JavaScript has an increment and a decrement operator.
They modify a variable in place.
++
adds one to a number, --
subtracts one from a number.
let i = 3;
i++;
// i is now 4
let j = 0;
j--;
// j is now -1
The example above shows the postfix increment/decrement operator. In this case the operator is placed after the variable and the return value of the expression is the value of the variable before the increase/decrease.
let a = 3;
let b = a++;
// a = 4
// b = 3
There is also the prefix variant where the operator is placed before the variable. Then the return value is the value of the variable after the increase/decrease.
let a = 3;
let b = ++a;
// a = 4
// b = 4