类型检查可以在编译时或运行时执行。静态类型语言(如 C++ 和 Java)在编译时执行类型检查。动态类型语言(如 Smalltalk 和 Python)在运行时执行类型检查。ActionScript 3.0 是动态类型的语言,它在运行时执行类型检查,同时也支持在名为“严格模式”的特殊编译器模式下在编译时执行类型检查。在严格模式下,类型检查既发生在编译时也发生在运行时,但是在标准模式下,类型检查仅发生在运行时。
在构造代码时,动态类型的语言带来了极大的灵活性,但代价是在运行时可能出现类型错误。静态类型的语言在编译时报告类型错误,但代价是要求类型信息在编译时是已知的。
Objects Are Reference Types
All JavaScript data types can be categorized as either primitive or reference types.
These two types correspond to the primitive and composite types discussed in Chapter 3.
Primitive types are the primitive data types: number, string, Boolean, undefined, and null.
These types are primitive in the sense that they are restricted to a set of specific values.
You can think of primitive data as stored directly in the variable itself.
Reference types are objects, including Objects, Arrays, and Functions. Because these types can
hold very large amounts of heterogeneous data, a variable containing a reference type does not
contain its actual value. It contains a reference to a place in memory that contains the actual data.
This distinction will be transparent to you the majority of the time.
But there are some situations when you need to pay particular attention
to the implications of these types. The first is when you create two or
more references to the same object. Consider the following example with primitive types:
var x = 10;
var y = x;
x = 2;
alert("The value of y is: " + y);
This code behaves as you would expect. Because x has a primitive type (number),
the value stored in it (10) is assigned to y on the second line. Changing the value
of x has no effect on y because y received a copy of x’s value. The result is shown here:
Now consider similar code using a reference type:
var x = [10, 9, 8];
var y = x;
x[0] = 2;
alert("The value of y's first element is: " + y[0]);
The result might be surprising:
Because arrays are reference types, the second line copies the reference to x’s data into y.
Now both x and y refer to the same data, so changing the value of this data using either
variable is naturally visible to both x and y.
