price = price - (price-discount_amount)
price = price-discount_amount;
Or, more succinctly:
price -= discount_amount;
By the same token, this code:
discount_amount = 0.10
price = price - (price*discount_amount)
Could be written:
discount_amount = 0.10;
price -= (price*discount_amount);
Or, even simpler,
discounted_rate = 0.90;
price *= discounted_rate;
No, it's not required. But there are a few cases where what YOU think is the end of the statement turns out not to be where JS thinks it is. Play it safe.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.