Numbers and booleans are primitive types in JavaScriptprimitive because they consist of nothing more than a small, fixed number of bytes that are easily manipulated at the low levels of the JavaScript interpreter. Objects, on the other hand, are
reference types. Arrays and functions, which are specialized types of objects, are therefore also reference
types. These datatypes can contain arbitrary numbers of properties or elements, so they cannot be manipulated as easily as fixed-size primitive values can. Since object and array values can become quite
large, it doesn't make sense to manipulate these types by value, because this could involve the inefficient
copying and comparing of large amounts of memory.
What about strings? A string can have an arbitrary length, so it would seem that strings should be
reference types. In fact, though, they are usually considered primitive types in JavaScript simply because
they are not objects. Strings don't actually fit into the primitive-versus-reference type dichotomy.
JavaScript strings are (presumably) copied and passed by reference, they are compared by value.
Numbers, boolean values, and the null and undefined types are primitive. Objects,
arrays, and functions are reference types.
reference types. Arrays and functions, which are specialized types of objects, are therefore also reference
types. These datatypes can contain arbitrary numbers of properties or elements, so they cannot be manipulated as easily as fixed-size primitive values can. Since object and array values can become quite
large, it doesn't make sense to manipulate these types by value, because this could involve the inefficient
copying and comparing of large amounts of memory.
What about strings? A string can have an arbitrary length, so it would seem that strings should be
reference types. In fact, though, they are usually considered primitive types in JavaScript simply because
they are not objects. Strings don't actually fit into the primitive-versus-reference type dichotomy.
JavaScript strings are (presumably) copied and passed by reference, they are compared by value.
Numbers, boolean values, and the null and undefined types are primitive. Objects,
arrays, and functions are reference types.
发表评论
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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 818If a function executes a return ... -
functions
2009-01-04 23:48 799[size=medium]Technically speaki ...
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