JavaScript syntax

The syntax of JavaScript is a set of rules that defines how a JavaScript program will be written and interpreted. The JavaScript syntax was influenced by several programming languages, including Java.

Origin of Syntax

Brendan Eich summarized the ancestry of the syntax in the first paragraph of the JavaScript 1.1 specification as follows:

JavaScript borrows most of its syntax from Java, but also inherits from Awk and Perl, with some indirect influence from Self in its object prototype system.

Variables

Variables in standard JavaScript have no type attached, and any value can be stored in any variable. Variables can be declared with a var statement. These variables are lexically scoped and once a variable is declared, it may be accessed anywhere inside the function where it is declared. Variables declared outside any function, and variables used without being declared with 'var', are global (can be used by the entire program).

Here is an example of variable declarations and global values:

x = 0; // A global variable
var y = 'Hello!'; // Another global variable

function f(){
 var z = 'foxes'; // A local variable
 twenty = 20; // Another global
 return x; // We can use x here because it is global
}
// The value of z is no longer available

Basic data types

Numbers

Numbers in JavaScript are represented in binary as IEEE-754 Doubles, which provides an accuracy to about 14 or 15 significant digits JavaScript FAQ 4.7. Because they are binary numbers, they do not always exactly represent decimal numbers, particularly fractions.

This becomes an issue when formatting numbers for output, which JavaScript has no built-in methods for. For example:

alert(0.94 - 0.01); // displays 0.9299999999999999

As a result, rounding should be used whenever numbers are formatted for output. The toFixed() method is not part of the ECMAScript specification and is implemented differently in various environments, so it can't be relied upon.

Arrays

An Array is a map from integers to values. In JavaScript, all objects can map from integers to values, but Arrays are a special type of object that has extra behavior and methods specializing in integer indices (e.g., join, slice, and push).

Arrays have a length property that is guaranteed to always be larger than the largest integer index used in the array. It is automatically updated if one creates a property with an even larger index. Writing a smaller number to the length property will remove larger indices. This length property is the only special feature of Arrays that distinguishes it from other objects.

Elements of Arrays may be accessed using normal object property access notation:

myArray[1]
myArray["1"]

The above two are equivalent. It's not possible to use the "dot"-notation or strings with alternative representations of the number:

myArray.1 (syntax error)
myArray["01"] (not the same as myArray[1])

Declaration of an array can use either an Array literal or the Array constructor:

myArray = [0,1,,,4,5]; (array with length 6 and 4 elements)
myArray = new Array(0,1,2,3,4,5); (array with length 6 and 6 elements)
myArray = new Array(365); (an empty array with length 365)

Arrays are implemented so that only the elements defined use memory; they are "sparse arrays". Setting myArray[10] = 'someThing' and myArray[57] = 'somethingOther' only uses space for these two elements, just like any other object. The length of the array will still be reported as 58.

Strings

Strings in Javascript are a sequence of characters. Strings in JavaScript can be created directly by placing the series of characters between double or single quotes.

var greeting = "Hello, ronald!";
var another_greeting = 'Greetings, people of Earth.';

Individual characters within a string can be accessed (as strings with only a single character) through the same notation as arrays:

var h = greeting[0]; // Now h contains 'H'

Objects

The most basic objects in JavaScript act as dictionaries. These dictionaries can have any type of value paired with a key, which is a string. Objects with values can be created directly through object literal notation:

var o = {name: 'My Object', purpose: 'This object is utterly without purpose.', answer: 42};

Properties of objects can be created, set, and read individually using the familiar dot ('.') notation or by a similar syntax to arrays:

var name = o.name; // name now contains 'My Object'
var answer = o['answer']; // answer now contains 42

Object literals and array literals allow one to easily create flexible data structures:

var myStructure = {
  name: {
    first: "Mel",
    last: "Smith"
  },
  age: 33,
  hobbies: [ "chess", "jogging" ]
};

This is the basis for JSON, which is a simple notation that uses JavaScript-like syntax for data exchange. Bold text

Operators

The '+' operator is overloaded; it is used for string concatenation and arithmetic addition and also to convert strings to numbers. It also has special meaning when used in a regular expression.

