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

It's converting the individual arrays to strings, then combining the strings.

Answer 2

Another result using just a simple "+" sign will be:

[1,2]+','+[3,4] === [1,2,3,4]

So something like this should work (but!):

var a=[1,2];
var b=[3,4];
a=a+','+b; // [1,2,3,4]

... but it will convert the variable a from an Array to String! Keep it in mind.

Answer 3

It adds the two arrays as if they were strings.

The string representation for the first array would be "1,2" and the second would be "3,4". So when the + sign is found, it cannot sum arrays and then concatenate them as being strings.

Answer 4

The + concats strings, so it converts the arrays to strings.

[1,2] + [3,4]
'1,2' + '3,4'
1,23,4

To combine arrays, use concat.

[1,2].concat([3,4])
[1,2,3,4]

Answer 5

[1,2]+[3,4] in JavaScript is same as evaluating:

new Array( [1,2] ).toString() + new Array( [3,4] ).toString();

and so to solve your problem, best thing would be to add two arrays in-place or without creating a new array:

var a=[1,2];
var b=[3,4];
a.push.apply(a, b);

Answer 6

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];