Question

Boot up your interpreter/console and try the comparison:

> ",,," == Array(4)
True

Why? At first I thought maybe since you could think of ",,," as an array of four characters with a ‘\0’ terminating slice, that might be why, but

> "..." == Array(4)

Returns "False". So… why? I know it’s some idiosyncratic bit of duck typing in JavaScript, but I am just curious what underlines this behavior. I gleaned this from Zed Shaw’s excellent presentation here, btw.

Answer 1

Because the right hand operand is converted to a string and the string representation of Array(4) is ,,,:

> Array(4).toString()
  ",,,"

If you use the array constructor function and pass a number, it sets the length of the array to that number. So you can say you have four empty indexes (same as [,,,]) and the default string representation of arrays is a comma-separated list of its elements:

> ['a','b','c'].toString()
  "a,b,c"

---

How the comparison works is described in section 11.9.3 of the specification. There you will see (x == y):

8. If Type(x) is either String or Number and Type(y) is Object,
return the result of the comparison x == ToPrimitive(y).

_{(arrays are objects in JavaScript)}

and if you follow the ToPrimitive method you will eventually find that it it calls toString.