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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.

fotofrank
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    You're correct. I'll change the code to the original one. – fotofrank Dec 08 '17 at 17:21
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    The `^` operator is the XOR, or EXCLUSIVE OR, operator. It only returns true if and only if one value is true. If both are true, or both are false, the result is false. – Derek Dec 08 '17 at 17:30
  • Above code implements the 'mathematical' modulo operator, see https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers – le_m Dec 08 '17 at 17:34

2 Answers2

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The ^ operator is the XOR, or EXCLUSIVE OR, bitwise operator. It only returns true if and only if one value is true. If both are true, or both are false, the result is false

From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators#Bitwise_XOR:

a b a XOR b 0 0 0 0 1 1 1 0 1 1 1 0

As to why your second example returns FALSE, it's because you're using AND logic:

FALSE && (FALSE ^ TRUE) && TRUE results in FALSE && TRUE && TRUE which equates to FALSE.

Derek
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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 ...

B M
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