// Concatenate 2 strings
var a = 'This';
var b = ' and that';
alert(a + b);  // displays 'This and that'
  
// Add two numbers
var x = 2;
var y = 6;
alert(x + y); // displays 8

// Adding a number and a string results in concatenation
alert( x + '2'); // displays 22

// Convert a string to a number
var z = '4';   // z is a string (the digit 4)
alert( z + x)  // displays 42
alert( +z + x) // displays 6

Arithmetic

Binary operators

+     Addition
-     Subtraction
*     Multiplication
/     Division (returns a floating-point value)
%     Modulus (returns the integer remainder)

Unary operators

-     Unary negation (reverses the sign)
++    Increment (can be prefix or postfix)
--    Decrement (can be prefix or postfix)

Assignment

=     Assign

+=    Add and assign

===Comparison===
<pre>
==    Equal
!=    Not equal
>     Greater than
>=    Greater than or equal to
<     Less than
<=    Less than or equal to

===   Identical (equal and of the same type)
!==   Not identical

Boolean

• Short-circuit logical operations which means it will evaluate from left to right until it can determine the answer. For example:

a || b is automatically true if a is true. There is no reason to evaluate b. a && b is false if a is false. There is no reason to evaluate b.

&&    and
||    or
!     not (logical negation)

Bitwise

Binary operators

&     And
|     Or
^     Xor

<<    Shift left  (zero fill)
>>    Shift right (sign-propagating); copies of the leftmost bit (sign bit) are shifted in from the
      left.
>>>   Shift right (zero fill)

      For positive numbers, >> and >>> yield the same result.

Unary operators

~     Not (inverts the bits)

String

=     Assignment
+     Concatenation
+=    Concatenate and assign

Examples

str = "ab" + "cd";   // "abcd"
str += "e";          // "abcde"

Control structures

If ... else

if (expr) {
  statements;
} else if (expr) {
  statements;
} else {
  statements;
}

• else statements must be cuddled (i.e. "} else {" , all on the same line), or else some browsers may not parse them correctly .

Switch statement

switch (expr) {
  case VALUE: 
    statements;
    break;
  case VALUE: 
    statements;
    break;
  default:
    statements;
    break;
}

• break; is optional; however, it's recommended to use it in most cases, since otherwise code execution will continue to the body of the next case block.
• Add a break statement to the end of the last case as a precautionary measure, in case additional cases are added later.
• Strings can be used for the case values.
• Braces are required.

For loop

for (initial-expr; cond-expr; expr evaluated after each loopround) {
  statements;
}

For ... in loop

for (var property-name in object-name) {
  statements using object-name[property-name];
}

• Iterates through all enumerable properties of an object, or the objects at all indices of an array.

While loop

while (cond-expr) {
  statements;
}

Do ... while

do {
  statements;
} while (cond-expr);

Functions

A function is a block with a (possibly empty) argument list that is normally given a name. A function may give back a return value.

function function-name(arg1, arg2, arg3) {
  statements;
  return expression;
}

Example: Euclid's original algorithm of finding the greatest common divisor. (This is a geometrical solution which subtracts the shorter segment from the longer):

function gcd(segmentA, segmentB) {
  while (segmentA != segmentB) {
    if (segmentA > segmentB)
      segmentA -= segmentB;
    else
      segmentB -= segmentA;
  }
  return segmentA;
}

The number of arguments given when calling a function may not necessarily correspond to the number of arguments in the function definition; a named argument in the definition that does not have a matching argument in the call will have the value undefined. Within the function the arguments may also be accessed through the arguments list (which is an array); this provides access to all arguments using indices (e.g. arguments[0], arguments[1], ... arguments[n]), including those beyond the number of named arguments.

Basic data types (strings, integers, ...) are passed by value whereas objects are passed by reference.

Objects

For convenience, Types are normally subdivided into primitives and objects. Objects are entities that have an identity (they are only equal to themselves) and that map property names to values, ("slots" in prototype-based programming terminology). JavaScript objects are often mistakenly described as associative arrays or hashes, but they are neither.

JavaScript has several kinds of built in objects, namely Array, Boolean, Date, Function, Math, Number, Object, RegExp and String. Other objects are "host objects", defined not by the language but by the runtime environment. For example, in a browser, typical host objects belong to the DOM (window, form, links etc.).

Creating objects

Objects can be created using a declaration, an initialiser or a constructor function:

// Declaration 
var anObject = new Object();

// Initialiser
var objectA = {};
var objectB = {'index1':'value 1','index2':'value 2'};

// Constructor (see below)

Constructors

Constructor functions are a way to create multiple instances or copies of the same object. JavaScript is a prototype based object-based language. This means that inheritance is between objects, not between classes (JavaScript has no classes). Objects inherit properties from their prototypes.

