Language Basics(Chapter 3 of Professional JavaScript® for Web Developers 2nd Edition)-优快云博客

本文深入探讨了JavaScript中变量类型的判断、数值与布尔值转换、字符串与数字转换、空值与未定义值的特性，以及不同类型的比较操作符在实际应用中的行为。通过具体示例代码，展示了等号（==）与恒等号（===）的区别，帮助开发者理解在进行变量比较时如何避免常见的陷阱。

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　　The end result was for ECMAScript to provide two sets of operators: equal and not equal to perform conversion before comparison, and identically equal and not identically equal to perform comparison without conversion.

equality

1 <html>
2 <head>
3 <script type="text/javascript">
4
5 window.onload = function()
6 {
7     console.log("------------Equality Operator-------------");
8     console.log("null == undefined: " + (null == undefined));
9     console.log("'NaN' == NaN: " + ("NaN" == NaN));
10     console.log("5 == NaN: " + (5 == NaN));
11     console.log("NaN == NaN: " + (NaN == NaN));
12     console.log("NaN != NaN: " + (NaN != NaN));
13     console.log("false == 0: " + (false == 0));
14     console.log("true == 1: " + (true == 1));
15     console.log("true == 2: " + (true == 2));
16     console.log("undefined == 0: " + (undefined == 0));
17     console.log("null == 0: " + (null == 0));
18     console.log("'5' == 5: " + ('5' == 5));
19
20     console.log("-----Identically Equality Operator------");
21     console.log("null === undefined: " + (null === undefined));
22     console.log("'NaN' === NaN: " + ("NaN" === NaN));
23     console.log("5 === NaN: " + (5 === NaN));
24     console.log("NaN === NaN: " + (NaN === NaN));
25     console.log("NaN !== NaN: " + (NaN !== NaN));
26     console.log("false === 0: " + (false === 0));
27     console.log("true === 1: " + (true === 1));
28     console.log("true === 2: " + (true === 2));
29     console.log("undefined === 0: " + (undefined === 0));
30     console.log("null === 0: " + (null === 0));
31     console.log("'5' === 5: " + ('5' === 5));
32 };
33 </script>
34 </head>
35 <body>
36 </body>
37 </html>

1 <html>
2 <head>
3 <script type="text/javascript">
4
5 function howManyArgs()
6 {
7     console.log("Count of arguments is " + arguments.length);
8 }
9
10 function addSomeNumber(num)
11 {
12     return num + 100;
13 }
14
15 function addSomeNumber(num)
16 {
17     return num + 200;
18 }
19
20 window.onload = function()
21 {
22     howManyArgs("string", 45);
23     howManyArgs();
24
25     console.log("addSomeNumber(100) is " + addSomeNumber(100));
26 };
27
28 function setName(obj)
29 {
30     obj.name = "Summer";
31     obj = new Object();
32     obj.name = "autumn";
33 }
34 var person = new Object();
35 setName(person);
36 console.log(person.name);
37
38 </script>
39 </head>
40 <body>
41 </body>
42 </html>

1 <html>
2 <head>
3 <script type="text/javascript">
4
5 window.onload = function()
6 {
7     console.log("---------------------typeof-----------------------");
8     var a;
9     console.log("typeof undefined is '" + typeof(a) + "'");
10
11     var b = null;
12     console.log("typeof null is '" + typeof(b) + "'");
13
14     var c = true;
15     console.log("typeof true is '" + typeof(c) + "'");
16
17     var d = 90;
18     console.log("typeof 90 is '" + typeof(d) + "'");
19
20     var e = "hello";
21     console.log("typeof hello is '" + typeof(e) + "'");
22
23     var o = new Object();
24     console.log("typeof Object is '" + typeof(o) + "'");
25
26     console.log("----------Boolean Conversion-------------");
27
28     var emptyString = "";
29     if(emptyString)
30     {
31         console.log("Empty string is converted to true");
32     }
33     else
34     {
35         console.log("Empty string is converted to false");
36     }
37
38     if(b)
39     {
40         console.log("null is converted to true");
41     }
42     else
43     {
44         console.log("null is converted to false");
45     }
46
47     if(a)
48     {
49         console.log("undefined is converted to true");
50     }
51     else
52     {
53         console.log("undefined is converted to false");
54     }
55
56     var zero = 0;
57     if(zero)
58     {
59         console.log("zero is converted to true");
60     }
61     else
62     {
63         console.log("zero is converted to false");
64     }
65
66     console.log("-----------------Number Conversion-----------------");
67
68     console.log(Number("Hello Ray"));
69     console.log(Number(""));
70     console.log(Number("0010"));
71     console.log(Number(null));
72     console.log(Number(true));
73
74     console.log("---------------parseInt---------------");
75     console.log(parseInt("123abc"));
76     console.log(parseInt("123abc", 16));
77     console.log(parseInt(""));
78     console.log(parseInt("0xA"));
79     console.log(parseInt(22.5));
80     console.log(parseInt("070"));
81
82     console.log("---------------parseFloat---------------");
83     console.log(parseFloat("22.34.5"));
84     console.log(parseFloat("3.12e4"));
85 }
86
87 </script>
88 </head>
89 <body>
90 </body>
91 </html>

转载于:https://www.cnblogs.com/zhtf2014/archive/2011/01/10/1932272.html