Grammar and types

本章討論 JavaScript 的基本語法與基礎資料類型,包括變數,常數, 字元常數

基礎知識(Basics)

JavaScript許多基本語法借鑒自C/C++和Java，但函數的用法、原型的繼承、正規表達亦受 Awk、Perl 和 Python 的影響。

JavaScript 是 case-sensitive(區分大小寫) 並使用  Unicode 編碼。

在 JavaScript 中，每行指令被稱為statements，並用分號(;)分隔。空格(spaces)、Tab與換行符號皆被視為空白(whitespace)。

註解(Comments)

註解語法跟C++和其他語言相同:

// a one line comment
 
/* this is a longer, 
   multi-line comments
 */
 
/* You can't, however, /* nest comments */ SyntaxError */

宣告(Declarations)

JavaScript有三種宣告方式

var

Declares a variable, optionally initializing it to a value.

let

Declares a block scope local variable, optionally initializing it to a value.

const

Declares a read-only named constant.

變數(Variables)

變數(variable)是對值(value)的引用，變數的名稱被稱為 identifiers 需要遵從一定的規則。

在JavaScript中，identifier 必須使用字母( letter )、下底線( _ )、錢號( $ ) 作為開頭；後面的字員組成可以包含數字(0-9)。JavaScript是區分大小寫(case secsitive)的，大寫字母('A'~'Z')和小寫字母('a'~'z')皆可使用且不相等。

數值

JavaScript 使用下列基礎資料類型

儘管這些變數關聯性很小, 他們可以讓你在你的應用程式中, 產生出有意義的函數. 

物件與 函數 在語言中是其它的基本元素. 你可以把物件想成是一個被命名過且用來裝數值的容器,以及函數則為你的應用程式所執行的步驟.  

資料型別轉換

JavaScript is a dynamically typed language. That means you don't have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. So, for example, you could define a variable as follows:

var answer = 42;

And later, you could assign the same variable a string value, for example:

answer = "Thanks for all the fish...";

Because JavaScript is dynamically typed, this assignment does not cause an error message.

In expressions involving numeric and string values with the + operator, JavaScript converts numeric values to strings. For example, consider the following statements:

x = "The answer is " + 42 // "The answer is 42"
y = 42 + " is the answer" // "42 is the answer"

In statements involving other operators, JavaScript does not convert numeric values to strings. For example:

"37" - 7 // 30
"37" + 7 // "377"

字串轉數值

In the case that a value representing a number is in memory as a string, there are methods for conversion.

parseInt() and parseFloat()

See: parseInt() and parseFloat() pages.

parseInt will only return whole numbers, so its use is diminished for decimals. Additionally, a best practice for parseInt is to always include the radix parameter. The radix parameter is used to specify which numerical system is to be used. 

Unary plus operator

An alternative method of retrieving a number from a string is with the + (unary plus) operator.

"1.1" + "1.1" = "1.11.1"
(+"1.1") + (+"1.1") = 2.2   // Note: the parentheses are added for clarity, not required.

Variables

You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules.

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

Starting with JavaScript 1.5, you can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the \uXXXX Unicode escape sequences as characters in identifiers.

Some examples of legal names are Number_hits, temp99, and _name.

Declaring variables

You can declare a variable in two ways:

• With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables.
• By simply assigning it a value. For example, x = 42. This always declares a global variable and cannot be changed at the local level. It generates a strict JavaScript warning. You shouldn't use this variant.

Evaluating variables

A variable declared using the var statement with no initial value specified has the value undefined.

An attempt to access an undeclared variable will result in a ReferenceError exception being thrown:

var a;
console.log("The value of a is " + a); // logs "The value of a is undefined"
console.log("The value of b is " + b); // throws ReferenceError exception

You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true.

var input;
if(input === undefined){
  doThis();
} else {
  doThat();
}

The following is related to "Variables" section as potential values in assignment.

The undefined value behaves as false when used in a boolean context. For example, the following code executes the function myFunction because the myArray element is not defined:

var myArray = new Array();
if (!myArray[0]) myFunction(); 

The undefined value converts to NaN when used in numeric context.

var a;
a + 2 = NaN

When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in boolean contexts. For example:

var n = null;
console.log(n * 32); // Will log 0 to the console

Variable scope

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function.

