How to determine whether a string is empty in js

JS method to determine whether a string is empty: 1. To determine whether a string is empty, the code is [if (string.length == 0)]; 2. To determine whether a string is an "empty" character That is, the user entered a space, and the code is [if (strings.replace(/(^s*)|(s*$)..].

this Tutorial operating environment: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer. This method is suitable for all brands of computers.

JS method to determine whether a string is empty:

Determine whether the string is empty

var strings = ''; 
if (string.length == 0) 
{ 
alert('不能为空'); 
}

Determine 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('不能为空'); 
}

Determine whether the input string is empty or all characters Space

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 cannot judge normally. The following code is the case when it is judged as null

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

When exp is undefined, the same result as null will be obtained, although Null and undefined are different.

Note: You can use this method when you want to judge null and undefined at the same time. The code is as follows

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

If exp is undefined, or the number zero, or false, you will also get The same result as null, although null is different from the two. Note: This method can be used when you want to judge null, undefined, number zero, and false at the same time. The code is as follows

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

For backward compatibility, exp is null When, typeof null always returns object, so it cannot be judged this way.

<script type="text/javascript">
function testuser(){
var i= document.getElementByIdx_x("aa");
if (i.value=="null")
{
alert("请登录后再发表留言!")
return false;
}
else
{
alert(i.value)
return true;
}
}
</script>

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