How to use += and -=

Question

So it's come to my attention that when I look at some code I see -= += and it makes sense how they used them but I try and I cant.. So when and how is the appropriate and best time to use them

Answer 1

you can think of it as an operator. The function operator+=(a,b) will add b to a and save the value in a; then, it just returns a. Here an example:
https://code.sololearn.com/c90im60eT7R9/?ref=app

Answer 2

An example is better than everything else...

x +=2 means x=x+2
x -=2 means x=x-2
x /=2 means x=x/2
x *=2 means x=x*2

those are short forms for operations and you can use them everytime you need to perform an operation...
clearer now? :)

Answer 3

a += 2;
is the shorthand of
a = a + 2;

a -= 2;
is the shorthand of
a = a - 2;

a *= 2;
is the shorthand of
a = a * 2;

a /= 2;
is the shorthand of
a = a / 2;

a %= 2;
is the shorthand of
a = a % 1;

a ^= 2;
is the shorthand of
a = a ^ 2;

a |= 2;
is the shorthand of
a = a | 2;

a **= 2;
is the shorthand of
a = a ** 2;
(** is an operator for exponentiation in ES6)