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When I have one plus, I get the wrong answer e.g.

var b = [069];
var total = 0;

total = total + b
console.log(total) // total = 069

However, when I add a second plus so the equation looks like this

total = total + + b  // total = 69

I get the correct answer of 69. The above is just a simplified example of my issue.

This works fine, however whilst using JSHint I get a warning saying 

confusing pluses

How can I get the correct answer without using the + + ? Also, what is this operator called?

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    [069] is an array; you're initializing b to an array value, and then trying to use it with the + operator. What is it that you expect that to do? Commented Oct 17, 2016 at 19:35
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    Why not just var b = 69;? Commented Oct 17, 2016 at 19:36
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    + in front of a variable would cast it to a number if I'm correct. Try this in your console "5" would return "5", where +"5" would return 5. You could use total = parseInt(total) + parseInt(b); to get a correct result. Commented Oct 17, 2016 at 19:36
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    This seems to be perfect Answer material @AlexSzabó, go for it :) Commented Oct 17, 2016 at 19:39
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    You can also get the same answer if you try with total +- b. You will not need the space between operators if you are are using the +-. You will need the space as + + since ++ is considered to be an increment operator. Put a + or a - before a symbol to convert to an integer. Commented Oct 17, 2016 at 20:20

2 Answers 2

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Javascript unfortunately does a lot of implicit conversions... with your code

b + [69]

what happens is that [69] (an array containing the number 69) is converted to a string, becoming "69". This is then concatenated to b that also is converted in this case to a string "0". Thus the result "069".

If however you add another unary + in front of the array the string gets converted back to a number and you get a numeric result added to b.

0 + [69] → 0 + "69" → "0" + "69" → "069"
0 + + [69] → 0 + + "69" → 0 + 69 → 69

Exact rules are quite complex, but you can be productive with Javascript just considering the simplified form that for binary +:

  • if both are numbers the result is numeric addition
  • otherwise they're converted both to string and result is concatenation

One thing that is somewhat surprising is that implicit conversion of an array to a string is just the conversion of elements to string adding "," between them as separator.

This means that the one-element array [1] is converted to "1"... and implies apparently crazy consequences like [1] == 1.

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13 Comments

"unfortunately" You might want to de-bias your answer :P
@EmileBergeron how advanced the answer has nothing to do with how skilled OP is. They asked a question, they got an answer.
that is irrelevant.
Did you just assume OPs knowledge?
@EmileBergeron unfortunately that really doesn't matter. It's a very correct answer to OPs question. You can't reduce an answer, potentially making it wrong in order to cater to OP.
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Posting my comment as an answer

+ in front of a variable would cast it to a number if I'm correct.

Try this in your console:

"5" would return "5" (string), where

+"5" would return 5 (number).

You could use total = parseInt(total) + parseInt(b); to get a correct result, as parseInt() would try to make a number out of any input parameter it gets.

Theoritecally, you could just parse the total as a number, but it would be prone to an error like "1" + "0" = "10" resulting in 10, which should mathematically be 1.

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