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    Valores, variables y literales

    This translation is incomplete. Please help translate this article from English.

    En este capítulo se habla sobre los valores reconocidos en JavaScript y describe los bloques fundamentales de creación de expresiones en JavaScript: variables, constantes y literales

    Valores

    JavaScript reconoce los siguientes cinco tipo de valores primitivos:

     
    Tipo Ejemplo de valores
    Números 42, 3.14159
    Lógico (Booleano) true / false (verdadero / falso)
    Cadena "Hola"
    null palabra clave especial que denota un valor nulo; null también es un valor primitivo. Debido a que JavaScript distingue entre mayúsculas y minúsculas, null no es lo mismo que NullNULL, o cualquier otra variable.
    undefined 
    (indefinido)
    una propiedad de primer nivel cuyo valor es indefinido; undefined también es un valor primitivo.

     

    Aunque estos tipos de dato son relativamente pocos, te permiten realizar funciones muy útiles en tus aplicaciones.

    Los objetos y las funciones son otros elementos fundamentales en este lenguaje. Puedes pensar en objetos como contenedores de valores, y funciones como procedimientos que tu aplicación puede realizar

    Conversión de Tipo de Dato

    JavaScript es un lenguaje de tipos dinámicos. Eso quiere decir que no tienes que especificar el tipo de dato de la variable cuando la declaras, y los tipos de dato son convertidos automáticamente de acuerdo a lo que se necesite en la ejecución del scrtip. Así, por ejemplo, puedes definir una variable de la siguiente manera:

    var respuesta = 42;
    

    Y luego, puedes asignarle una cadena a esa misma variable, por ejemplo:

    respuesta = "Gracias por todo el pescado...";
    

    Debido a que es un lenguajede tipos dinámicos, esta asignación no causa un mensaje de error.

    En expresiones que involucran valores numéricos y de cadena con el operador +, JavaScript convierte valores numéricos a cadena. Por ejemplo, considera las siguientes declaraciones:

    x = "La respuesta es " + 42 // "La respuesta es 42"
    y = 42 + " es la respuesta" // "42 es la respuesta"
    

    En declaraciones que involucran otros operadores, JavaScript no convierte los valores numéricos a cadenas. Por ejemplo:

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

    Convertir cadenas a números

    En el caso que un valor representando un número está en memoria como cadena, hay métodos para la conversión.

    parseInt() y parseFloat()

    Ver páginas parseInt() y parseFloat().

    parseInt sólo retornará valores enteros, de manera que su uso se verá disminuído por los decimales. Adicionalmente, una buena práctica para parseInt es incluir siempre el parámetro radix. El parámetro rádix es usado para especificar qué sistema numérico será usado.

    Operador Unario más

    Un método alternativo para recibir un número de una cadena es con el operador unario + (más).

    "1.1" + "1.1" = "1.11.1"
    (+"1.1") + (+"1.1") = 2.2   // Nota: los paréntesis son para fines demostrativos, no son oblitarios.

    Variables

    Usas variables como nombres simbólicos para valores en tu aplicación. Los nombres de las variables, llamados identificadores, se rigen por ciertas reglas.

    Un identificador en JavaScript tiene que empezar con una letra, un guión bajo (_) o un símbolo de dólar ($); los valores subsiguientes puede ser números. Debido a que JavaScript diferencia entre mayúsculas y minúsculas, las letras incluyen tanto desde la A hasta la Z como de la a hasta la z.

    A partir de JavaScript 1.5 puedes usar el ISO 8859-1 o letras Unicode tales como å y ü en un identificador. 

    Algunos ejemplos de nombre permitidos son Numero_Visitas, temp99, y _nombre.

    Declarando Variables

    Puedes declarar una variable de dos maneras:

    • Con la palabra clave var. Por ejemplo, var x = 42. Esta sintaxis puede ser usada para declarar tanto variables locales como globales.
    • Simplemente asignandole un valor. Por ejemplo, x = 42. Esto siempre declara una variable global y no puede ser cambiado a nivel local. Esto genera una advertencia strict de JavaScript. Esta variante no debe usarse.

    Evaluar variables

    Una variable declarada usando var y sin asignarle un valor inicial tiene el valor undefined.

    Al intentar acceder a una variable no declarada, se lanzara la excepcion ReferenceError:

    var a;
    console.log("El valor de a es " + a); // muestra "El valor de a es undefined"
    console.log("El valor de b es " + b); // lanza la excepcion ReferenceError
    

    Se puede usar undefined para determinar si una variable tiene un valor. En el siguiente codigo a la variable input no se le asigna ningun valor y la sentencia de control if la evalua a true.

    var input;
    if(input === undefined){
      hazEsto();
    } else {
      hazEso();
    }
    

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

    El valor undefined se comporta como un false cuando se usa en una operacion logica. Por ejemplo, el siguiente codigo ejecuta la funcion myFunction porque el elemento myArray no ha sido definido:

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

    El valor undefined se convierte en NaN, no numerico, cuando se usa en una operacion aritmetica.

    var a;
    a + 2 = NaN

    Cuando se evalua una variable nula, el valor null se comporta como el 0 en operaciones aritmeticas y como false en operaciones logicas. Por ejemplo:

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

    Ambito de las variables

    Cuando usted declara una variable fuera de una función, esta es llamada una variable global, porque esta disponible para cualquier otro codigo en el documento actual. Cuando usted declara una variable dentro de una función, esta es llamada variable local, porque esta disponible solo dentro de la función.

    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

    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:

    console.log("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.

    Table 2.1 JavaScript special characters
    Character Meaning
    \b Backspace
    \f Form feed
    \n New line
    \r Carriage return
    \t Tab
    \v Vertical tab
    \' Apostrophe or single quote
    \" Double quote
    \\ Backslash character
    \XXX The character with the Latin-1 encoding specified by up to three octal digits XXX between 0 and 377. For example, \251 is the octal sequence for the copyright symbol.
    \xXX The character with the Latin-1 encoding specified by the two hexadecimal digits XX between 00 and FF. For example, \xA9 is the hexadecimal sequence for the copyright symbol.
    \uXXXX The Unicode character specified by the four hexadecimal digits XXXX. For example, \u00A9 is the Unicode sequence for the copyright symbol. See Unicode escape sequences.

    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.

    Table 2.2 Unicode values for special characters
    Category Unicode value Name Format name
    White space values \u0009 Tab <TAB>
    \u000B Vertical Tab <VT>
    \u000C Form Feed <FF>
    \u0020 Space <SP>
    Line terminator values \u000A Line Feed <LF>
    \u000D Carriage Return <CR>
    Additional Unicode escape sequence values \u0008 Backspace <BS>
    \u0009 Horizontal Tab <HT>
    \u0022 Double Quote "
    \u0027 Single Quote '
    \u005C Backslash \

    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.

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    Contributors to this page: vinixio, teoli, diegogaysaez, Amatos
    Última actualización por: vinixio,