sum of variable in for(variable in object) loop is resulting in concatenation JavaScript [duplicate]

Question

i'm new to JavaScript. Im trying to make a loop that prints every month and also numbers them, but i end up getting a concatenation of the numbers. here's my code

var months = ['january', 'february', 'march','april',
                    'may', 'june', 'july', 'august',
                    'september', 'october', 'november, 'december'];
        for(i in months){
            document.write((i+1)+'.- '+months[i]);
            document.write('<br >')
        }

and my output is like this:

01.- janury
11.- february
21.- march
.... etc

Don’t use for in to iterate over arrays – it’s for iterating over properties of objects. for (var i = 0; i < months.length; i++) to match the existing code, but really, you need to find a more up-to-date resource for learning JavaScript. Anything that teaches document.write is probably outdated. — Ry-
– Ry- ♦, Commented Sep 15, 2020 at 22:47
The index of a for-in loop is always a string, and when you add a string to a number it concatenates them. — John Montgomery
– John Montgomery, Commented Sep 15, 2020 at 22:47
for(i in obj) iterates keys, and they are Strings. When used on Arrays, they are still Strings. That's why they "concatenate"... + the order is not guaranteed. Try (+i+1). — iAmOren
– iAmOren, Commented Sep 15, 2020 at 22:47
(The accepted answer on the linked duplicate offers several options, but only #3 is correct. Don’t continue using a for in loop with a cast to number.) — Ry-
– Ry- ♦, Commented Sep 15, 2020 at 22:47
thanks for the anwsers! it definetly got mee more aware of the course im following. thnaks again! — Santiago A
– Santiago A, Commented Sep 15, 2020 at 23:42

PsiKai · Accepted Answer · 2020-09-15 23:49:17Z

0

This code will give you the desired result, changing it to a standard for loop:

var months = ['january', 'february', 'march','april', 'may', 'june', 'july', 'august' 'september', 'october', 'november', 'december'];

for (i=0; i < months.length; i++) {
    document.write(i+1 + ".- " + months[i] + "<br>") 
};

edited Sep 15, 2020 at 23:49

answered Sep 15, 2020 at 23:15

PsiKai

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