学习JavaScript（二）

Checking if a Variable Exists

var result = '';
if (somevar){result = 'yes';}

A better way to check if a variable is defined is to use typeof.

if (typeof somevar !== "undefined"){result = 'yes';}
result

Because somevar is undefined.

Agruments

var result = sum(1, 2);
 result;

JavaScript is not picky at all when it comes to parameters. If you pass more parameters than the function expects, the extra parameters will be silently ignored:
>>> sum(1, 2, 3, 4, 5)
3

>>> function args() { return arguments; }
>>>args();
[]
args(1,2,3,4,true,"ninja")
[1, 2, 3, 4, true, "ninja"]
By using the arguments array you can improve the sum() function to accept any number of parameters and add them all up.

function sumOnSteroids() {
var i, res = 0;
var number_of_params = arguments.length;
for (i = 0; i < number_of_params; i++) { 
res += arguments[i];
}
return res;
}

sumOnSteroids(1, 1, 1);=3
sumOnSteroids(1, 2, 3, 4);=10

var scope

var a = 123;
function f() {
 
alert(a);
var a = 1;
alert(a);
}
f();

You might expect that the first alert() will display 123 (the value of the global variable a)
the second will display 1 (the local a).

The first alert will show "undefined". This is because inside the function the local scope is more important than the global scope. So a local variable overwrites any global variable with the same name. At the time of the first alert() a was not yet defined (hence the value undefined) but it still existed in the local space.[color=red][/color]


CallBack Function

function invoke_and_add(a, b){
return a() + b();
}
function one() {
return 1;
}
function two() {
return 2;
}
invoke_and_add(one, two);

When you pass a function A to another function B and B executes A, it's often said that A is a callback function. If A doesn't have a name, then you can say that it's an anonymous callback function.

function multiplyByTwo(a, b, c, callback) {
var i, ar = [];
for(i = 0; i < 3; i++) {
ar[i] = callback(arguments[i] * 2);
}
return ar;
}
function addOne(a) {
return a + 1;
}

myarr = multiplyByTwo(1, 2, 3, addOne);


Self-invoking Functions

(
function(name){
alert('Hello ' + name + '!');
}
)('dude')

One good reason for using self-invoking anonymous functions is to have some
work done without creating global variables.
A drawback, of course, is that you cannot execute the same function twice