tailieunhanh - JavaScript Bible 5th Edition 2004 phần 2

Báo cáo bên trong các thiết lập thẻ thực hiện chỉ khi bắn của sự kiện. Không có định nghĩa chức năng được yêu cầu. Bằng cách này, ràng buộc một sự kiện của đối tượng một kịch bản có nghĩa là không có xử lý sự kiện được xác định trong thẻ của phần tử. | 146 Part III Document Objects Reference you design a script to perform some action upon a mousedown event in a paragraph whose ID is myParagraph the script statements are enclosed in the following tag set script for myParagraph event onmousedown type text javascript Script Statements inside the tag set execute only upon the firing of the event. No function definitions are required. This way of binding an object s event to a script means that there is no event handler defined in the element s tag. Therefore it guarantees that only IE4 or later can carry out the script when the event occurs. But the tag and attributes contain a lot of source code overhead for each object s script so this is not a technique that you should use for script statements that need to be called by multiple objects. Also be aware that you cannot use this tag variation if non-IE or pre-IE4 browsers load the page. In such browsers script statements execute as the page loads which certainly causes script errors. Hiding script statements from older browsers The number of people using old Web browsers that don t support scripting languages has diminished considerably in the past few years. However new devices such as mobile phones and pocket-sized computers often employ compact browsers that don t have built-in JavaScript interpreters. Nonscriptable browsers do not know about the script tag. Normally browsers ignore tags they don t understand. That s fine when a tag is just one line of HTML but a script tag sets off any number of script statement lines in a document. Old and compact browsers don t know to expect a closing script tag. Therefore their natural inclination is to render any lines they encounter after the opening script tag. Unfortunately this places script code squarely in the document surely to confuse anyone who sees such gibberish on the page. You can however exercise a technique that tricks most nonscriptable browsers into ignoring the script statements surround the script .