JavaScript increment/decrement operators are used to increase or decrease a variable and/or value, and is often used in loops as a way to count iterations.

The ++ can be placed before or after the variable. This is the same as x = x + 1.

let x = 1;
x++; // x now equals 2

let x = 1;
++x; // x now equals 2

The — can be placed before or after the variable. This is the same as x = x – 1.

let x = 10;
x--; // x now equals 9

let x = 10;
--x; // x now equals 9

Although the ++/– signs can occur before or after the variable, there can be a big difference in the results. When placing the ++ after the variable the value will be returned before the operand is increased.

So in the following z is equal to x BEFORE the incrementation (i.e., z = x THEN x is incremented).

// post incrementation: the ++ is AFTER the variable
let x = 1;
let z = x++; // z = 1, x = 2

And in the following z is equal to x AFTER the incrementation (i.e., z = x AFTER x is incremented).

// pre incrementation: the ++ is BEFORE the variable
let x = 1;
let z = ++x; // z = 2, x = 2

The above similarly applies to post and pre decrement.

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