A re-introduction to JavaScript (JS tutorial)

Pengantar

Mengapa dilakukan pengenalan ulang? Karena JavaScript menjadi bahasa pemprograman yang paling salah dipahami di dunia. Sering kali JavaScript dicemooh sebagai sebuah mainan,  tetapi dibalik kesederhanaannya yang menipu tersebut, terdapat beberapa fitur yang sangat handal. Saat ini JavaScript digunakan oleh aplikasi high-profile yang luar biasa banyaknya, hal ini menunjukan pengetahuan mendalam untuk teknologi ini adalah salah satu skill penting untuk para pengembang web atau mobile.

Mengetahui sekilas tentang sejarah bahasa pemograman JavaScript sangat berguna. JavaScript dibuat pada tahun 1995 oleh Brendan Eich ketika dia menjadi seorang praktisi di Netscape. Pertama kali JavaScript dirilis bersamaan dengan Netscape 2 di awal tahun 1996. Pada awalnya dinamakan LiveScript, tetapi diganti karena keputusan pemasaran yang  mencoba memanfaatkan ketenaran bahasa pemograman Java dari Sun Microsystem, walaupun keduanya memiliki sedikit sekali kesamaan. Hal inilah yang menjadi sumber kebingungan sejak saat itu.

Beberapa bulan kemudian, Microsoft merilis JScript yang tertanam di dalam Internet Explore 3. Sebagian besar JScript kompatible dan mirip dengan JavaScript. Beberapa bulan setelahnya Netscape mengajukan JavaScript ke  Ecma International, Organisasi Eropa yang melakukan standarisasi sistem informasi dan komunikasi, yang kemudian menghasilkan edisi pertama dari standar ECMAScript pada tahun itu. Standar ini memperoleh pembaharuan yang signifikan pada ECMAScript edisi ke-3 pada tahun 1999, dan sejak saat itu standar tersebut tidak terlalu banyak mengalami perubahan. Edisi ke-4 ECMAScript diabaikan dengan alasan perbedaan pandangan terhadap kompleksitas bahasa. Beberapa bagian dari edisi ECMAScript ke-4  membentuk dasar ECMAScript edisi ke-5, yang dipublikasi pada tahun 2009 and edisi ECMAScript ke-6 dirilis pada bulan Juni tahun 2015.

Tidak seperti kebanyakan bahasa pemograman, JavaScript tidak mempunyai konsep input atau output (I/O). Didesain untuk dapat dieksekusi sebagai bahasa scripting pada lingkungan pengguna dan mekanisme komunikasi dengan dunia luar diserahkan kepada lingkungan pengguna. Browser adalah lingkungan pengguna yang sangat umum, tetapi Interpreter JavaScript dapat juga ditemukan ditempat lain, termasuk Adobe Acrobat, Adobe Photshop, SVG images, Yahoo Widget engine, lingkungan server seperti Node.js, database NoSQL  seperti Apache CouchDB, komputer tertanam, lingkungan desktop seperti GNOME (salah satu antarmuka yang paling terkenal pada sustem operasi GNU/Linux), dan masih banyak lagi yang lainnya.
 

Ikhtisar

JavaScript adalah bahasa pemograman dinamis berorientasi objek dengan types dan operator, objek bawaan standar (objek literal),  dan methods. Syntax JavaScript dipengaruhi oleh bahasa Java dan C - sangat banyak struktur dalam bahasa tersebut yang digunakan dalam JavaScript. Perbedaan yang paling mendasar adalah JavaScript tidak mempunyai kelas, melainkan fungsionalitas kelas yang dicapai dengan obyek prototipe (Lebih lanjut tentang ES6 Classes). Perbedaan utama lainnya adalah fungsi sebagai objek, yaitu fungsi mempunyai kapasitas untuk menahan kode yang dapat dieksekusi dan dilewatkan seperti objek yang lain.

