Why is [1,2] + [3,4] = “1,23,4” in JavaScript?

Question

I wanted to add the elements of an array into another, so I tried this:

[1,2] + [3,4]

It responded with:

\"1,23,4\"
         


        

Answer 1

In JavaScript, the binary addition operator (+) performs both numerical addition and string concatenation. However, when it's first argument is neither a number nor a string then it converts it into a string (hence "1,2") then it does the same with the second "3,4" and concatenates them to "1,23,4".

Try using the "concat" method of Arrays instead:

var a = [1, 2];
var b = [3, 4];
a.concat(b) ; // => [1, 2, 3, 4];