Javascript Further

1. 類型轉換

Value	Context in which value is used
	String	Number	Boolean	Object
Undefined value	"undefined"	NaN	false	Error
null	"null"	0	false	Error
Nonempty string	As is	Numeric value of string or NaN	true	String object
Empty string	As is	0	false	String object
0	"0"	As is	false	Number object
NaN	"NaN"	As is	false	Number object
Infinity	"Infinity"	As is	true	Number object
Negative infinity	"-Infinity"	As is	true	Number object
Any other number	String value of number	As is	true	Number object
true	"true"	1	As is	Boolean object
false	"false"	0	As is	Boolean object
Object	toString( )	valueOf( ) or toString( ) or NaN	true	As is

2. 值傳遞和引用傳遞

The basic rule inJavaScript is this: primitive types are manipulated by value, and referencetypes, as the name suggests, are manipulated by reference. Numbers and booleansare primitive types in JavaScript -- primitive because they consist of nothingmore than a small, fixed number of bytes that are easily manipulated at the low(primitive) levels of the JavaScript interpreter. Objects, on the other hand,are reference types. Arrays and functions, which are specialized types ofobjects, are therefore also reference types. These data types can containarbitrary numbers of properties or elements, so they cannot be manipulated aseasily as fixed-size primitive values can. Since object and array values canbecome quite large, it doesn't make sense to manipulate these types by value,as this could involve the inefficient copying and comparing of large amounts ofmemory.

 

What about strings? Astring can have an arbitrary length, so it would seem that strings should bereference types. In fact, though, they are usually considered to be primitivetypes in JavaScript simply because they are not objects. Strings don't actuallyfit into the primitive versus reference type dichotomy. We'll have more to sayabout strings and their behavior a little later

值傳遞：數字，布爾值。

引用傳遞：對象，數組，函數。

字符串比較特別，屬於值傳遞。

3. 閉包Lexical Scoping and Nested Functions

var x = "global";
function f(  ) {
    var x = "local";
    function g(  ) { alert(x); }
    g(  );
}
f(  );  // Calling this function displays "local"

4. Function構造函數 The Function( ) Constructor and Function Literals

var y = "global";

function constructFunction(  ) {

    var y = "local";

    return new Function("return y");  // Does not capture the local scope!

}

// This line displays "global", because the function returned by the

// Function(  ) constructor does not use the local scope. Had a function

// literal been used instead, this line would have displayed "local".

alert(constructFunction()(  ));  // Displays "global"