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

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"

What is going on?

The + operator is not defined for arrays.

What happens is that Javascript converts arrays into strings and concatenates those.

 

Update

Since this question and consequently my answer is getting a lot of attention I felt it would be useful and relevant to have an overview about how the + operator behaves in general also.

So, here it goes.

Excluding E4X and implementation-specific stuff, Javascript (as of ES5) has 6 built-in data types:

1. Undefined
2. Null
3. Boolean
4. Number
5. String
6. Object

Note that although typeof somewhat confusingly returns object for Null and function for callable Objects, Null is actually not an Object and strictly speaking, in specification-conforming Javascript implementations all functions are considered to be Objects.

That's right - Javascript has no primitive arrays as such; only instances of an Object called Array with some syntactic sugar to ease the pain.

Adding more to the confusion, wrapper entities such as new Number(5), new Boolean(true) and new String("abc") are all of object type, not numbers, booleans or strings as one might expect. Nevertheless for arithmetic operators Number and Boolean behave as numbers.

Easy, huh? With all that out of the way, we can move on to the overview itself.

Different result types of + by operand types

            || undefined | null   | boolean | number | string | object |
=========================================================================
 undefined  || number    | number | number  | number | string | string | 
 null       || number    | number | number  | number | string | string | 
 boolean    || number    | number | number  | number | string | string | 
 number     || number    | number | number  | number | string | string | 
 string     || string    | string | string  | string | string | string | 
 object     || string    | string | string  | string | string | string | 

^{* applies to Chrome13, FF6, Opera11 and IE9. Checking other browsers and versions is left as an exercise for the reader.}

Note: As pointed out by CMS, for certain cases of objects such as Number, Boolean and custom ones the + operator doesn't necessarily produce a string result. It can vary depending on the implementation of object to primitive conversion. For example var o = { valueOf:function () { return 4; } }; evaluating o + 2; produces 6, a number, evaluating o + '2' produces '42', a string.

To see how the overview table was generated visit http://jsfiddle.net/1obxuc7m/

JavaScript's + operator has two purposes: adding two numbers, or joining two strings. It doesn't have a specific behaviour for arrays, so it's converting them to strings and then joining them.

If you want to join two arrays to produce a new one, use the .concat method instead:

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

If you want to efficiently add all elements from one array to another, you need to use the .push method:

var data = [1, 2];

// ES6+:
data.push(...[3, 4]);
// or legacy:
Array.prototype.push.apply(data, [3, 4]);

// data is now [1, 2, 3, 4]

The behaviour of the + operator is defined in ECMA-262 5e Section 11.6.1:

> ### 11.6.1 The Addition operator ( + ) > > The addition operator either performs string concatenation or numeric addition. The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows: > > 1. Let lref be the result of evaluating AdditiveExpression. > 1. Let lval be GetValue(lref). > 1. Let rref be the result of evaluating MultiplicativeExpression. > 1. Let rval be GetValue(rref). > 1. Let lprim be ToPrimitive(lval). > 1. Let rprim be ToPrimitive(rval). > 1. If Type(lprim) is String or Type(rprim) is String, then > 1. Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim) > 1. Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

You can see that each operand is converted ToPrimitive. By reading further we can find that ToPrimitive will always convert arrays to strings, producing this result.

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.

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]

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];

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

It looks like JavaScript is turning your arrays into strings and joining them together. If you want to add tuples together, you'll have to use a loop or a map function.

[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);

It's doing exactly what you asked it to do.

What you're adding together are array references (which JS converts to strings), not numbers as it seems. It's a bit like adding strings together: "hello " + "world" = "hello world"

It is because, + operator assumes that the operands are string, if they are not numbers. So, it first converts them to string and concats to give the final result , if its not a number. Also, it does not support arrays.

would be nice if you could overload operators in JavaScript but you can't: https://stackoverflow.com/questions/4700085/can-i-define-custom-operator-overloads-in-javascript you can only hack the "==" operator which converts to strings before comparing: http://blogger.xs4all.nl/peterned/archive/2009/04/01/462517.aspx

Some answers here have explained how the unexpected undesired output ('1,23,4') happens and some have explained how to obtain what they assume to be the expected desired output ([1,2,3,4]), i.e. array concatenation. However, the nature of the expected desired output is actually somewhat ambiguous because the original question simply states "I wanted to add the elements of an array into another...". That could mean array concatenation but it could also mean tuple addition (e.g. here and here), i.e. adding the scalar values of elements in one array to the scalar values of the corresponding elements in the second, e.g. combining [1,2] and [3,4] to obtain [4,6].

Assuming both arrays have the same arity/length, here is one simple solution:

const arr1 = [1, 2];
const arr2 = [3, 4];

const add = (a1, a2) => a1.map((e, i) => e + a2[i]);

console.log(add(arr1, arr2)); // ==> [4, 6]

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.