javascript中的+ +运算符

Question

When I have one plus, I get the wrong answer eg

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?

Answer 1

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 .

Answer 2

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 .