Properties and methods can be added by the constructor, or they can be added and removed after the object has been created. To do this for all instances created by a single constructor function, the prototype property of the constructor is used to access the prototype object. Object deletion is not mandatory as the scripting engine will garbage collect any variables that are no longer being referenced.

Example: Manipulating an object

// constructor function
function MyObject(attributeA, attributeB) {
  this.attributeA = attributeA;
  this.attributeB = attributeB;
}

// create an Object
obj = new MyObject('red', 1000);

// access an attribute of obj
alert(obj.attributeA);

// access an attribute using square bracket notation
alert(obj["attributeA"]);

// add a new property
obj.attributeC = new Date();

// remove a property of obj
delete obj.attributeB;

// remove the whole Object
delete obj;

Inheritance

JavaScript supports inheritance hierarchies through prototyping. For example:

function Base() {
  this.Override = function() {
    alert("Base::Override()");
  }

  this.BaseFunction = function() {
    alert("Base::BaseFunction()");
  }
}

function Derive() {
  this.Override = function() {
    alert("Derive::Override()");
  }
}

Derive.prototype = new Base();

d = new Derive();
d.Override();
d.BaseFunction();
d.__proto__.Override(); // mozilla only

will result in the display:

Derive::Override()
Base::BaseFunction()
Base::Override()  // mozilla only

Exceptions

Newer versions of JavaScript (as used in Internet Explorer 5 and Netscape 6) include a try ... catch ... finally exception handling statement. Purloined from the Java programming language, this is intended to help with run-time errors but does so with mixed results.

The try ... catch ... finally statement catches exceptions resulting from an error or a throw statement. Its syntax is as follows:

try {
  // Statements in which exceptions might be thrown
} catch(error) {
  // Statements that execute in the event of an exception
} finally {
  // Statements that execute afterward either way
}

Initially, the statements within the try block execute. If an exception is thrown, the script's control flow immediately transfers to the statements in the catch block, with the exception available as the error argument. Otherwise the catch block is skipped. Once the catch block finishes, or the try block finishes with no exceptions thrown, then the statements in the finally block execute. This is generally used to free memory that may be lost if a fatal error occurs—though this is less of a concern in JavaScript. This figure summarizes the operation of a try...catch...finally statement:

try {
  // Create an array
  arr = new Array();
  // Call a function that may not succeed
  func(arr);
}
catch (...) {
  // Recover from error
  logError();
}
finally {
  // Even if a fatal error occurred, we can still free our array
  delete arr;
} 

The finally part may be omitted:

try {
  statements
}
catch (err) {
  // handle errors
}

In fact, the catch part may also be omitted:

try {
  statements
}
finally {
  // ignore potential errors and just go directly to finally
}

Miscellaneous

Case sensitivity

JavaScript is case sensitive.

Whitespace

Spaces, tabs and newlines used outside of string constants are called whitespace. Unlike C, whitespace in JavaScript source can directly impact semantics. Because of a technique called "semicolon insertion", any statement that is well formed when a newline is parsed will be considered complete (as if a semicolon were inserted just prior to the newline). Programmers are advised to supply statement terminating semicolons explicitly to enhance readability and lessen unintended effects of the automatic semicolon insertion.

Unnecessary whitespace, whitespace characters that are not needed for correct syntax, can increase the amount of wasted space, and therefore the file size of .js files. The easiest way to address the problem of file size is to set the server to use zip compression. This compression will work far better than any whitespace parser and will reduce the size of all other source your server uploads. This method will work with or without semi-colons, but they should be used anyway for readability.

Comments

Comment syntax is the same as in C++.

// comment

/* multi-line
   comment */

See also

• JavaScript

References

• David Flanagan, Paula Ferguson: JavaScript: The Definitive Guide, O'Reilly & Associates, ISBN 0-596-10199-6
• Danny Goodman, Brendan Eich: JavaScript Bible, Wiley, John & Sons, ISBN 0-7645-3342-8
• Thomas A. Powell, Fritz Schneider: JavaScript: The Complete Reference, McGraw-Hill Companies, ISBN 0-07-219127-9
• Emily Vander Veer: JavaScript For Dummies, 4th Edition, Wiley, ISBN 0-7645-7659-3

External links

Reference Material

• Core References for JavaScript versions 1.5, 1.4, 1.3 and 1.2

Resources

• Mozilla JavaScript Language Documentation