Given this piece of code :

var n = 1;
console.log(n);
console.log(n--);
console.log(n);

The output is

1

1

0

And for this one

var n = 1;
console.log(n);
console.log(--n);
console.log(n);

The output is

1

0

0

What is happening?

Given this piece of code :

var n = 1;
console.log(n);
console.log(n--);
console.log(n);

The output is

1

1

0

And for this one

var n = 1;
console.log(n);
console.log(--n);
console.log(n);

The output is

1

0

0

What is happening?

• javascript

• 2 If you want it to be done before, use --n instead. – Etheryte Commented Sep 14, 2015 at 10:43
• 7 n-- is not --n - and both variants work identically in javascript as they do in C/C++/C# and all the other spinoffs and variations – Jaromanda X Commented Sep 14, 2015 at 10:43
• 1 I thought the usual behavior of n-- was to decrement n as the last operation on the line. Maybe you're thinking of --n which decrements right away. – Davide Commented Sep 14, 2015 at 10:44
• @JaromandaX Strictly speaking, C# does it differently to C – Rowland Shaw Commented Sep 14, 2015 at 10:48

3 Answers 3

If you want the value to update immediately you can move the -- to the front of the variable name:

var n = 1;
console.log(n);
console.log(--n);
console.log(n);

1

0

0

This is also how it works in C.

The decrement operator decrements (subtracts one from) its operand and returns a value.

• If used postfix (for example, x--), then it returns the value before decrementing.
• If used prefix (for example, --x), then it returns the value after decrementing.

— MDN's notes on Arithmetic Operators

First it's not only javascript who does this. All other programming languages including C, C++, PHP etc does the same. Check the following code:

var i = 1;
console.log(i) // Prints 1
console.log(i--) // It first prints the variable i then decrements it by 1. Therefore the result would be 1.
i = 1;
console.log(--i) // It FIRST decrements the variable i by 1 then prints its decremented value which prints the number 0

There are two types of increment / dectement operators: prefix and postfix. Increment prefix adds 1 immediately, before the operand will be used in current code string operation. So n=++x will first increment x and then puts the incremented x to the n Increment postfix adds 1 after the value of the operand was used in current code string operation. So n = x++ will first puts x to n and then x will be incremented

Decrement operator acts identically