Javascript 笔记(1)----函数
• You can pass an anonymous function as a parameter to another function. The receiving function can do something useful with the function that you pass.
• You can define an anonymous function and execute it right away.
- Callback Functions
Advantages:
- They let you pass functions withot the need to name them(which means there are less global variables)更少的全部变量
- You can delegate the responsibility of calling a function to another function(which means there is less code to write)你能指派一个回调函数给另一个函数,意味着你可以写更少的代码
- they can help with performance
function multiplyByTwo(a, b, c) {
var i, ar = [];
for(i = 0; i < 3; i++) {
ar[i] = arguments[i] * 2;
}
return ar;
}
function addOne(a) {
return a + 1;
}
>>>var myarr = [];
>>>myarr = multiplyByTwo(10.20,30);
[20,40,60]
>>for(var i=0; i<3; i++){myarr[i] = addNoe(myarr[i]);}
myarr
[21,41,61]
Loops can be expensive if they go through a lot or repetitions
匿名函数的改进:
>>>myarr = mulitiplybyTwo(1,2,3,function(a){return a+1;});
[4,6,8]
- Slef-invoking Functions:calling this functinon right after it was defined.
()();
(
function() {
alert('hello');
}
)();
you simply place an anonymous function definition inside parentheses(括号) followed by another set of parentheses. The second set basically says "execute now" and is also the place to put any parameters that your anonymous function might accept.
One good reason for using self-invoking anonymous functions is to have some work done without creating global variables. A drawback(缺陷), of course, is that you cannot execute the same function twice (unless you put it inside a loop or another function). This makes the anonymous self-invoking functions best suited for one-off(一次性) or initialization tasks.
- Inner (Private) Functions
Bearing in mind that a function is just like any other value.
function a() {
function b() {......}
}
a() is gloable function, and then b() is a local function, which canot accessible outside a(); likes a private function
Advantage
-
- You keep the global namespace clean (smaller chance of naming collisions).减少命名空间的冲突
- Privacy—you expose(暴露) only the functions you decide to the "outside world"(外部接口),keeping to yourself functionality that is not meant to be consumed(消耗) by the rest of the application.(让自己的功能不至于被外界的程序或则应用猜测到)
- Function that Return Functions
前面说过,指明return的时候函数会返回相应的值/或则数组.当没指明时通常是undefined.同样,我们可以返回一个函数!
function a(){
alert('A');
return function () {
alert('B');
};
}
>>>var newFunc = a();
>>>>newFunc();
//A
//B
若想立即执行返回的函数,还可以这么做
>>>a()();
- Function,Rewrite Thyself(你自己)!
Countiuing with the previous example:
>>>a = a();
//A (consider this as being the one-off preparatory work)
//B (Redefine the global variable a,assigning a new function to it)
This is usefull when a function has some initial one-off woek to do, The function overwrites itself after the first it back to the function.
var a = function () { //最重要的,a在执行一次后就变样了,便成了a -> actualWork();
function someSetup() { //private
var setup = 'done';
}
function actualWork() { //private
alert('Work-worky');
}
someSetup(); //调用私有函数
return actualWork; //return actualWork();区别在于前者返回的是一个函数,后者返回的是函数执行的结果
}(); //self-invoking function only excute once
好处:在解决浏览器的兼容性的问题中,根据不同的浏览器选择不同的环境,就像上面一样,在执行一次后a就变了.
