you are assigning to an empty position at the end of the array.

Arrays come with a number of methods:

a.toString(), a.toLocaleString(), a.concat(item, ..), a.join(sep),
a.pop(), a.push(item, ..), a.reverse(), a.shift(), a.slice(start, end),
a.sort(cmpfn), a.splice(start, delcount, [item]..), a.unshift([item]..)

• concat returns a new array with the items added on to it.
• pop removes and returns the last item
• push adds one or more items to the end (like our ar[ar.length] idiom)
• slice returns a sub-array
• sort takes an optional comparison function
• splice lets you modify an array by deleting a section and replacing it with more items
• unshift prepends items to the start of the array

Functions

Along with objects, functions are the core component in understanding JavaScript. The most basic function couldn't be much simpler:

This demonstrates everything there is to know about basic functions. A JavaScript function can take 0 or more named parameters. The function body can contain as many statements as you like, and can declare its own variables which are local to that function. The return statement can be used to return a value at any time, terminating the function. If no return statement is used (or an empty return with no value), JavaScript returns undefined.

The named parameters turn out to be more like guidelines than anything else. You can call a function without passing the parameters it expects, in which case they will be set to undefined.

You can also pass in more arguments than the function is expecting:

That may seem a little silly, but functions have access to an additional variable inside their body called arguments, which is an array-like object holding all of the values passed to the function. Let's re-write the add function to take as many values as we want:

That's really not any more useful than writing 2 + 3 + 4 + 5 though. Let's create an averaging function:

This is pretty useful, but introduces a new problem. The avg() function takes a comma separated list of arguments — but what if you want to find the average of an array? You could just rewrite the function as follows:

But it would be nice to be able to reuse the function that we've already created. Luckily, JavaScript lets you call a function and call it with an arbitrary array of arguments, using the apply() method of any function object.

The second argument to apply() is the array to use as arguments; the first will be discussed later on. This emphasizes the fact that functions are objects too.

JavaScript lets you create anonymous functions.

This is semantically equivalent to the function avg() form. It's extremely powerful, as it lets you put a full function definition anywhere that you would normally put an expression. This enables all sorts of clever tricks. Here's a way of "hiding" some local variables — like block scope in C:

JavaScript allows you to call functions recursively. This is particularly useful for dealing with tree structures, such as you get in the browser DOM.

This highlights a potential problem with anonymous functions: how do you call them recursively if they don't have a name? The answer lies with the arguments object, which in addition to acting as a list of arguments also provides a property called arguments.callee. The arguments.callee usage is depreciated and even disallowed in strict mode. Instead, you should use "named anonymous functions" as below:

The name provided to an anonymous function as above is(or at least should be) only availabe to the function's own scope. This both allows more optimizations to be done by the engine and a more readable code.

Custom objects

In classic Object Oriented Programming, objects are collections of data and methods that operate on that data. JavaScript is a prototype-based language which contains no class statement, such as is found in C++ or Java. (This is sometimes confusing for programmers accustomed to languages with a class statement.) Instead, JavaScript uses functions as classes. Let's consider a person object with first and last name fields. There are two ways in which the name might be displayed: as "first last" or as "last, first". Using the functions and objects that we've discussed previously, here's one way of doing it:

This works, but it's pretty ugly. You end up with dozens of functions in your global namespace. What we really need is a way to attach a function to an object. Since functions are objects, this is easy:

There's something here we haven't seen before: the 'this' keyword. Used inside a function, 'this' refers to the current object. What that actually means is specified by the way in which you called that function. If you called it using dot notation or bracket notation on an object, that object becomes 'this'. If dot notation wasn't used for the call, 'this' refers to the global object. This is a frequent cause of mistakes. For example:

When we call fullName(), 'this' is bound to the global object. Since there are no global variables called first or last we get undefined for each one.

We can take advantage of the 'this' keyword to improve our makePerson function:

We've introduced another keyword: 'new'. new is strongly related to 'this'. What it does is it creates a brand new empty object, and then calls the function specified, with 'this' set to that new object. Functions that are designed to be called by 'new' are called constructor functions. Common practise is to capitalise these functions as a reminder to call them with new.

Our person objects are getting better, but there are still some ugly edges to them. Every time we create a person object we are creating two brand new function objects within it — wouldn't it be better if this code was shared?

That's better: we are creating the method functions only once, and assigning references to them inside the constructor. Can we do any better than that? The answer is yes:

Person.prototype is an object shared by all instances of Person. It forms part of a lookup chain (that has a special name, "prototype chain"): any time you attempt to access a property of Person that isn't set, JavaScript will check Person.prototype to see if that property exists there instead. As a result, anything assigned to Person.prototype becomes available to all instances of that constructor via the this object.

This is an incredibly powerful tool. JavaScript lets you modify something's prototype at any time in your program, which means you can add extra methods to existing objects at runtime:

Interestingly, you can also add things to the prototype of built-in JavaScript objects. Let's add a method to String that returns that string in reverse:

Our new method even works on string literals!

As I mentioned before, the prototype forms part of a chain. The root of that chain is Object.prototype, whose methods include toString() â€” it is this method that is called when you try to represent an object as a string. This is useful for debugging our Person objects:

Remember how avg.apply() had a null first argument? We can revisit that now. The first argument to apply() is the object that should be treated as 'this'. For example, here's a trivial implementation of 'new':

This isn't an exact replica of new as it doesn't set up the prototype chain. apply() is difficult to illustrate — it's not something you use very often, but it's useful to know about.

apply() has a sister function named call, which again lets you set 'this' but takes an expanded argument list as opposed to an array.

Inner functions

JavaScript function declarations are allowed inside other functions. We've seen this once before, with an earlier makePerson() function. An important detail of nested functions in JavaScript is that they can access variables in their parent function's scope:

This provides a great deal of utility in writing more maintainable code. If a function relies on one or two other functions that are not useful to any other part of your code, you can nest those utility functions inside the function that will be called from elsewhere. This keeps the number of functions that are in the global scope down, which is always a good thing.

This is also a great counter to the lure of global variables. When writing complex code it is often tempting to use global variables to share values between multiple functions — which leads to code that is hard to maintain. Nested functions can share variables in their parent, so you can use that mechanism to couple functions together when it makes sense without polluting your global namespace — 'local globals' if you like. This technique should be used with caution, but it's a useful ability to have.