Why is my Javascript increment operator (++) not working properly in my addOne function [duplicate]

Question

Please can someone explain to me why my addOne function doesn't work with the increment operator (++). Please see my code below.

// addOne Function

function addOne(num){
  return num + 1
}

log(addOne(6)) => 7


// same function with ++ operator

function addOne(num){
  return num++
}

log(addOne(6)) => 6


// Question - why am I getting 6 instead of 7 when I use ++ operator?

Anse

The post increment operator does exactly what it should: it returns the original value and then increments the variable. – Percival

at first it returns and then increments. use prefix notation in your case – Tidwell

Your explanation makes absolute sense. Thank you so much. – Anse

Answer 1

There are two increment operators: prefix and postfix.

The postfix operator increments the variable after it is evaluated. For example, the following code produces 11, because it adds 5 and 6:

var a = 5;
(a++) + (a++)

The prefix operator increments the variable before it is evaluated. Sounds like that is what you want. The following code produces 13, because it adds 6 and 7:

var a = 5;
(++a) + (++a)

So your code should be:

function addOne(num) {
  return ++num;
}

console.log(addOne(6));

Ulrikaumeko

Thank you so much for your explanation. You really broke it down. – Anse

Answer 2

That is not the correct use of ++, but also a lot of people would not recommend using ++ at all. ++ mutates the variable and returns its previous value. Try the example below.

var two = 2;
var three = two += 1;
alert(two + ' ' + three);

two = 2;
three = two++;
alert(two + ' ' +  three);

two = 2;
three = two + 1;
alert(two + ' ' +  three);

Militarism

Answer 3

num+1 increments the number before the current expression is evaluted so log will be the number after increment, but num++ increments the number after the expression is evaluated, so log will log the num before increment then increment it.

if you like to do the same functionality as num+1 you may use ++num and it will do the same.

They both increment the number. ++i is equivalent to i = i + 1.

i++ and ++i are very similar but not exactly the same. Both increment the number, but ++i increments the number before the current expression is evaluted, whereas i++ increments the number after the expression is evaluated. See this question

Ferrante