Arithmetic operators and math

- [Instructor] As I explained previously, programming, in its most basic form, is taking some data, doing something with it, and putting the results back. This is done using operators, and in JavaScript, we have a bunch of operators that look familiar, but don't always do exactly what you expect. You've already seen one of these, the equals symbol. Outside of programming, an equals symbol denotes the result of a math equation, but in Javascript, it is an assignment. If we put a variable on the left of an equals sign and something else on the right, whatever is on the right will be assigned to that variable, effectively put into the variable container.

In Javascript, we have the standard math operators, plus, minus, multiply, and divide. We use these pretty much the way you'd expect. If you want to calculate the sum of a and b, you just type in a + b. Subtract one from the other by typing in a - b, multiply, a * b, so the asterisk is multiply, and divide a / b. JavaScript follows standard algebra rules, so if I type in a + b * c, operator precedence kicks in and the multiplication of b * c is done first before a is added to it.

So basically, parentheses are wrapped around b and c. This happens with multiplication and division which have precedence over addition and subtraction, just like in regular math. If you want to do something like multiplying the sum of a + b by c, you do what you'd do in standard algebra, wrap a + b in parentheses. Because we typically worth with variables, there's a good chance you may want to do a calculation on an existing variable, and then store the result, something like a = a + 4.

Since the same variable is on both sides of this assignment we can make a shorthand and say a += 4. This takes the value of a, adds 4 to it, and returns the result to a. The same can be done with -=, *=, and /=. If you have a value and you want to incrementally increase or decrease it by adding or subtracting one, there's a super shorthand you can use called the unary operator.

So instead of doing this, a = a + 1, we simply type in a ++. You can do the same thing with minus as well. Instead of a = a - 1, we just say a --. What we're doing here is taking a, then adding or subtracting 1, and then return the value into a again. We can also reverse this operator, writing ++ or -- a instead. This produces the same result, but in a different order, and to see what I mean here, we need to go into the console in the browser.

In my console I'll first setup a new variable a, and set it 1. Now I can console log a, and I get 1. If I console log ++a, I get 2. That's because we took 1 and added it to the original value before printing out a. Remember how I said in JavaScript, whatever happens first happens first and then the next thing happens afterwards? Well here, ++ happens before a, so therefore we take a value and add it into a, then print out a.

If on the other hand, I say console log a ++, the result is still 2, but if I console log a, you will see that a is 3. That's because this time we console logged out the value of a first, then added 1 to it, so that means we get the output 2, and then afterwards 1 is added to it, and the end result is a is 3. This may seem like a weird little quirk, but it's actually a super useful tool when you start working with increments, because it can choose when you want to add 1 to any item.