Decisions and Loops - Beginning JavaScript - page 64

evaluate to true , it won't make any difference to the final result—it'll still be false . So to avoid
wasting time, if the left‐hand side is false , JavaScript doesn't even bother checking the right‐hand
side and just returns a result of false .

Or
Just like AND , OR also works much as it does in English. For example, you might say that if it is
raining or if it is snowing, then you'll take an umbrella. If either of the conditions “it is raining” or
“it is snowing” is true, you will take an umbrella.

Again, just like AND , the OR operator acts on two boolean values (one from its left‐hand side and one
from its right‐hand side) and returns another boolean value. If the left‐hand side evaluates to true
or the right‐hand side evaluates to true , the result returned is true . Otherwise, the result is false .
The following table shows the possible results:

left‐hand side right‐hand side result

true true true

false true true

true false true

false false false

As with the AND operator, JavaScript likes to avoid doing things that make no difference to the final
result. If the left‐hand side is true , then whether the right‐hand side is true or false makes no
difference to the final result—it'll still be true . So, to avoid work, if the left‐hand side is true , the
right‐hand side is not evaluated, and JavaScript simply returns true . The end result is the same—the
only difference is in how JavaScript arrives at the conclusion. However, it does mean you should not
rely on the right‐hand side of the OR operator to be executed.

NOt
In English, we might say, “If I'm not hot, then I'll eat soup.” The condition being evaluated is whether
we're hot. The result is true or false, but in this example we act (eat soup) if the result is false.

However, JavaScript is used to executing code only if a condition is true . So if you want a false
condition to cause code to execute, you need to switch that false value to true (and any true value
to false ). That way you can trick JavaScript into executing code after a false condition.

You do this using the NOT operator. This operator reverses the logic of a result; it takes one boolean
value and changes it to the other boolean value. So it changes true to false and false to true .
This is sometimes called negation .

To use the NOT operator, you put the condition you want reversed in parentheses and put the !
symbol in front of the parentheses. For example:

if (!(degCent < 100)) {
// Some code
}