String <-> Number conversion in Javascript

Question

All,

Below is the code that I have written in JS.

<html>
   <head>
      <title>Fibonacci trial</title>
   </head>
   <body>
      <script type="text/javascript">
            var strRepresen = new String();
            var numberReprsen = new Number();
            strRepresen = "XXX";
            numberReprsen = 10;
            strRepresen = 5;
            numberReprsen = "XXX again";
            document.write(strRepresen);
            document.write(numberReprsen);
      </script>
   </body>
</html>

I have following queries:

1 > I am not able to understand how the line numberReprsen = "XXX again"; does not give an error or NaN when I have defined numberReprsen as a Number object.
2 > I have seen many scripts not writing <script type = "text/javascript">. Is it a standard or mandate to mention the script type?

Note: I am a beginner in Javascript.

w3schools.com/js/default.asp is a good resource for beginners. — zzzzBov
– zzzzBov, Commented Nov 11, 2010 at 17:46

cdhowie · Accepted Answer · 2010-11-11 17:57:37Z

2

I am not able to understand how the line numberReprsen = "XXX again"; does not give an error or NaN when I have defined numberReprsen as a Number object.

Variables in JavaScript are not typed. You have not defined numberReprsen as a Number variable. You have defined it as a variable. That it at one time happened to contain a Number object is irrelevant. You can store anything in it.

I have seen many scripts not writing <script type = "text/javascript">. Is it a standard or mandate to mention the script type?

I believe type is an attribute required by the more recent (X)HTML specifications. But yes, generally, it is a good idea to specify. I always do.

edited Nov 11, 2010 at 17:57

answered Nov 11, 2010 at 17:45

cdhowie

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3 Comments

name_masked Over a year ago

Thank you. So is it possible to check the type of a variable at any point ? something like isInteger() ?

cdhowie Over a year ago

Yes, but only in the most basic sense. For example, if you do var a = 1; then alert(typeof a); will display "number". But if you did var a = new Number(1); then alert(typeof a); will display "object" because the Number class essentially boxes a number in an object.

cdhowie Over a year ago

Oh, as a side note, there are specific techniques you can use to determine the type of an object, or if it is convertible to a type. For example, !isNaN(a) will determine if a is a number, a Number object, or a string that can be successfully converted to a number.

Vivin Paliath · Accepted Answer · 2010-11-11 17:44:36Z

1

As far as your first question is concerned, Javascript is not a statically-typed language; it is a dynamically-typed language. Hence, types are associated with values and not with variables.

Initially numberReprsen is bound to a number, and later you are rebinding it to a string. Here, you're seeing exactly what I described; Javascript is typing the variable based on the value (which was initially a number, and then a string).

JavaScript has various ways to test the type of an object, one of which is duck typing.

To answer your second question, I think it is mandated by [X]HTML specs. Either way, I always specify a type attribute and I believe it is best practice.

answered Nov 11, 2010 at 17:44

Vivin Paliath

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3 Comments

name_masked Over a year ago

yes. But the link[w3schools.com/jsref/jsref_Number.asp] shows that passing a String value to Number object gives a NaN

cdhowie Over a year ago

But you did not say Number("XXX ..."). You said "XXX ...".

Vivin Paliath Over a year ago

You said "XXX ...." and not Number("XXX ..."). The latter will give you an error because the "constructor" expects a number.

webSol · Accepted Answer · 2010-11-11 17:45:11Z

1

It won't raise an error because when you do:

numberReprsen = "XXX again";

You are actually changing its type to string and overwriting its previous value.

answered Nov 11, 2010 at 17:45

webSol

1 Comment

cdhowie Over a year ago

Technically, you are not changing the type of anything. You are discarding one value and replacing it with another. The variable has no type to begin with.

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