Mari mulai dengan melihat blok pembangun dari bahasa pemograman: types (tipe-tipe data). Program pada JavaScript memanipulasi nilai dan nilai tersebut termasuk ke dalam sebuah tipe. Beberapa tipe dalam JavaScript adalah:
 

• Number
• String
• Boolean
• Function
• objek
• Symbol (Baru ditambahkan pada ES6)

...ooo, ada lagi undefined and null keduanya mempunyai karakteristik yang sedikit ganjil, Array merupakan objek spesial, Date dan RegExp merupakan objek yang dapat digunakan langsung dan untuk lebih tepatnya, fungsi termasuk dalam objek spesial. Jadi, diagramnya kira-kira seperti ini:

• Number
• String
• Boolean
• Symbol (new in Edition 6)
• objek
  • Function
  • Array
  • Date
  • RegExp
• null
• undefined

Ada beberapa tipe Error bawaan juga, akan lebih memudahkan jika kita mempelajari terlebih dahulu diagram pertama, bagaimanapun juga kita akan membahas tipe - tipe yang telah dituliskan diatas.

Numbers

Menurut spesifikasi, tipe nomer pada JavaScript menggunakan "format IEEE 754 64-bit double-presisi". Hal ini menyebabkan beberapa konsekuensi, diantaranya tidak adanya tipe integer pada JavaScript. Jadi programmer harus agak teliti dengan operasi aritmatika jika sebelumnya berangkat dari C atau Java.

Perlu juga memperhatikan kasus seperti berikut:

0.1 + 0.2 == 0.30000000000000004

In practice, integer values are treated as 32-bit ints, and some implementations even store it that way until they are asked to perform an instruction that's valid on a Number but not on a 32-bit integer. This can be important for bit-wise operations.

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

Math.sin(3.5);
var circumference = Math.PI * (r + r);

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:

parseInt("123", 10); // 123
parseInt("010", 10); // 10

In older browsers, strings beginning with a "0" are assumed to be in octal (radix 8), but this hasn't been the case since 2013 or so. Unless you're certain of your string format, you can get surprising results on those older browsers:

parseInt("010");  //  8
parseInt("0x10"); // 16

Here, we see the parseInt() function treat the first string as octal due to the leading 0, and the second string as hexadecimal due to the leading "0x". The hexadecimal notation is still in place; only octal has been removed.

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

parseInt("11", 2); // 3

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

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

+ "42";   // 42
+ "010";  // 10
+ "0x10"; // 16

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

parseInt("hello", 10); // NaN

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

NaN + 5; // NaN

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

isNaN(NaN); // true

JavaScript also has the special values Infinity and -Infinity:

 1 / 0; //  Infinity
-1 / 0; // -Infinity

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

isFinite(1/0); // false
isFinite(-Infinity); // false
isFinite(NaN); // false

Strings

Strings in JavaScript are sequences of Unicode characters. This should be welcome news to anyone who has had to deal with internationalization. More accurately, they are sequences of UTF-16 code units; each code unit is represented by a 16-bit number. Each Unicode character is represented by either 1 or 2 code units.

If you want to represent a single character, you just use a string consisting of that single character.

To find the length of a string (in code units), access its length property:

"hello".length; // 5

There's our first brush with JavaScript objeks! Did we mention that you can use strings like objeks too? They have methods as well that allow you to manipulate the string and access information about the string:

"hello".charAt(0); // "h"
"hello, world".replace("hello", "goodbye"); // "goodbye, world"
"hello".toUpperCase(); // "HELLO"

Other types

JavaScript distinguishes between null, which is a value that indicates a deliberate non-value (and is only accessible through the null keyword), and undefined, which is a value 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. undefined is actually a constant.

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, empty strings (""), NaN, null, and undefined all become false.
2. All other values become true.

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

Boolean("");  // false
Boolean(234); // true

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:

var a;
var name = "simon";

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

An important difference between JavaScript and 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. However, starting with ECMAScript Edition 6, let and const declarations allow you to create block-scoped variables.

