w3r-javascript07: Functions

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JavaScript : Function

Syntax

function functionName(argument1,argument2..argumentN)
{// block of code in JavaScript
}

Declare a JavaScript function

you can do it in one of two ways:

方式一

function cube(n) 
{    
return n*n*n; 
}

方式二

var cube = function(n) 
{    
return n*n*n; 
};

Calling JavaScript functions

cube(2);

Function scope

// The following variables are defined in the global scope
var x = 12, y = 25;// This function is defined in the global scope
function add(){return (x + y);
}add(); // Returns 37

Parameters

If you pass a primitive parameter to a function by value and if the function changes the value of the parameter, the changes is not reflected globally or in the calling function.

函数内部对原始类型数据进行改变，不会对全局变量的值造成影响

function primitive_parameter(a)
{a = 100;alert("Inside the function a is: " + a);//100
}var a = 20;
alert("Before calling the function a is: " + a);//20
primitive_parameter(a);
alert("After calling the function a is: " + a);//20

If you pass an object as an non primitive parameter and the function changes the object's properties, the changes is reflected globally i.e. it is visible outside the function.

函数的参数是一个对象，在函数内部改变对象的属性值，这个改变会同步到全局的对象上。

function student(theObject)
{theObject.name = "Sara";
}var student1= {name: "Scott", standard: "V", roll_no: 1};
var x,y;
x = student1.name;     // x gets the value "Scott"
student(student1);
y = student1.name;     // y gets the value "Sara"
// (the name property was changed by the function)

function student(theObject)
{theObject = {name: "Sara", standard: "VI", roll_no: 1};// a new Object
}var student1= {name: "Scott", standard: "V", roll_no: 1};
var x, y;x = student1.name;  // x gets the value "Scott"
student(student1);
y = student1.name;  // y still gets the value "Scott"

Using the arguments object

Using the arguments object, you can call a function with more arguments and it is useful if you don't know in advance how many arguments will be passed to the function. The individual arguments contained in the arguments object can be accessed in the same way array elements are accessed. You can access each argument in the following way :

arguments[i]

function adding_numbers(numbers) {var result = 0, i;// iterate through argumentsfor (i = 0; i < arguments.length; i++) {result += arguments[i];}return result;
}
alert(adding_numbers(1,2,3,-4));

Recursive function

递归函数

例一

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>JavaScript: Recursive function.</title>
<script type="text/javascript">
function factorial(n)
{
if ((n==0) || (n==1))
return 1;
else 
facn = (n * factorial(n-1))
return facn;
}
</script>
</head>
<body>
<h1 style="color: red">JavaScript Recursive function example</h1>
<hr />
<script type="text/javascript">
function factcal()
{
n=document.form1.text1.value;
result = factorial(n);
alert(" Factorial of "+n+" = "+result);
}
</script>
<form name="form1" action="#">
Input a number :
<input type="text" name="text1" size="5" />
<br />
<input type="button" value="Result" οnclick="factcal()" />
</form>
</body>
</html>

一个小例子

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Cube of a Number</title>
<link rel='stylesheet' href='form-style.css' type='text/css' />
</head>
<body οnlοad='document.form1.text1.focus()'>
<div class="mail">
<h2>Input a number and get it's Cube</h2>
<form name="form1" action="#">
<ul>
<li><input type='text' name='text1'/></li>
<li>&nbsp;</li>
<li><input type="submit" name="submit" value="Submit" οnclick="cube(document.form1.text1)" /></li>
<li>&nbsp;</li>
</ul>
</form>
</div>
<script src="cube-number.js"></script>
</body>
</html>

function cube(inputtxt){var numbers = /^[-+]?[0-9]+$/;  var x;if(inputtxt.value.match(numbers)){alert('Cube of '+inputtxt.value+' is '+inputtxt.value*inputtxt.value*inputtxt.value);document.form1.text1.focus();return true;}else{alert('Please input numeric characters only');document.form1.text1.focus();return false;}}

PreDefined Functions---Javascript预定义的函数

isFinite()

判断一个数是否是有限范围内的

The isFinite is used to determine whether a specified number is finite or not. isFinite is a top-level function and is not associated with any object.

isFinite(number)

<script type="text/javascript">
//This is done to make the following JavaScript code compatible to XHTML. <![CDATA[
document.write(isFinite("Good Morning")+ "<br />");//false
document.write(isFinite("2009/01/01")+ "<br />");//false
document.write(isFinite(455)+ "<br />");//true
document.write(isFinite(-9.34)+ "<br />");//true
document.write(isFinite(15-12)+ "<br />");//true
document.write(isFinite(0)+ "<br />");//true
//]]>
</script>

isNaN()

The isNaN function is used to determine whether a value is "NaN" (not a number) or not. isNaN is a top-level function and is not associated with any object.

判断是否为非数字类型的值

<script type="text/javascript">
//This is done to make the following JavaScript code compatible to XHTML. <![CDATA[
document.write(isNaN("Good Morning")+ "<br />");//true
document.write(isNaN("2009/01/01")+ "<br />");//true
document.write(isNaN(455)+ "<br />");//false
document.write(isNaN(-9.34)+ "<br />");//false
document.write(isNaN(15-12)+ "<br />");//false
document.write(isNaN(0)+ "<br />");//false
//]]>
</script>

parseInt()

The parseInt is used to get a numeric value from a string.

parseInt is a top-level function and is not associated with any object.

parseInt(numString, radix)

parseFloat()

The parseFloat is used to get a floating value from a string.

parseFloat is a top-level function and is not associated with any object.

parseFloat(string)

NaN

NaN is a value represents not a number.

NaN不是一个数值

NaN is a top level property and is not associated with any object.

NaN不与任何对象关联，它是一个顶级属性

NaN is never equal to any other number, including NaN itself. It is not possible to check whether a value is number by comparing it with NaN using equal to operator. You have to use the isNaN function instead.

NaN与任何数字都不相等，包括它自己。不能通过与NaN比较是否相等来判断是否为数字类型

Methods like Number constructor, parseFloat, and parseInt return NaN if the argument passed to them are not numbers.

对于操作数字的方法，Number constructor,parseInt,parseFloat,如果发生异常，将返回NaN

NaN property can be used to refer an error for a function, provided the function returns a valid number.

NaN更多的是用来在发生异常的时候，返回一个数据给调用者

Number

The number function is used to converts a specified object to a number..