JavaScript does not have block statement scope; rather, it will be local to the code that the block resides within. For example the following code will log 5, because the scope of x is the function (or global context) within which x is declared, not the block, which in this case is an if statement.

if (true) {
  var x = 5;
}
console.log(x);

Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception. This concept is known as hoisting; variables in JavaScript are in a sense "hoisted" or lifted to the top of the function or statement. However, variables that aren't initialized yet will return a value of undefined.

/**
 * Example 1
 */
console.log(x === undefined); // logs "true"
var x = 3;

/**
 * Example 2
 */
// will return a value of undefined
var myvar = "my value";
 
(function() {
  console.log(myvar); // undefined
  var myvar = "local value";
})();

The above examples will be interpreted the same as:

/**
 * Example 1
 */
var x;
console.log(x === undefined); // logs "true"
x = 3;
 
/**
 * Example 2
 */
var myvar = "my value";
 
(function() {
  var myvar;
  console.log(myvar); // undefined
  myvar = "local value";
})();

Because of hoisting, all var statements in a function should be placed as near to the top of the function as possible. This best practice increases the clarity of the code.

Global variables

need links to pages discussing scope chains and the global object Global variables are in fact properties of the global object. In web pages the global object is window, so you can set and access global variables using the window.variable syntax.

Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a FRAMESET document, you can refer to this variable from a child frame as parent.phoneNumber.

Constants

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter, underscore or dollar sign and can contain alphabetic, numeric, or underscore characters.

const prefix = '212';

A constant cannot change value through assignment or be re-declared while the script is running.

The scope rules for constants are the same as those for variables, except that the const keyword is always required, even for global constants. If the keyword is omitted, the identifier is assumed to represent a variable.

You cannot declare a constant with the same name as a function or variable in the same scope. For example:

// THIS WILL CAUSE AN ERROR
function f() {};
const f = 5;

// THIS WILL CAUSE AN ERROR ALSO
function f() {
  const g = 5;
  var g;

  //statements
}

Literals

You use literals to represent values in JavaScript. These are fixed values, not variables, that you literally provide in your script. This section describes the following types of literals:

• Array literals
• Boolean literals
• Floating-point literals
• Integers
• Object literals
• String literals

Array literals

An array literal is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets ([]). When you create an array using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified.

The following example creates the coffees array with three elements and a length of three:

var coffees = ["French Roast", "Colombian", "Kona"];

Note An array literal is a type of object initializer. See Using Object Initializers.

If an array is created using a literal in a top-level script, JavaScript interprets the array each time it evaluates the expression containing the array literal. In addition, a literal used in a function is created each time the function is called.

Array literals are also Array objects. See Array Object for details on Array objects.

Extra commas in array literals

You do not have to specify all elements in an array literal. If you put two commas in a row, the array is created with undefined for the unspecified elements. The following example creates the fish array:

var fish = ["Lion", , "Angel"];

This array has two elements with values and one empty element (fish[0] is "Lion", fish[1] is undefined, and fish[2] is "Angel").

If you include a trailing comma at the end of the list of elements, the comma is ignored. In the following example, the length of the array is three. There is no myList[3]. All other commas in the list indicate a new element. (Note: trailing commas can create errors in older browser versions and it is a best practice to remove them.)

var myList = ['home', , 'school', ];

In the following example, the length of the array is four, and myList[0] and myList[2] are missing.

var myList = [ , 'home', , 'school'];

In the following example, the length of the array is four, and myList[1] and myList[3] are missing. Only the last comma is ignored.

var myList = ['home', , 'school', , ];

Understanding the behavior of extra commas is important to understanding JavaScript as a language, however when writing your own code: explicitly declaring the missing elements as undefined will increase your code's clarity and maintainability.

Boolean literals

The Boolean type has two literal values: true and false.

Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object. The Boolean object is a wrapper around the primitive Boolean data type. See Boolean Object for more information.

Integers

Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8).

• Decimal integer literal consists of a sequence of digits without a leading 0 (zero).
• Leading 0 (zero) on an integer literal indicates it is in octal. Octal integers can include only the digits 0-7.
• Leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F.

Octal integer literals are deprecated and have been removed from the ECMA-262, Edition 3 standard (in strict mode). JavaScript 1.5 still supports them for backward compatibility.

Some examples of integer literals are:

0, 117 and -345 (decimal, base 10)
015, 0001 and -077 (octal, base 8) 
0x1123, 0x00111 and -0xF1A7 (hexadecimal, "hex" or base 16)

Floating-point literals

A floating-point literal can have the following parts:

• A decimal integer which can be signed (preceded by "+" or "-"),
• A decimal point ("."),
• A fraction (another decimal number),
• An exponent.

