Re: Conditional shortcut (?:) limitations?

On Sun, 1 Feb 2004 17:05:41 -0500, TheKeith <no (AT) spam (DOT) com> wrote:

[snip]

Much appreciated--but I'm still confused as to why that is required. If I
can do conditionals in the if...else format without the else statement,
why can't I do the same with ?: operator shortcut?

It's not a shortcut; it's a ternary operator. Unary operators, like !,
require one operand. Binary operators, like +, require two operands.
Ternary operators, like ?:, require three. Only using two operands with
them is like trying to use:

var a = b *; // or a = * b;

Also, why does it have to be the word *true*--I tried it with just
putting anything behind the *:*, like *: var hello = "hello"* for
example, and that still causes an error--why is this?

The syntax of the operator is:

(expr) ? (if-true expr) : (if-false expr)

The first expression is evaluated. If it is considered true, the second
operand (if-true expr) is evaluated and returned. If the first expression
is considered false, the third operand (if-false expr) is evaluated and
returned.

The main use for the operator is conditional assignment. Say, for example,
you wanted to ensure that an argument in a function is always valid[1]:

function a( x ) {
x = (undefined == x) ? 0 : x;
}

a();

Here, no argument was passed to function a(), so the expression (undefined
== x) will evaluate to true. This means that the operator, as a whole,
will evaluate to 0 and 0 will be assigned to x.

a( 6 );

Here, (undefined == x) will evaluate as false because x is 6. The
operator, as a whole, will evaluate to 6, so 6 will be assigned to x.

If you are just wanted to do perform something based on a certain
condition, then just use an if statement. If you want an expression to
evaluate to a different value based on a condition, then use the
conditional operator.

I suggest you take a look at a JavaScript reference. Try:

http://devedge.netscape.com/library/...1.5/reference/

Beware of URL wrap.

Mike

[1] This isn't a particularly good example, but it's late and I'm tired,
so bear with me

--
Michael Winter
M.Winter (AT) blueyonder (DOT) co.invalid (replace ".invalid" with ".uk" to reply)

"Michael Winter" <M.Winter (AT) blueyonder (DOT) co.invalid> wrote

On Sun, 1 Feb 2004 17:05:41 -0500, TheKeith <no (AT) spam (DOT) com> wrote:

[snip]

Much appreciated--but I'm still confused as to why that is required. If
I
can do conditionals in the if...else format without the else statement,
why can't I do the same with ?: operator shortcut?

It's not a shortcut; it's a ternary operator. Unary operators, like !,
require one operand. Binary operators, like +, require two operands.
Ternary operators, like ?:, require three. Only using two operands with
them is like trying to use:

var a = b *; // or a = * b;

Also, why does it have to be the word *true*--I tried it with just
putting anything behind the *:*, like *: var hello = "hello"* for
example, and that still causes an error--why is this?

The syntax of the operator is:

(expr) ? (if-true expr) : (if-false expr)

The first expression is evaluated. If it is considered true, the second
operand (if-true expr) is evaluated and returned. If the first expression
is considered false, the third operand (if-false expr) is evaluated and
returned.

The main use for the operator is conditional assignment. Say, for example,
you wanted to ensure that an argument in a function is always valid[1]:

function a( x ) {
x = (undefined == x) ? 0 : x;
}

a();

Here, no argument was passed to function a(), so the expression (undefined
== x) will evaluate to true. This means that the operator, as a whole,
will evaluate to 0 and 0 will be assigned to x.

a( 6 );

Here, (undefined == x) will evaluate as false because x is 6. The
operator, as a whole, will evaluate to 6, so 6 will be assigned to x.

If you are just wanted to do perform something based on a certain
condition, then just use an if statement. If you want an expression to
evaluate to a different value based on a condition, then use the
conditional operator.

that cleared it up--thanks a lot.