Summary of js's methods for determining whether an input string is empty blank or null

Determine whether a string is empty

var strings = ''; 
if (string.length == 0) 
{ 
alert(' Cannot be empty '); 
}

Determines whether the string is an empty character, that is, the user has entered a space

var strings = ' '; 
if (strings.replace(/(^s*)|(s*$)/g, "").length ==0) 
{ 
alert(' Cannot be empty '); 
}

Determine if the input string is empty or all are spaces

function isNull( str ){
if ( str == "" ) return true;
var regu = "^[ ]+$";
var re = new RegExp(regu);
return re.test(str);
}

If there is null, the above code will not judge properly. The following code is the case for null

var exp = null; 
if (exp == null) 
{ 
alert("is null"); 
}

When exp is undefined, the same results are obtained as null, although null and undefined are not the same.

Note: This law can be used to determine both null and undefined.The code is as follows

var exp = null; 
if (!exp) 
{ 
alert("is null"); 
}

If exp is undefined, or a number of zeros, or false, you will get the same results as null, although null and 2 are not the same.Note: This law can be used to judge null, undefined, Number Zero, false at the same time.The code is as follows

var exp = null; 
if (typeof exp == "null") 
{ 
alert("is null"); 
}

For downward compatibility, when exp is null, typeof null always returns object, so this cannot be judged.

<script type="text/javascript">
function testuser(){
var i= document.getElementByIdx_x("aa");
if (i.value=="null")
{
alert(" Please sign in before posting a message !")
return false;
}
else
{
alert(i.value)
return true;
}
}
</script>