The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").

Some examples of floating-point literals are 3.1415, -3.1E12, .1e12, and 2E-12.

More succinctly, the syntax is:

[(+|-)][digits][.digits][(E|e)[(+|-)]digits]

For example:

3.14
2345.789
.3333333333333333333
-.283185307179586

Object literals

An object literal is a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). You should not use an object literal at the beginning of a statement. This will lead to an error or not behave as you expect, because the { will be interpreted as the beginning of a block.

The following is an example of an object literal. The first element of the car object defines a property, myCar, and assigns to it a new string, "Saturn"; the second element, the getCar property, is immediately assigned the result of invoking the function (CarTypes("Honda")); the third element, the special property, uses an existing variable (Sales).

var Sales = "Toyota";

function CarTypes(name) {
  if (name == "Honda") {
    return name;
  } else {
    return "Sorry, we don't sell " + name + ".";
  }
}

var car = { myCar: "Saturn", getCar: CarTypes("Honda"), special: Sales };

console.log(car.myCar);   // Saturn
console.log(car.getCar);  // Honda
console.log(car.special); // Toyota 

Additionally, you can use a numeric or string literal for the name of a property or nest an object inside another. The following example uses these options.

var car = { manyCars: {a: "Saab", "b": "Jeep"}, 7: "Mazda" };

console.log(car.manyCars.b); // Jeep
console.log(car[7]); // Mazda

Object property names can be any string, including the empty string. If the property name would not be a valid JavaScript identifier, it must be enclosed in quotes. Property names that would not be valid identifiers also cannot be accessed as a dot (.) property, but can be accessed and set with the array-like notation("[]").

var unusualPropertyNames = {
  "": "An empty string",
  "!": "Bang!"
}
console.log(unusualPropertyNames."");   // SyntaxError: Unexpected string
console.log(unusualPropertyNames[""]);  // "An empty string"
console.log(unusualPropertyNames.!);    // SyntaxError: Unexpected token !
console.log(unusualPropertyNames["!"]); // "Bang!"

Please note:

var foo = {a: "alpha", 2: "two"};
console.log(foo.a);    // alpha
console.log(foo[2]);   // two
//console.log(foo.2);  // Error: missing ) after argument list
//console.log(foo[a]); // Error: a is not defined
console.log(foo["a"]); // alpha
console.log(foo["2"]); // two

String literals

A string literal is zero or more characters enclosed in double (") or single (') quotation marks. A string must be delimited by quotation marks of the same type; that is, either both single quotation marks or both double quotation marks. The following are examples of string literals:

• "foo"
• 'bar'
• "1234"
• "one line \n another line"
• "John's cat"

You can call any of the methods of the String object on a string literal value—JavaScript automatically converts the string literal to a temporary String object, calls the method, then discards the temporary String object. You can also use the String.length property with a string literal:

print("John's cat".length) // Will print the number of symbols in the string including whitespace. In this case, 10.

You should use string literals unless you specifically need to use a String object. See String Object for details on String objects.

Using special characters in strings

In addition to ordinary characters, you can also include special characters in strings, as shown in the following example.

"one line \n another line"

The following table lists the special characters that you can use in JavaScript strings.

Escaping characters

For characters not listed in Table 2.1, a preceding backslash is ignored, but this usage is deprecated and should be avoided.

You can insert a quotation mark inside a string by preceding it with a backslash. This is known as escaping the quotation mark. For example:

var quote = "He read \"The Cremation of Sam McGee\" by R.W. Service.";
console.log(quote);

The result of this would be:

He read "The Cremation of Sam McGee" by R.W. Service.

To include a literal backslash inside a string, you must escape the backslash character. For example, to assign the file path c:\temp to a string, use the following:

var home = "c:\\temp";

You can also escape line breaks by preceding them with backslash. The backslash and line break are both removed from the value of the string.

var str = "this string \
is broken \
across multiple\
lines."
console.log(str);   // this string is broken across multiplelines.

Although JavaScript does not have "heredoc" syntax, you can get close by adding a linebreak escape and an escaped linebreak at the end of each line:

var poem = 
"Roses are red,\n\
Violets are blue.\n\
I'm schizophrenic,\n\
And so am I."

