jQuery: checking if the value of a field is null (empty)

Is this a good way to check if the value of a field is null?

if($('#person_data[开发者_开发知识库document_type]').value() != 'NULL'){}

Or is there a better way?

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The value of a field can not be null, it's always a string value.

The code will check if the string value is the string "NULL". You want to check if it's an empty string instead:

if ($('#person_data[document_type]').val() != ''){}

or:

if ($('#person_data[document_type]').val().length != 0){}

If you want to check if the element exist at all, you should do that before calling val:

var $d = $('#person_data[document_type]');
if ($d.length != 0) {
  if ($d.val().length != 0 ) {...}
}

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I would also trim the input field, cause a space could make it look like filled

if ($.trim($('#person_data[document_type]').val()) != '')
{

}

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Assuming

var val = $('#person_data[document_type]').value();

you have these cases:

val === 'NULL';  // actual value is a string with content "NULL"
val === '';      // actual value is an empty string
val === null;    // actual value is null (absence of any value)

So, use what you need.

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that depends on what kind of information are you passing to the conditional..

sometimes your result will be null or undefined or '' or 0, for my simple validation i use this if.

( $('#id').val() == '0' || $('#id').val() == '' || $('#id').val() == 'undefined' || $('#id').val() == null )

NOTE: null != 'null'

---

_helpers: {
        //Check is string null or empty
        isStringNullOrEmpty: function (val) {
            switch (val) {
                case "":
                case 0:
                case "0":
                case null:
                case false:
                case undefined:
                case typeof this === 'undefined':
                    return true;
                default: return false;
            }
        },

        //Check is string null or whitespace
        isStringNullOrWhiteSpace: function (val) {
            return this.isStringNullOrEmpty(val) || val.replace(/\s/g, "") === '';
        },

        //If string is null or empty then return Null or else original value
        nullIfStringNullOrEmpty: function (val) {
            if (this.isStringNullOrEmpty(val)) {
                return null;
            }
            return val;
        }
    },

Utilize this helpers to achieve that.

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jquery provides val() function and not value(). You can check empty string using jquery

if($('#person_data[document_type]').val() != ''){}

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Try

if( this["person_data[document_type]"].value != '') { 
  console.log('not empty');
}

<input id="person_data[document_type]" value="test" />

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My Working solution is

var town_code = $('#town_code').val(); 
 if(town_code.trim().length == 0){
    var town_code = 0;
 }

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In your code 'NULL' is a string. So its wrong.

You can use exclamation mark ! to check if it is null.

Here is the code:

if(!$('#person_data[document_type]').val()){

}

The above code means that if the $('#person_data[document_type]') has no value (if the value is null).