[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:
These unusual results occur because function definition occurs at a different time than variable definition.
Fortunately, these situations do not arise very often.[/size]
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]
发表评论
-
Extending Built-in Types
2009-01-08 13:55 728The Function.apply( ) method is ... -
Breakpoints using closures
2009-01-07 13:43 870// // This function implements ... -
Private properties with closures
2009-01-07 13:13 768// // This function adds proper ... -
Constructor Functions
2009-01-07 00:42 763the new operator creates a new ... -
Functions as Methods
2009-01-07 00:39 809When a function is invoked as a ... -
The callee Property
2009-01-07 00:02 667In addition to its array elemen ... -
Variable-Length Argument Lists: The Arguments Obje
2009-01-06 23:06 794The Arguments object has one ve ... -
Function Literals
2009-01-06 21:50 705Although function literals crea ... -
Nested Functions
2009-01-06 21:09 770Nested functions may be defined ... -
Deleting Array Elements
2009-01-05 22:11 572The delete operator sets an arr ... -
Reading and Writing Array Elements
2009-01-05 22:00 771Note that array indexes must be ... -
The valueOf() Method
2009-01-05 18:05 748... -
The toLocaleString() Method
2009-01-05 18:02 858In ECMAScript v3 and JavaScript ... -
The constructor Property
2009-01-05 17:51 794[size=medium]Since constructor ... -
the empty statement
2009-01-05 00:29 788[size=medium]When you intention ... -
with
2009-01-05 00:27 687[size=medium]The with statement ... -
try/catch/finnaly
2009-01-05 00:17 892If control leaves the try bloc ... -
throw
2009-01-04 23:56 694The tHRow statement has the fol ... -
return
2009-01-04 23:52 819If a function executes a return ... -
Labels
2009-01-04 23:01 712Label names are distinct from v ...
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