js的類型轉換是js的基礎，必須要掌握的，我做了一下例子分析，大家可以直接復制運行：
<script type="text/javascript">
var str =
"100";
var num = 100;
var bool = true;
var obj =
{};
//string transform to number
var str_num1 =
Number(str);
var str_num2 = str - 2;
var str_num3 =
parseInt(str,8);//parseInt只能用于字符串轉為數字
//string transform to
bool
var str_bool1 = Boolean(str);
var str_bool2 =
!!(str);
//string transform to object
var str_obj1=
Object(str);
document.write("string to num:<br>type:"+
typeof(str_num1) + " str_num1:" +
str_num1);
document.write("<br>type:"+ typeof(str_num2) +
" str_num2:" +
str_num2);
document.write("<br>type:"+ typeof(str_num3) +
" str_num3:" +
str_num3);
document.write("<br><br>string to
bool:<br>type:"+ typeof(str_bool1) +
" str_bool1:" +
str_bool1);
document.write("<br>type:"+ typeof(str_bool2) +
" str_bool2:" +
str_bool2);
document.write("<br><br>string to
object:<br>type:"+ typeof(str_obj1) +
" str_obj1:" +
str_obj1);
//number transform to string
var num_str1
= String(num);
var num_str2 = num.toFixed(2);
var num_str3 =
num + "1";
var num_str4 = num.toString();
//number transform
to bool
var num_bool1 = Boolean(num);
var num_bool2 =
!!(num);
//number transform to object
var num_obj1 =
Object(num);
document.write("<br><br><br><br>num
to string:<br>type:"+ typeof(num_str1) +
" num_str1:" +
num_str1);
document.write("<br>type:"+ typeof(num_str2) +
" num_str2:" +
num_str2);
document.write("<br>type:"+ typeof(num_str3) +
" num_str3:" +
num_str3);
document.write("<br>type:"+ typeof(num_str4) +
" num_str4:" +
num_str4);
document.write("<br><br>num to
bool:<br>type:"+ typeof(num_bool1) +
" num_bool1:" +
num_bool1);
document.write("<br>type:"+ typeof(num_bool2) +
" num_bool2:" +
num_bool2);
document.write("<br><br>num to
object:<br>type:"+ typeof(num_obj1) +
" num_obj1:" + num_obj1);
//bool
transform to num
var bool_num1 = Number(bool);
var bool_num2 =
bool - 2;
var bool_num3 = parseInt(bool);
//bool transform to
string
var bool_str1 = String(bool);
var bool_str2 = bool +
"2";
var bool_str3 = bool.toString();
//bool transform to
object
var bool_obj1 =
Object(bool);
document.write("<br><br><br><br>bool
to number:<br>type:"+ typeof(bool_num1) +
" bool_num1:" +
bool_num1);
document.write("<br>type:"+ typeof(bool_num2) +
" bool_num2:" +
bool_num2);
document.write("<br>type:"+ typeof(bool_num3) +
" bool_num3:" +
bool_num3);
document.write("<br><br>bool to
string:<br>type:"+ typeof(bool_str1) +
" bool_str1:" +
bool_str1);
document.write("<br>type:"+ typeof(bool_str2) +
" bool_str2:" +
bool_str2);
document.write("<br>type:"+ typeof(bool_str3) +
" bool_str3:" +
bool_str3);
document.write("<br><br>bool to
object:<br>type:"+ typeof(bool_obj1) +
" bool_obj1:" +
bool_obj1);
//object transform to num
var obj_num1 =
Number(obj);
var obj_num2 = obj - 2;
var obj_num3 =
parseInt(obj);
//object transform to string
var obj_str1 =
String(obj);
var obj_str2 = obj + "2";
var obj_str3 =
obj.toString();
//object transform to bool
var obj_bool1 =
Boolean(obj);
var obj_bool2 =
!!(obj);
document.write("<br><br><br><br>object
to number:<br>type:"+ typeof(obj_num1) +
" obj_num1:" +
obj_num1);
document.write("<br>type:"+ typeof(obj_num2) +
" obj_num2:" +
obj_num2);
document.write("<br>type:"+ typeof(obj_num3) +
" obj_num3:" +
obj_num3);
document.write("<br><br>object to
string:<br>type:"+ typeof(obj_str1) +
" obj_str1:" +
obj_str1);
document.write("<br>type:"+ typeof(obj_str2) +