Unicode

Unicode is a universal character-coding standard for the interchange and display of principal written languages. It covers the languages of the Americas, Europe, Middle East, Africa, India, Asia, and Pacifica, as well as historic scripts and technical symbols. Unicode allows for the exchange, processing, and display of multilingual texts, as well as the use of common technical and mathematical symbols. It hopes to resolve internationalization problems of multilingual computing, such as different national character standards. Not all modern or archaic scripts, however, are currently supported.

The Unicode character set can be used for all known encoding. Unicode is modeled after the ASCII (American Standard Code for Information Interchange) character set. It uses a numerical value and name for each character. The character encoding specifies the identity of the character and its numeric value (code position), as well as the representation of this value in bits. The 16-bit numeric value (code value) is defined by a hexadecimal number and a prefix U, for example, U+0041 represents A. The unique name for this value is LATIN CAPITAL LETTER A.

Unicode is not supported in versions of JavaScript prior to 1.3.

Unicode compatibility with ASCII and ISO

Unicode is fully compatible with the International Standard ISO/IEC 10646-1; 1993, which is a subset of ISO 10646.

Several encoding standards (including UTF-8, UTF-16 and ISO UCS-2) are used to physically represent Unicode as actual bits.

The UTF-8 encoding of Unicode is compatible with ASCII characters and is supported by many programs. The first 128 Unicode characters correspond to the ASCII characters and have the same byte value. The Unicode characters U+0020 through U+007E are equivalent to the ASCII characters 0x20 through 0x7E. Unlike ASCII, which supports the Latin alphabet and uses a 7-bit character set, UTF-8 uses between one and four octets for each character ("octet" meaning a byte, or 8 bits.) This allows for several million characters. An alternative encoding standard, UTF-16, uses two octets to represent Unicode characters. An escape sequence allows UTF-16 to represent the whole Unicode range by using four octets. The ISO UCS-2 (Universal Character Set) uses two octets.

JavaScript and Navigator support for UTF-8/Unicode means you can use non-Latin, international, and localized characters, plus special technical symbols in JavaScript programs. Unicode provides a standard way to encode multilingual text. Since the UTF-8 encoding of Unicode is compatible with ASCII, programs can use ASCII characters. You can use non-ASCII Unicode characters in the comments, string literals, identifiers, and regular expressions of JavaScript.

Unicode escape sequences

You can use the Unicode escape sequence in string literals, regular expressions, and identifiers. The escape sequence consists of six ASCII characters: \u and a four-digit hexadecimal number. For example, \u00A9 represents the copyright symbol. Every Unicode escape sequence in JavaScript is interpreted as one character.

The following code returns the copyright symbol and the string "Netscape Communications".

var x = "\u00A9 Netscape Communications";

The following table lists frequently used special characters and their Unicode value.

The JavaScript use of the Unicode escape sequence is different from Java. In JavaScript, the escape sequence is never interpreted as a special character first. For example, a line terminator escape sequence inside a string does not terminate the string before it is interpreted by the function. JavaScript ignores any escape sequence if it is used in comments. In Java, if an escape sequence is used in a single comment line, it is interpreted as an Unicode character. For a string literal, the Java compiler interprets the escape sequences first. For example, if a line terminator escape character (e.g., \u000A) is used in Java, it terminates the string literal. In Java, this leads to an error, because line terminators are not allowed in string literals. You must use \n for a line feed in a string literal. In JavaScript, the escape sequence works the same way as \n.

Unicode characters in JavaScript files

Earlier versions of Gecko assumed the Latin-1 character encoding for JavaScript files loaded from XUL. Starting with Gecko 1.8, the character encoding is inferred from the XUL file's encoding. Please see International characters in XUL JavaScript for more information.

Displaying characters with Unicode

You can use Unicode to display the characters in different languages or technical symbols. For characters to be displayed properly, a client such as Mozilla Firefox or Netscape needs to support Unicode. Moreover, an appropriate Unicode font must be available to the client, and the client platform must support Unicode. Often, Unicode fonts do not display all the Unicode characters. Some platforms, such as Windows 95, provide partial support for Unicode.

To receive non-ASCII character input, the client needs to send the input as Unicode. Using a standard enhanced keyboard, the client cannot easily input the additional characters supported by Unicode. Sometimes, the only way to input Unicode characters is by using Unicode escape sequences.

For more information on Unicode, see the Unicode Home Page and The Unicode Standard, Version 2.0, published by Addison-Wesley, 1996.

Resources

• Text Escaping and Unescaping in JavaScript – an utility to convert characters in JavaScript unicode values