`
pstinghua
  • 浏览: 24235 次
  • 性别: Icon_minigender_1
  • 来自: 北京
最近访客 更多访客>>
社区版块
存档分类
最新评论

functions

阅读更多
[size=medium]Technically speaking, the function statement is not a statement. Statements cause dynamic behavior in a
JavaScript program, while function definitions describe the static structure of a program. Statements are
executed at runtime, but functions are defined when JavaScript code is parsed, or compiled, before it is
actually run. When the JavaScript parser encounters a function definition, it parses and stores (without
executing) the statements that comprise the body of the function. Then it defines a property (in the call
object if the function definition is nested in another function; otherwise, in the global object) with the
same name as the function to hold the function.
The fact that function definitions occur at parse time rather than at runtime causes some surprising
effects. Consider the following code:
alert(f(4)); // Displays 16. f( ) can be called before it is defined.
var f = 0; // This statement overwrites the property f.
function f(x) { // This "statement" defines the function f before either
  return x*x; // of the lines above are executed.
}
alert(f); // Displays 0. f( ) has been overwritten by the variable f.

These unusual results occur because function definition occurs at a different time than variable definition.
Fortunately, these situations do not arise very often.[/size]
分享到:
评论

相关推荐

Global site tag (gtag.js) - Google Analytics