CODEWITHSUNDEEP
javascript
Tom
Tom

Reputation: 845

var x; ... x.trim(); Why sometimes it allows but sometimes it makes the rest of the code stop working?

This minor issue causes me 5 hours to fix. Finally I figured out. See this code:

<script language="JavaScript" type="text/javascript">
   var x;
   .... // a lot of codes here
   var k=x.trim();

</script>

The above code made the whole app stop working!

I remembered that I used to do like that before but got no problem.

So, about var x; ... x.trim();, Why sometimes it allows but sometimes it makes the rest of the code stop working?

And what is the best code practice for it?

Upvotes: 0

Views: 76

Answers (3)

Vasim Shaikh
Vasim Shaikh

Reputation: 4512

You can do like this:

if(typeof x === 'undefined'){
    // your get an error message
}
else
{
     var k=x.toString().trim();
}

Using strict equality operator === above is good idea there because in JS, you can name a variable as undefined too:

var undefined = "something";

So using === makes sure that you are really checking against undefined value for a variable.

Upvotes: 1

rishat
rishat

Reputation: 8376

I would expand Rajesh's answer. He's right, when you try to call a method that does not exist, a TypeError is thrown. The easiest and fool-proof approach would be to use try/catch to ensure that the rest of the code would be executed as it should. But it's likely that even if it does, you don't get the result you want.

I believe the best way to do would be to wrap the value you're having into String object. It's as easy as

var k = String(x).trim();

It does several important things:

  1. Converts the value of x, whatever it be, into a string, i.e. when you check its type, it's always 'string' and is always an instance of the String object.
  2. Ensures that the resulting value has the method trim which does what it should.
  3. Doesn't throw any error, so the rest of the code is executed.

There may be several pitfalls. If x is undefined, null, NaN or an object, the result of String(x) would be, correspondingly, 'undefined', 'null', 'NaN', or '[object Object]'. If x is an array, it's a specific case, and the value would be the same as if you call x.join(','), for example

x = [1, 2, 3];
var k = String(x).trim; // k is now '1,2,3'

So always keep in mind what types you're dealing with.

Just as with String, you can cast variables to other types, but naïvely converting anything into a Number, a String or an Array is considered a very bad practice. You should always be somewhat sure what type you're working with.

Upvotes: 0

Rajesh
Rajesh

Reputation: 24925

trim is a function of String. Refer MDN - String.trim().

So when you apply it to an integer, it fails and throws error, causing you code to stop work

Example

try{
  var a = 1;
  console.log(a.trim());
}
catch(ex){
  console.log(ex);
}

You can try to convert number to string using .toString() and then apply .trim()

try{
  var a = 1;
  console.log(a.toString().trim());
}
catch(ex){
  console.log(ex);
}

Upvotes: 0

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