Why does [1,2] + [3,4] = “1,23,4” in JavaScript?
I wanted to add the elements of an array into another, so I tried this simple sentence in our beloved Firebug:
[1,2] + [3,4]
It responded with:

"1,23,4"
What is going on?

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JavaScript has 6 built-in data types:

undefined
boolean
number
string
function
object

Note that neither null nor [] is a separate type - both return object when fed to typeof. However + works differently in either case.

That's right - JavaScript has no primitive arrays as such; only instances of a class 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:
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            | undefined | boolean | number | string | function | object | null   | array  |
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undefined   | number    | number  | number | string | string   | string | number | string |

boolean     | number    | number  | number | string | string   | string | number | string |

number      | number    | number  | number | string | string   | string | number | string |

string      | string    | string  | string | string | string   | string | string | string |

function    | string    | string  | string | string | string   | string | string | string |

object      | string    | string  | string | string | string   | string | string | string |

null        | number    | number  | number | string | string   | string | number | string |

array       | string    | string  | string | string | string   | string | string | string |

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* this applies to Chrome 13, Firefox 6, Opera 11 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.