Possible Duplicate:
What does this mean? (function (x,y)){…}){a,b); in JavaScript
Please anyone explain and give example about the Javascript below, I can't find any document that explain about this:
(function([arg, arg ...]) {
//javascript code here
})([str, str ...]);
I'm asking this question because I see in the Google also Facebook javascript code, most of the them using this syntax.
It is anonymous function invocation
the arguments values str, str are passed to the function as arguments arg, arg and then the javascript code is executed.
it is the same as
function foo([arg, arg ...]) {
//javascript code here
}
foo([str, str, ...]);
What's happening is, an anonymous function expression is defined (in parentheses) -
(function([arg, arg ...]) {
//javascript code here
})
The function is then immediately called using the () at the end of the function, just as as you'd call a normal function var result = someFunction(). So , for example -
(function() {
alert('Called by "()" at end of function');
})()
You can also pass arguments to the anoymous function, via the parentheses -
(function(username) {
alert('Called by "()" at end of function. Hello ' + username + '!');
})('Dave')
(function(a, b) {
var x;
})
returns an anonymous function. For technical/historical reasons, anonymous functions must be wrapped in parentheses.
Notably, the names a,b and x are not visible to outside code. In contrast, just writing var x; would make x accessible and thereby pollute the global namespace. This is the equivalent of private variables in classical object-orientated languages.
We could also write:
var func = function(a, b) {var x;};
// or equivalently:
function foo(a, b) {var x;};
func('arg1', 2);
but that would again pollute the global namespace by making a function func visible for everyone else. Since we don't want to call func more than once anyway,
(function(a, b) {var x;}) ('arg1', 2);
is the perfect way to execute code with variables without making these variables(or any variable names) visible to the rest of the world.
undefined, not a function.
The function in round brackets is called just once with args defined by strs.
The round brackets containing the function are used to limit the scope of the function, i.e. the function cannot be called externally.
The str variables can be global variables (e.g. jQuery) and can be aliased by their corresponding arg (e.g. jQuery -> $) for the sole use of the function.
Generally, this is a good, safe way to write javascript, e.g.
(function($) {
// code
})(jQuery)
For the function, the local variable '$' means the global variable 'jQuery'. The means you can have '$' defined to be something else in another script (or function) and that variable will not affect '$' in this function.
