Andrea's Hello world

每次都忘記
紀錄一下

var a =null; // a is not true //a is not "char" // a is undefined 

var b= '';  // b is not true // b is not null // b is not undefined

var c ='test';  //c ok  //c is not null //c is not undefined //c is not empty char

var d = undefined;  // d is not true // d is not "char" // d is null

 

if(d) { alert('d OK'); }

if(!d) { alert('d is not true') };

if(d==""){ alert('d is empty char ')};

if(d!=""){ alert('d is not empty char ')}

if(d==null) { alert('d is null ') };

if(d!=null){ alert('d is not null ') };

// d is not true

// d is not empty char

// d is null

if(a) { alert('a OK'); }

if(!a) { alert('a is not true') };

if(a==""){ alert('a is empty char ')};

if(a!=""){ alert('a is not empty char ')}

if(a==undefined) { alert('a is undefined') };

if(a!=undefined){ alert('a is not undefined') };

// a is not true

//a is not empty char

// a is undefined 

if(b) { alert('b OK'); }

if(!b) { alert('b is not true') };

if(b==null){ alert('a is null ')};

if(b!=null){ alert('a is not null ')}

if(b==undefined) { alert('a is undefined') };

if(b!=undefined){ alert('a is not undefined') };

// b is not true

// b is not null

// b is not undefined

if(c){ alert('c ok');}
if(!c){alert('c is not true')};
if(c==null){alert('c is null')}
if(c!=null){alert('c is not null ')}
if(c==undefined){alert('c is undefined ')}
if(c!=undefined){ alert('c is not undefined ')}
if(c==""){alert('c is empty char ')}
if(c!=""){ alert('c is not empty char ')}

//c ok

//c is not null

//c is not undefined

//c is not empty char