PDF In-Depth

Forms - More About JavaScript Types

February 01, 2001

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Data types are implicit and (to a degree) contextual in JavaScript, which makes life easy on the programmer, for the most part. But there are times when it is helpful or even essential to know whether two variables are of the same type. In recognition of this, the folks who certify and modify the core language have introduced a new operator (or pair of operators) that, while not in the version of JavaScript used in Acrobat 4.0, will without a doubt be in the next version of Acrobat JavaScript. I am talking, of course, about the strict-equality operator. Some people call it the "identity operator," but I prefer the term strict equality, which to me is just as strict (!) and more descriptive since it admits the possibility of a not-so-strict equaity operator (which is in fact what we're all accustomed to using now).

In JavaScript 1.3 and 1.4, the strict equality operator looks like three equal signs in a row. You can use it to test things like:

if (1 === '1')   app.beep(0);

In real life, you wouldn't test a statement like that one, necessarily. I chose it because it illustrates a point. The point is that currently, if you do:

if (1 == '1')   app.beep(0);

you will get a beep, because with the less-strict equality operator (that we're all accustomed to using), the comparison is permissively type-resolved. That is, JavaScript converts the string to a number and then does the comparison. Whereas, with the strict-equality version (further above), the comparison fails because the items being compared are of different types. Naturally, there is also a strict inequality operator, !==, which can be used in much the same way and circumstances as the strict equality operator. Again, if the parameters differ in type, !== will return true on that basis alone.

So, if you want to compare two items by value, just use == or != per usual. If you want to compare them by value and type, use === or !==. But don't use these latter two yet, because they don't work in Acro 4. (You'll get a syntax error.)

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