[javascript] How to check for empty value in Javascript?

I am working on a method of retrieving an array of hidden inputs in my form like so

<input type="hidden" value="12:34:00" name="timetemp0">
<input type="hidden" value="14:45:00" name="timetemp1">
<input type="hidden" value="15:12:00" name="timetemp2">
<input type="hidden" value="16:42:12" name="timetemp3">
<input type="hidden" value="16:54:56" name="timetemp4">
<input type="hidden" value="17:03:10" name="timetemp5">

My javascript function retrieves these individually by using getElementsByName('timetemp'+i)

    for (i ; i < counter[0].value; i++)
        {
//finds hidden element by using concatenation of base name plus counter

var timetemp = document.getElementsByName('timetemp'+i);

//if there is a value alert that value to user - this is just for testing purposes at the moment
//because there is only one of timetemp.i then it occupies position 0 in array
            if (timetemp[0].value == null)
            {
                alert ('No value');

            }
            else
            {
                alert (timetemp[0].value);

            }
}

So what should happen is it will alert the user of the value in that hidden input but if it comes accross an input with no value like this:

<input type="hidden" value="" name="timetemp16">

Then it will say "No value"

However th if function cannot seem to work with this:

I have tried:

1. (timetemp[0].value == null)
2. (timetemp[0].value === null)
3. (timetemp[0].value == undefined)
4. (timetemp[0].value == '')

It always seems to default to else clause.

Any ideas?

In my opinion, using "if(value)" to judge a value whether is an empty value is not strict, because the result of "v?true:false" is false when the value of v is 0(0 is not an empty value). You can use this function:

const isEmptyValue = (value) => {
    if (value === '' || value === null || value === undefined) {
        return true
    } else {
        return false
    }
}

Your script seems incorrect in several places.

Try this

var timetemp = document.getElementsByTagName('input');

for (var i = 0; i < timetemp.length; i++){
    if (timetemp[i].value == ""){
        alert ('No value');
    }
    else{
        alert (timetemp[i].value);
    }
}

Example: http://jsfiddle.net/jasongennaro/FSzT2/

Here's what I changed:

1. started by getting all the inputs via TagName. This makes an array
2. initialized i with a var and then looped through the timetemp array using the timetemp.length property.
3. used timetemp[i] to reference each input in the for statement

The counter in your for loop appears incorrect (or we're missing some code). Here's a working Fiddle.

This code:

//Where is counter[0] initialized?
for (i ; i < counter[0].value; i++)

Should be replaced with:

var timeTempCount = 5; //I'm not sure where you're getting this value so I set it.

for (var i = 0; i <= timeTempCount; i++)

First, I would check what i gets initialized to, to see if the elements returned by getElementsByName are what you think they are. Maybe split the problem by trying it with a hard-coded name like timetemp0, without the concatenation. You can also run the code through a browser debugger (FireBug, Chrome Dev Tools, IE Dev Tools).

Also, for your if-condition, this should suffice:

if (!timetemp[0].value) {
    // The value is empty.
}
else {
    // The value is not empty.
}

The empty string in Javascript is a falsey value, so the logical negation of that will get you into the if-block.