Operators

JavaScript's numeric operators are +, -, *, / and % — which is the remainder operator (which is not the same as modulo.) Values are assigned using =, and there are also compound assignment statements such as += and -=. These extend out to x = x operator y.

x += 5
x = x + 5

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

The + operator also does string concatenation:

"hello" + " world"; // "hello world"

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

"3" + 4 + 5;  // "345"
 3 + 4 + "5"; // "75"

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

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:

123 == "123"; // true
1 == true; // true

To avoid type coercion and make sure your comparisons are always accurate, you should always use the triple-equals operator:

123 === "123"; // false
1 === true;    // false

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:

var name = "kittens";
if (name == "puppies") {
  name += "!";
} else if (name == "kittens") {
  name += "!!";
} else {
  name = "!" + name;
}
name == "kittens!!"

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:

while (true) {
  // an infinite loop!
}

var input;
do {
  input = get_input();
} while (inputIsNotValid(input))

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.

for (var i = 0; i < 5; i++) {
  // Will execute 5 times
}

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 objeks before accessing their attributes:

var name = o && o.getName();

Or for setting default values:

var name = otherName || "default";

JavaScript has a ternary operator for conditional expressions:

var allowed = (age > 18) ? "yes" : "no";

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

switch(action) {
  case 'draw':
    drawIt();
    break;
  case 'eat':
    eatIt();
    break;
  default:
    doNothing();
}

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 labeling deliberate fallthrough with a comment if you really meant it to aid debugging:

switch(a) {
  case 1: // fallthrough
  case 2:
    eatIt();
    break;
  default:
    doNothing();
}

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:

switch(1 + 3) {
  case 2 + 2:
    yay();
    break;
  default:
    neverhappens();
}

objeks

JavaScript objeks can be thought of as simple 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 objek, 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 objeks. This allows you to build data structures of arbitrary complexity.

There are two basic ways to create an empty objek:

var obj = new objek();

And:

var obj = {};

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

objek literal syntax can be used to initialize an objek in its entirety:

var obj = {
  name: "Carrot",
  "for": "Max",
  details: {
    color: "orange",
    size: 12
  }
}

Attribute access can be chained together:

obj.details.color; // orange
obj["details"]["size"]; // 12

The following example creates an objek prototype, Person, and instance of that prototype, You.

function Person(name, age) {
  this.name = name;
  this.age = age;
}

// Define an objek
var You = new Person("You", 24); 
// We are creating a new person named "You" 
// (that was the first parameter, and the age..)

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

obj.name = "Simon";
var name = obj.name;

And...

obj["name"] = "Simon";
var name = obj["name"];

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. However, 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:

obj.for = "Simon"; // Syntax error, because 'for' is a reserved word
obj["for"] = "Simon"; // works fine

For more on objeks and prototypes see: objek.prototype.

Arrays

Arrays in JavaScript are actually a special type of objek. They work very much like regular objeks (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.

One way of creating arrays is as follows:

var a = new Array();
a[0] = "dog";
a[1] = "cat";
a[2] = "hen";
a.length; // 3

A more convenient notation is to use an array literal:

var a = ["dog", "cat", "hen"];
a.length; // 3

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

var a = ["dog", "cat", "hen"];
a[100] = "fox";
a.length; // 101

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

If you query a non-existent array index, you'll get a value of undefined returned:

typeof a[90]; // undefined

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

for (var i = 0; i < a.length; i++) {
  // Do something with a[i]
}

You can iterate over an array using a for...in loop. Note that if someone added new properties to Array.prototype, they will also be iterated over by this loop.  Therefore this method is "not" recommended.

Another way of iterating over an array that was added with ECMAScript 5 is forEach():

["dog", "cat", "hen"].forEach(function(currentValue, index, array) {
  // Do something with currentValue or array[index]
});

If you want to append an item to an array simply do it like this:

a.push(item);

Arrays come with a number of methods. See also the full documentation for array methods.

