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javascriptboolean-expression

javascript (boolean ^ boolean)

I'm working on rewriting some Javascript-code to work in Excel VBA. I've manage to translate almost all of the Javascript-code thanks to searching on this website! However, there is some code I can't understand what it's doing precisely. This is the bit of Javascript-code:

var k = x % y;
return (k != 0 && (k > 0 ^ y > 0) && isFinite(y)) ? k + y : k;

The first line is clear to me. k is the remainder of x/y. In the second line, modulo gets the value of either k+y or k. This is decided by (k!=0 && (k > 0 ^ y > 0) && isFinite(y))

Could somebody explain to me step by step how I should be reading this code? Especially the &&(k>0^y>0)&& is puzzling me.

This is how far I seem to be getting till now:

Example 1: x=9 and y=4

k = 9%4 = 9/4=2 with remainder 1    So: k=1
(1!=0&&(1>0^4>0)&&isFinite(4))   (True AND (True^True) AND True)  =  True?

Example 1: x=9 and y=1

k = 9%1 = 9/1=9 with remainder 0    So: k=0
(0!=0&&(0>0^1>0)&&isFinite(1))  (False AND (False^True) AND True)  =  False?

If I would think purely mathematically, then I would guess that the part (False^True) should be read like (0^1) = 0 = False.

Solution

  • Step by step explanation of the critical part as evaluated by the language:

    (k != 0 && (k > 0 ^ y > 0) && isFinite(y)) ? k + y : k;

    • (k != 0 && (k > 0 ^ y > 0): true if and only if 1. and 2. below is true
      1. k != 0: true if k is not in 0, '0', 0.0, '0.0'
      2. (k > 0 ^ y > 0): true if and only if one of 1. and 2. is true
        1. k > 0: true if and only if k is a Number or numeric String above 0
        2. y > 0: true if and only if y is a Number or numeric String above 0

    • isFinite(y)): true if y is not one of undefined, Object, Array, Infinity, -Infinity, ... (further native objects) or a non-numeric String

    • (k != 0 && (k > 0 ^ y > 0) && isFinite(y)): true if and only if both parts above were true. The ternary ? operator will return the 1st post-expression k + y if the pre-expression is true and the 2nd post-expression k otherwise.

    In more general terms the take-away lessons are:

    • comparisons in JS are complex: String is often cast to Number and different functions have different behavior on native objects (isFinite(null) === true but isFinite(undefined) === false). Therefore generally always prefer using === and !==.

    • JS uses short-circuiting on boolean operators

    • ^ is the exclusive OR aka XOR operator

    • ? is the ternary if operator which equals if ... do ... else do ...