" obj_str2:" +
obj_str2);
document.write("<br>type:"+ typeof(obj_str3) +
" obj_str3:" +
obj_str3);
document.write("<br><br>object to
bool:<br>type:"+ typeof(obj_bool1) +
" obj_bool1:" +
obj_bool1);
document.write("<br>type:"+ typeof(obj_bool2) +
" obj_bool2:" + obj_bool2);
//Boolean()用法
document.write("<br><br><br>Boolean()的用法");
document.write(Boolean("")
+ "<br>"); //false
document.write(Boolean(null) + "<br>");
//false
document.write(Boolean("hi") +
"<br>");//true
document.write(Boolean(200) +
"<br>");//true
document.write(Boolean(0) +
"<br>");//false
document.write(Boolean(undefined) +
"<br>");//false
document.write(Boolean(new Object()) +
"<br>");//true
//Number()用法
document.write("<br><br><br>Number()的用法");
document.write(Number("")
+ "<br>"); //0
document.write(Number(null) + "<br>");
//0
document.write(Number("hi") +
"<br>");//NaN
document.write(Number(200) +
"<br>");//200
document.write(Number(0) +
"<br>");//0
document.write(Number(undefined) +
"<br>");//NaN
document.write(Number(new Object()) +
"<br>");//NaN
document.write(false + "<br>");
//0
document.write(true + "<br>");
//1
//Object()的用法
document.write("<br><br><br>Object()的用法");
document.write(Object("")
+ "<br>"); //返回空格
document.write(Object(null) + "<br>");
//[object Object]
document.write(Object("hi") +
"<br>");//h3
document.write(Object(200) +
"<br>");//200
document.write(Object(0) +
"<br>");//0
document.write(Object(undefined) +
"<br>");//[object Object]
document.write(Object(new Object()) +
"<br>");//[object Object]
document.write(false + "<br>");
//false
document.write(true + "<br>");
//true
</script>
結果如下：
string to
num:
type:number str_num1:100
type:number str_num2:98
type:number str_num3:64
string
to
bool:
type:boolean str_bool1:true
type:boolean str_bool2:true
string
to object:
type:object str_obj1:100
num to
string:
type:string num_str1:100
type:string num_str2:100.00
type:string num_str3:1001
type:string num_str4:100
num
to
bool:
type:boolean num_bool1:true
type:boolean num_bool2:true
num
to object:
type:object num_obj1:100
bool to
number:
type:number bool_num1:1
type:number bool_num2:-1
type:number bool_num3:NaN
bool
to
string:
type:string bool_str1:true
type:string bool_str2:true2
type:string bool_str3:true
bool
to object:
type:object bool_obj1:true
object
to
number:
type:number obj_num1:NaN
type:number obj_num2:NaN
type:number obj_num3:NaN
object
to string:
type:string obj_str1:[object
Object]
type:string obj_str2:[object
Object]2
type:string obj_str3:[object Object]
object
to
bool:
type:boolean obj_bool1:true
type:boolean obj_bool2:true
Boolean()的用法false
false
true
true
false
false
true
Number()的用法0
0
NaN
200
0
NaN
NaN
false
true
Object()的用法
[object
Object]
hi
200
0
[object Object]
[object
Object]
false
true
下面是實驗總結：
類型轉換分為三種方式：（1）強類型轉換（2）轉換函數轉換（3）弱類型轉換
- 強類型轉換：Number(value); String(value); Boolean(value); Object(value)
- 轉換函數轉換：parseInt(value); parseFloat(value)
- 弱類型轉換：var str = str - 0; var num = num + "2"; var num = !!num;
- 轉換為number類型方法： Number(value); parseInt(value);
parseFloat(value),注意：只有對String類型調用這些方法，這兩個函數才能正確運行；對其他類型返回的是NaN(Not a Number)
var value = value - 1; 若value為非數字形式的字符串或對象，返回NaN
- 轉換為string類型方法： String(value); var value = value + "A"
若value為對象類型，則對象首先執行toString()方法，轉化為字符串再與"A"相連; value.toString();
- 轉換為boolean類型方法： Boolean(value); !!(value);
- 轉換為object類型方法： Object(value)
另外，Number(),Boolean(),Object()的用法可以參考實驗結果。