Functions

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

function add(x, y) {
  var total = x + y;
  return total;
}

This demonstrates a basic function. 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.

add(); // NaN 
// You can't perform addition on undefined

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

add(2, 3, 4); // 5 
// added the first two; 4 was ignored

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

function add() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum;
}

add(2, 3, 4, 5); // 14

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

function avg() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
}

avg(2, 3, 4, 5); // 3.5

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:

function avgArray(arr) {
  var sum = 0;
  for (var i = 0, j = arr.length; i < j; i++) {
    sum += arr[i];
  }
  return sum / arr.length;
}

avgArray([2, 3, 4, 5]); // 3.5

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 objek.

avg.apply(null, [2, 3, 4, 5]); // 3.5

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 objeks too.

JavaScript lets you create anonymous functions.

var avg = function() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
};

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:

var a = 1;
var b = 2;

(function() {
  var b = 3;
  a += b;
})();

a; // 4
b; // 2

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

function countChars(elm) {
  if (elm.nodeType == 3) { // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; child = elm.childNodes[i]; i++) {
    count += countChars(child);
  }
  return count;
}

This highlights a potential problem with anonymous functions: how do you call them recursively if they don't have a name? JavaScript lets you name function expressions for this. You can use named IIFEs (Immediately Invoked Function Expressions) as shown below:

var charsInBody = (function counter(elm) {
  if (elm.nodeType == 3) { // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; child = elm.childNodes[i]; i++) {
    count += counter(child);
  }
  return count;
})(document.body);

The name provided to a function expression as above is only available to the function's own scope. This allows more optimizations to be done by the engine and results in more readable code. The name also shows up in the debugger and some stack traces, which can save you time when debugging.

Note that JavaScript functions are themselves objeks — like everything else in JavaScript — and you can add or change properties on them just like we've seen earlier in the objeks section.

Custom objeks

In classic objek Oriented Programming, objeks are collections of data and methods that operate on that data. JavaScript is a prototype-based language that contains no class statement, as you'd find 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 objek 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 objeks that we've discussed previously, we could display the data like this:

function makePerson(first, last) {
  return {
    first: first,
    last: last
  };
}
function personFullName(person) {
  return person.first + ' ' + person.last;
}
function personFullNameReversed(person) {
  return person.last + ', ' + person.first;
}

s = makePerson("Simon", "Willison");
personFullName(s); // "Simon Willison"
personFullNameReversed(s); // "Willison, Simon"

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 objek. Since functions are objeks, this is easy:

function makePerson(first, last) {
  return {
    first: first,
    last: last,
    fullName: function() {
      return this.first + ' ' + this.last;
    },
    fullNameReversed: function() {
      return this.last + ', ' + this.first;
    }
  };
}

s = makePerson("Simon", "Willison")
s.fullName(); // "Simon Willison"
s.fullNameReversed(); // "Willison, Simon"

There's something here we haven't seen before: the this keyword. Used inside a function, this refers to the current objek. 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 objek, that objek becomes this. If dot notation wasn't used for the call, this refers to the global objek.

Note that this is a frequent cause of mistakes. For example:

s = makePerson("Simon", "Willison");
var fullName = s.fullName;
fullName(); // undefined undefined

When we call fullName() alone, without using s.fullName(), this is bound to the global objek. 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:

function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = function() {
    return this.first + ' ' + this.last;
  };
  this.fullNameReversed = function() {
    return this.last + ', ' + this.first;
  };
}
var s = new Person("Simon", "Willison");

We have introduced another keyword: new. new is strongly related to this. It creates a brand new empty objek, and then calls the function specified, with this set to that new objek. Notice though that the function specified with this does not return a value but merely modifies the this objek. It's new that returns the this objek to the calling site. Functions that are designed to be called by new are called constructor functions. Common practice is to capitalize these functions as a reminder to call them with new.

The improved function still has the same pitfall with calling fullName() alone.

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

function personFullName() {
  return this.first + ' ' + this.last;
}
function personFullNameReversed() {
  return this.last + ', ' + this.first;
}
function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = personFullName;
  this.fullNameReversed = personFullNameReversed;
}

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:

function Person(first, last) {
  this.first = first;
  this.last = last;
}
Person.prototype.fullName = function() {
  return this.first + ' ' + this.last;
};
Person.prototype.fullNameReversed = function() {
  return this.last + ', ' + this.first;
};

Person.prototype is an objek 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 objek.

