Why is comparing with just "!=" not good enough?

Hello,

I've been running my scripts through Douglas Crockford's JSLint. I
notice that it keeps complaining when I do this:

if (da != null) {

it says "Use !== to compare with null".

It does the same for comparison with undefined.

It also complains when I check whether soemthign is true or not:

Use '===' to compare with 'true'.

My question is, what is the reason behind it? Won't != and == work
equally well in the cases above?

Thanks!
Ray

re: Why is comparing with just "!=" not good enough?

On Feb 10, 4:05 pm, "Ray" <ray_use...@yahoo.comwrote:Because it wants you to think about why you're comparing to null:

var x = undefined;
alert( x == null ) // shows true
alert( x === null ) // shows false

It depends on whether you want x to be equivalent to null or *exactly*
null. It's called the strict equality operator for a reason! :-)


--
Rob

re: Why is comparing with just "!=" not good enough?

On Feb 10, 12:05 am, "Ray" <ray_use...@yahoo.comwrote:Using !== is the only way to insure the if what you have is null. For
example:

var x = getNumberOrNull();
if (x != null) {

}

In this example, getNumberOrNull() could return zero. Zero and null
both evaluate to false so the if block would be skipped if x was zero
(false eqauls false). However, !== tests for type and boolean value so
only a true null value would cause the expression to return false.

re: Why is comparing with just "!=" not good enough?

Many thanks, Rob, Benjamin! I learned something new :)

re: Why is comparing with just "!=" not good enough?

If you just want to know if "x" is truish, use
if (x) {

If you want to know if it's falsey, use
if (!x) {

(x == null) only tests if x is falsey, since (false == 0) and ('' ==
false) and (undefined == null). If you're going to bother writing out
(x == true), then it must matter that x is boolean "true" and not 1 or
"asdf" or [1,2,3], so you should use ===. If it doesn't matter, then
skip the "== true" to make it clearer.

undefined and null are oddballs, since they're falsey, but != false.
(null == undefined) but (null !== undefined).

--
Isaac Z. Schlueter
http://isaacschlueter.com

re: Why is comparing with just "!=" not good enough?

On Feb 10, 10:06 am, "Benjamin" <musiccomposit...@gmail.comwrote:That's not true in Javascript. undefined and null are their own
special class of things that don't get automatically typecast to
either true or false in equivalence tests.

alert( 0 != null ); // true
alert( 1 != null ); // true
alert( 0 == null ); // false
alert( undefined == null ); // true
alert( null == false ); // false
alert( null == true ); // false
alert( !null == true ); // true
alert( !0 == !null ); // true
alert( '' == 0 ); // true

--
Isaac Z. Schlueter
http://isaacschlueter.com