JavaScript Advanced Programming (Third edition) seventh chapter function expressions

One is a function declaration, and the other is a function expression.

function declaration Elevation:

Say Hi ();

function say Hi () {

Alert ("hi!");

}//This example does not throw an error, because the function declaration is read before the code executes.

anonymous function: var functionname=function (arg0,arg1,arg2) {function Body};

Sayhi (); Error: function does not yet exist

var sayhi=function () {

Alert ("hi!");

};

Don't do this//you can do this

if (condition) {var sayhi ();

function Sayhi () {if (condition) {

Alert ("hi!"); Sayhi=function () {

}else{alert ("hi!");

Alert ("yo!"); };

}}else{

} alert ("yo!");

};

}

7.1 Recursion

 function   factorial (num) {  if  (num <= 1) { return  1;  else   { return  num * factorial (num-1 var  anotherfactorial = factorial;            Factorial  = null  ;  Alert (Anotherfactorial ( 4)); // error!  

7.2 Closures

Closures are functions that have access to variables in another function scope. A common way to create closures is to create another function inside one function.

function fn (PropertyName) {

return function (Object1) {

var v1 = Object1[propertyname]; Accessed the external Variable propertyname variable

}

}

7.2.1 Closures and variables

function Createfunctions () {

var result = [];

for (var i=0; i<10; i++) {

Result[i] = function (num) {

return function () {

return num;

}

} (i);

}

}

var funcs = createfunctions ();

for (var i=0; i<funcs.length; i++) {

document.write (Funcs[i] + ' <br/> '); Output 0,1,2,3 ... 9

}

7.2.2 This object

If an HTML element is stored in the scope of the closure, it means that the element cannot be destroyed.

function Assignhandler () {

var element = document.getElementById (' someelement ');

var id = element.id;

Element.onclick = function () {

alert (ID);

};

element = null; variable is set to NULL to dismiss a reference to a DOM object, reduce the number of references, and ensure that the memory it consumes is properly reclaimed

}

7.2.3 Memory leaks

If an HTML element is stored in the scope of the closure, it means that the element cannot be destroyed.

function Assignhandler () {

var element = document.getElementById (' someelement ');

var id = element.id;

Element.onclick = function () {

alert (ID);

};

element = null; variable is set to NULL to dismiss a reference to a DOM object, reduce the number of references, and ensure that the memory it consumes is properly reclaimed

}

JavaScript Advanced Programming (Third edition) seventh chapter function expressions