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JavaScript FOR ™ DUMmIES phần 3

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Như một vấn đề của thực tế, bằng cách sử dụng JavaScript là cách dễ nhất để thực hiện các hiệu ứng, chẳng hạn như lật, như bạn có thể nhìn thấy trong Chương 8.Tôi hy vọng bạn đang chomping tại bit để có được bắt đầu trên trang web đầu tiên của bạn hỗ trợ JavaScript! Trước tiên, mặc dù. | Chapter 3 JavaScript Language Basics 57 As of this writing the next version of JavaScript version 2.0 due to be finalized later this year and with luck supported by upcoming browser versions provides for the strongly typed variables with which C and C programmers are familiar. What this means to you is that when browsers support JavaScript 2.0 you may use variable descriptors such as integer and number to declare upfront precisely what kind of value you want each variable to contain. Until then however no variable descriptors are necessary. After you declare a variable whether you use the var keyword or not you can reset its value later in the script by using the assignment operator . The name of the variable can be any legal identifier you want to use letters and numbers not special characters and the value can be any legal expression. A legal expression is any properly punctuated expression that you see represented in this chapter an if-else expression an assignment expression and so on. A variable is valid only when it s in scope. When a variable is in scope it s been declared between the same curly brace boundaries as the statement that s trying to access it. For example if you define a variable named firstName inside a function called displayReport you can refer to the variable only inside the displayReport function s curly braces. If you try to access firstName inside another function you get an error. If you want to reuse a variable among functions shudder that way lies madness you can declare that variable near the top of your script before any functions are declared. That way the variable s scope is the entire script and all the functions get to see it. Take a look at the following code example function displayReport var firstName document.myForm.givenName.value . alertC Click OK to see the report for firstName Using firstName here is fine it was declared inside the same set of curly braces. . function displayGraph alert Here s the graph for firstName .