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: http://www.arcetri.astro.it/irlab/doc/library/javascript/clientguide/fcns.htm
Дата изменения: Thu Oct 7 13:13:31 1999 Дата индексирования: Sat Dec 22 13:57:22 2007 Кодировка: Поисковые слова: ion drive |
function keyword, followed by
square:
function square(number) {
return number * number;
}
The function square takes one argument, called number. The function consists of one statement that indicates to return the argument of the function multiplied by itself. The return statement specifies the value returned by the function.
return number * numberfunction myFunc(theObject) {
theObject.make="Toyota"
}
mycar = {make:"Honda", model:"Accord", year:1998}
x=mycar.make // returns Honda
myFunc(mycar) // pass object mycar to the function
y=mycar.make // returns Toyota (prop was changed by the function)
In addition to defining functions as described here, you can also define Function objects, as described in "Function Object" on page 114.
A method is a function associated with an object. You'll learn more about objects and methods in Chapter 7, "Working with Objects."
square, you could call it as follows.
square(5)The preceding statement calls the function with an argument of five. The function executes its statements and returns the value twenty-five. The arguments of a function are not limited to strings and numbers. You can pass whole objects to a function, too. The
show_props function (defined in "Objects and Properties" on page 100) is an example of a function that takes an object as an argument.
A function can even be recursive, that is, it can call itself. For example, here is a function that computes factorials:
function factorial(n) {
if ((n == 0) || (n == 1))
return 1
else {
result = (n * factorial(n-1) )
return result
}
}
You could then compute the factorials of one through five as follows:
a=factorial(1) // returns 1
b=factorial(2) // returns 2
c=factorial(3) // returns 6
d=factorial(4) // returns 24
e=factorial(5) // returns 120
arguments[i]where
functionName.arguments[i]
i is the ordinal number of the argument, starting at zero. So, the first argument passed to a function would be arguments[0]. The total number of arguments is indicated by arguments.length.
Using the arguments array, you can call a function with more arguments than it is formally declared to accept. This is often useful if you don't know in advance how many arguments will be passed to the function. You can use arguments.length to determine the number of arguments actually passed to the function, and then treat each argument using the arguments array.
For example, consider a function that concatenates several strings. The only formal argument for the function is a string that specifies the characters that separate the items to concatenate. The function is defined as follows:
function myConcat(separator) {
result="" // initialize list
// iterate through arguments
for (var i=1; i<arguments.length; i++) {
result += arguments[i] + separator
}
return result
}
You can pass any number of arguments to this function, and it creates a list using each argument as an item in the list.
// returns "red, orange, blue, "
myConcat(", ","red","orange","blue")
// returns "elephant; giraffe; lion; cheetah;"
myConcat("; ","elephant","giraffe","lion", "cheetah")
// returns "sage. basil. oregano. pepper. parsley. "See the
myConcat(". ","sage","basil","oregano", "pepper", "parsley")
Function object in the Client-Side JavaScript Reference for more information.
eval function evaluates a string of JavaScript code without reference to a particular object. The syntax of eval is:
eval(expr)where
expr is a string to be evaluated.
If the string represents an expression, eval evaluates the expression. If the argument represents one or more JavaScript statements, eval performs the statements. Do not call eval to evaluate an arithmetic expression; JavaScript evaluates arithmetic expressions automatically.
isFinite is:
isFinite(number)where
number is the number to evaluate.
If the argument is NaN, positive infinity or negative infinity, this method returns false, otherwise it returns true.
The following code checks client input to determine whether it is a finite number.
if(isFinite(ClientInput) == true)
{
/* take specific steps */
}
isNaN function evaluates an argument to determine if it is "NaN" (not a number). The syntax of isNaN is:
isNaN(testValue)where
testValue is the value you want to evaluate.
The parseFloat and parseInt functions return "NaN" when they evaluate a value that is not a number. isNaN returns true if passed "NaN," and false otherwise.
The following code evaluates floatValue to determine if it is a number and then calls a procedure accordingly:
floatValue=parseFloat(toFloat)
if (isNaN(floatValue)) {
notFloat()
} else {
isFloat()
}
parseInt and parseFloat, return a numeric value when given a string as an argument.
The syntax of parseFloat is
parseFloat(str)where
parseFloat parses its argument, the string str, and attempts to return a floating-point number. If it encounters a character other than a sign (+ or -), a numeral (0-9), a decimal point, or an exponent, then it returns the value up to that point and ignores that character and all succeeding characters. If the first character cannot be converted to a number, it returns "NaN" (not a number).
The syntax of parseInt is
parseInt(str [, radix])
parseInt parses its first argument, the string str, and attempts to return an integer of the specified radix (base), indicated by the second, optional argument, radix. For example, a radix of ten indicates to convert to a decimal number, eight octal, sixteen hexadecimal, and so on. For radixes above ten, the letters of the alphabet indicate numerals greater than nine. For example, for hexadecimal numbers (base 16), A through F are used.
If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point. If the first character cannot be converted to a number in the specified radix, it returns "NaN." The parseInt function truncates the string to integer values.
Number and String functions let you convert an object to a number or a string. The syntax of these functions is:
Number(objRef)where
String(objRef)
objRef is an object reference.
The following example converts the Date object to a readable string.
D = new Date (430054663215)
// The following returns
// "Thu Aug 18 04:37:43 GMT-0700 (Pacific Daylight Time) 1983"
x = String(D)
escape and unescape functions let you encode and decode strings. The escape function returns the hexadecimal encoding of an argument in the ISO Latin character set. The unescape function returns the ASCII string for the specified hexadecimal encoding value.
The syntax of these functions is:
escape(string)These functions are used primarily with server-side JavaScript to encode and decode name/value pairs in URLs.
unescape(string)
Last Updated: 05/27/99 21:21:27