JavaScript Advanced Programming---Learning Notes (iv)

1. Global variables cannot be deleted through the delete operator, whereas properties defined directly on the Window object can.

    var age =;     = "Red";     Delete age; // Cannot delete    delete color; // can delete     alert (window.age); //  in    alert (Window.color); // undefined

2. Attempting to access an undeclared variable throws an error, but by querying the window object you can know if a variable that might not be declared exists.

    var newvalue = OldValue; // will throw the wrong, because OldValue is    undefined var newvalue = Window.oldvalue; // There is no error, because this is a property query, and the result of newvalue is undefined

3.location Object Query string parameters

Although Location.search can return everything from the question mark to the end of the URL, there is no way to access each of these query string parameters individually. To do this, you can create a function that parses the query string and then returns an object that contains all the parameters:

functionGetquerystringargs () {//get the query string and remove the question mark from the beginning    varQS = (location.search.length > 0? location.search.substring (1): ""); //objects that hold data    varargs = []; //get each item    varItems = qs.length? Qs.split ("&") : []; varitem =NULL; varName =NULL; varValue =NULL; //add each item individually to the Args object     for(vari=0;i< items.length;i++) {Item= Items[i].split ("="); Name= decodeURIComponent (item[0]); Value= decodeURIComponent (item[1]); if(name.length) {Args[name]=value; }    }    returnargs;}

Suppose the query string is? q=javascript&num=10
    var args = Getquerystringargs ();
    Alert (args["Q"]);//javascript
    Alert (args["num"]);//10

The first step in this function is to first remove the question mark at the beginning of the query string, provided that the Location.search must contain one or more characters. All parameters are then saved in the Args object, which is created in literal form. Next, divide the query string by the number (&) and return an array of strings in the Name=value format. The For loop then iterates over the array, dividing each item according to the equals sign, returning the first item to the parameter name, and the second item as an array of parameter values. Then use Decodecomponent () to decode name and value separately (because the query string should be encoded), and then name as the attribute of the Args object and value as the corresponding property. Finally, the function is called, and each query string parameter is a property of the returned object, facilitating access to each parameter.

JavaScript Advanced Programming---Learning Notes (iv)