判断 undefined and ( == null) and (!something) and ( == null)

1、判断 undefined

var a = '',
    b = ' ', 
    c = undefined ,
    d = null ,
    e = 1 ,
    f = false ,
    g = 0 ,
    h = "0";
    
    if(a === void 0){
      console.log("come in");
    }else{
      console.log("not in here");
    }
    
    //a  print: not in here
    //b  print: not in here
    //c  print: come in
    //d  print: not in here
    //e  print: not in here
    //f  print: not in here
    //g  print: not in here
    //h  print: not in here

2、判断 (!something)

var exp = null ,
    und = undefined ,
    no = '' ,
    space = ' ',
    fal = false ,
    zero = 0;
    if (!exp)
    {
        console.log("is null or nudefined or '' or false or 0");
    }else{
        console.log("is something");
    }
    
    //exp    print: is null or nudefined or '' or false or 0
    //und    print: is null or nudefined or '' or false or 0
    //no     print: is null or nudefined or '' or false or 0
    //space  print: is something
    //fal    print: is null or nudefined or '' or false or 0
    //zero   print: is null or nudefined or '' or false or 0

如果判断条件为 undefined，或数字零，或 false，也会得到与 null 相同的结果，虽然 null 和二者不一样。 注意：要同时判断 null、undefined、数字零、false 时可使用本法。

　　　　javascript中以下值会被转换为false

• false
• undefined
• null
• 0
• -0
• NaN
• ""

 

3、判断 ( == null)

var exp = null ,
    und = undefined ,
    no = '' ,
    space = ' ',
    fal = false ,
    zero = 0;
    if (zero == null)
    {
        console.log("is null");
    }else{
        console.log("here is another");
    }
    
    //exp    print: is null
    //und    print: is null
    //no     print: here is another
    //space  print: here is another
    //fal    print: here is another
    //zero   print: here is another

判断条件为 undefined 时，也会得到与 null 相同的结果，虽然 null 和 undefined 不一样。 注意：要同时判断 null 和 undefined 时可使用本法。

 

4、判断( == null) 方法1

var exp = null ,
    und = undefined ,
    undStr = 'undefined' ,
    no = '' ,
    space = ' ',
    fal = false ,
    zero = 0;
    
    if (!undStr && typeof undStr != "undefined" && undStr != 0)
    {
        console.log("is null");
    }else{
        console.log("is not null")
    }
    
    //exp       print: is null
    //und       print: is not null
    //undStr    print: is not null
    //no        print: is not null
    //space     print: is not null
    //fal       print: is not null
    //zero      print: is not null

typeof exp != "undefined" 排除了 undefined； exp != 0 排除了数字零和 false。

 

5、判断null 方法2

var exp = null ,
    und = undefined ,
    no = '' ,
    space = ' ',
    fal = false ,
    zero = 0;
    if (exp === null)
    {
        console.log("is null");
    }else{
        console.log("is not null")
    }
    
    //exp       print: is null
    //und       print: is not null
    //no        print: is not null
    //space     print: is not null
    //fal       print: is not null
    //zero      print: is not null

尽管如此，我们在 DOM 应用中，一般只需要用 (!exp) 来判断就可以了，因为 DOM 应用中，可能返回 null，可能返回 undefined，如果具体判断 null 还是 undefined 会使程序过于复杂。