Casting a number to a string by adding an array in JavaScript

Question

In JavaScript:

var stringVar = 12345 + []

Will cast the number to a string.

Small demo: https://jsfiddle.net/ce9bjcwo/

Why does this happen?

Answer 1

Due to Array.prototype.toString().
It converts it to a string representation when trying to concatenate, and since 12345 is not a string, it's type-cast to one.

Examples:
[1,2].toString() -> "1,2"
[].toString() -> ""
12345 + "" -> "12345"

Answer 2

It happens because:

There is no way to apply the + operator to an array, so Javascript stringifies the array. Since it's an empty array, it stringifies as ''. Then you have 12345 + ''

And in javascirpt "number" + "string" returns a string, by casting the number to a string and treating + as concatenation operator.

Answer 3

it's due to the fact it is trying to concatenate it to an empty string You may understand it better if you try something like

  var stringVar = ""+123;

the amazing thing is that if you do

  stringVar = stringVar - 0 ;

it turns to an integer

then if you evaluate

stringVar == '123'

you get true

but if you evaluate

stringVar === '123'

you get false

so cool