javascript - fill in the values of an array using the array.map -

i'm trying understand code uses array.map()

var char_set = array.apply(null, array(256)).map(boolean.prototype.valueof, false);  

the above code creating array indexes 0-255 , each value set false

could explain how array being created map method. syntax of map method

arr.map(callback[, thisarg]) 

in case thisarg set false?

array.apply(null, array(256)) : create array of 256 elements value undefined in elements

map(…) : used initialize values false

the first argument in map takes callback function; boolean.prototype.valueof function act callback.

true.valueof() returns true has invoked thisarg.
    |
    ---------- true thisarg

false.valueof() returns false has invoked thisarg.
    |
    ---------- false thisarg

when map(boolean.prototype.valueof, false) invoked each element in array pass false this boolean.prototype.valueof method. invoking false.valueof() hence returns false.

thus map(boolean.prototype.valueof, false) equivalent to:
map(function(item){ return false.valueof() });