javascript

CHAPTER 3 ■ OPERATORS
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Figure 3–5. JavaScript converts NaN to a wrapper object if you query it with the . or [] operator.
Finally, it’s vital to remember that the left operand to +=, -=, *=, or /= must be a variable, member,
element, or parameter. Otherwise, JavaScript will slap you upside the head by returning a SyntaxError
noting, "invalid assignment left-hand side", which means the left operand must be one of these
things.
3 -= 1;
// SyntaxError: invalid assignment left-hand side { message="invalid assignment left-hand
side" }
"blue" += "berries";
// SyntaxError: invalid assignment left-hand side { message="invalid assignment left-hand
side" }
Incrementing or Decrementing Values
In the event that you are adding or subtracting 1 from a value with += or -=, you may even more
succinctly do so with the ++ increment and -- decrement unary operators, which convert it to a number
if necessary. Note that although += may do addition or concatenation, ++ always does addition.
So in Firebug, let’s double saigonCinnamon with ++ and halve mincedLemonZest with --, verifying our
work with Figure 3–6.
var dough = {
pastryFlour: [1 + 3/4, "cup"],
almondFlour: [1/3, "cup"],
saigonCinnamon: [1, "tsp"],
mincedLemonZest: [2, "tsp"],
seaSalt: [1/4, "tsp"],
soda: [1, "tsp"],
tartar: [1, "tsp"],
pourableVanillaYogurt: [1, "cup"],
egg: [1],
wildBlueberries: [1 + 1/4, "cup"]
};
dough.saigonCinnamon[0] ++;
dough.mincedLemonZest[0] --;