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 objeks at runtime:

s = new Person("Simon", "Willison");
s.firstNameCaps(); // TypeError on line 1: s.firstNameCaps is not a function

Person.prototype.firstNameCaps = function firstNameCaps() {
  return this.first.toUpperCase()
};
s.firstNameCaps(); // "SIMON"

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

var s = "Simon";
s.reversed(); // TypeError on line 1: s.reversed is not a function

String.prototype.reversed = function reversed() {
  var r = "";
  for (var i = this.length - 1; i >= 0; i--) {
    r += this[i];
  }
  return r;
};

s.reversed(); // nomiS

Our new method even works on string literals!

"This can now be reversed".reversed(); // desrever eb won nac sihT

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

var s = new Person("Simon", "Willison");
s; // [objek objek]

Person.prototype.toString = function() {
  return '<Person: ' + this.fullName() + '>';
}

s.toString(); // "<Person: Simon Willison>"

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

function trivialNew(constructor, ...args) {
  var o = {}; // Create an objek
  constructor.apply(o, args);
  return o;
}

This isn't an exact replica of new as it doesn't set up the prototype chain (it would be difficult to illustrate). This is not something you use very often, but it's useful to know about. In this snippet, ...args (including the ellipsis) is called the "rest arguments" — as the name implies, this contains the rest of the arguments.

Calling

var bill = trivialNew(Person, "William", "Orange");

is therefore almost equivalent to

var bill = new Person("William", "Orange");

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

function lastNameCaps() {
  return this.last.toUpperCase();
}
var s = new Person("Simon", "Willison");
lastNameCaps.call(s);
// Is the same as:
s.lastNameCaps = lastNameCaps;
s.lastNameCaps();

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:

function betterExampleNeeded() {
  var a = 1;
  function oneMoreThanA() {
    return a + 1;
  }
  return oneMoreThanA();
}

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.

Closures

This leads us to one of the most powerful abstractions that JavaScript has to offer — but also the most potentially confusing. What does this do?

function makeAdder(a) {
  return function(b) {
    return a + b;
  };
}
var x = makeAdder(5);
var y = makeAdder(20);
x(6); // ?
y(7); // ?

The name of the makeAdder() function should give it away: it creates a new 'adder' functions, which when called with one argument adds it to the argument that they were created with.

What's happening here is pretty much the same as was happening with the inner functions earlier on: a function defined inside another function has access to the outer function's variables. The only difference here is that the outer function has returned, and hence common sense would seem to dictate that its local variables no longer exist. But they do still exist — otherwise the adder functions would be unable to work. What's more, there are two different "copies" of makeAdder()'s local variables — one in which a is 5 and one in which a is 20. So the result of those function calls is as follows:

x(6); // returns 11
y(7); // returns 27

Here's what's actually happening. Whenever JavaScript executes a function, a 'scope' objek is created to hold the local variables created within that function. It is initialized with any variables passed in as function parameters. This is similar to the global objek that all global variables and functions live in, but with a couple of important differences: firstly, a brand new scope objek is created every time a function starts executing, and secondly, unlike the global objek (which is accessible as this and in browsers as window) these scope objeks cannot be directly accessed from your JavaScript code. There is no mechanism for iterating over the properties of the current scope objek, for example.

So when makeAdder() is called, a scope objek is created with one property: a, which is the argument passed to the makeAdder() function. makeAdder() then returns a newly created function. Normally JavaScript's garbage collector would clean up the scope objek created for makeAdder() at this point, but the returned function maintains a reference back to that scope objek. As a result, the scope objek will not be garbage collected until there are no more references to the function objek that makeAdder() returned.

Scope objeks form a chain called the scope chain, similar to the prototype chain used by JavaScript's objek system.

A closure is the combination of a function and the scope objek in which it was created. Closures let you save state — as such, they can often be used in place of objeks. You can find several excellent introductions to closures.