How to 'null' check in javascript(node-webkit)?

Question

I'm developing window application for comparing source code with node-webkit and I want to check null function.

my code is

function click1() {
      if(swap == true)
        var lines1 = $('textarea1').val().split('\n'); //compare1
        var lines2 = $('textarea2').val().split('\n'); //compare2

        if (lines1.length == lines2.length) {
             for (var i=0;i<lines1.length;i++) {
                 if(lines1[i] == lines2[i]) {
                      var keys = lines1[i].match(/\b[\w\d]*/g);
                      if(keys[0] == null) { //**problem line**
                          alert('This line is null');
                      } else {
                          alert(keys[0]);
                      }
                 }
             }

If i execute this click event, error occured.

Uncaught TypeError : cannot read property '0' of null

How I can fix this problem..

help me

ps. I tried keys[0] === null, typeof keys[0] == 'null', !keys[0] etc...

Dr. Stitch · Accepted Answer · 2016-01-19 05:28:44Z

1

just use this:

if(keys[0]) {
  alert(keys[0]);
} else {
  alert('This line is null');
}

answered Jan 19, 2016 at 5:28

Dr. Stitch

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4 Comments

elclanrs Over a year ago

.match may return null, null[0] will throw that error. You have to guard against that case too.

Dr. Stitch Over a year ago

i believe keys is not array. remove the "[0]".

J.W.S Over a year ago

Oh, Thanks Dr.Stitch!! i solved problem thanks to your solution. but if key is not null, keys[0] is worked

Dr. Stitch Over a year ago

great! happy to help. :)

Antiokus · Accepted Answer · 2016-01-19 05:34:32Z

0

its not keys[0] that's null. its keys. Look at the error, it can't read property 0 of null meaning it can't read the 0 property of the null variable keys. Your regular expression isn't matching anything, so it returns null.

solution:

if (keys && keys[0])

answered Jan 19, 2016 at 5:34

Antiokus

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2 Comments

J.W.S Over a year ago

Thanks to your adivce. I understand keys not array, but if keys is not null, keys[0] is exist. Why existed [0] ?

Antiokus Over a year ago

I think I understand your question. For the answer I shall point you to this link here. match will return an array if it finds a match and null if no matches are found. It's best to learn how the method works before you agonize over it.

Jakir Hossain · Accepted Answer · 2016-01-19 05:39:47Z

0

array[index] is null or undefined.

if (keys[0] == 'undefined' || keys[0] == null) {
   alert('This line is null');
} else {
   alert(keys[0]);
}

array[index] is null and undefined.

if (keys[0] == 'undefined' && keys[0] == null) {
   alert('This line is null');
} else {
   alert(keys[0]);
}

answered Jan 19, 2016 at 5:39

Jakir Hossain

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1 Comment

J.W.S Over a year ago

Thanks to advice Jakir!

Pardeep Dhingra · Accepted Answer · 2016-01-19 08:01:54Z

0

Try this:

if(!keys || keys.length === 0) { //**problem line**
  alert('This line is null');
}else {
  alert(keys[0]);
}

edited Jan 19, 2016 at 8:01

Pardeep Dhingra

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answered Jan 19, 2016 at 5:43

saikumar

1,0616 silver badges10 bronze badges

1 Comment

J.W.S Over a year ago

Thanks to your adivice saikumar!

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