Why Does JavaScript\'s Modulus Operator Produce Unexpected Results with Negative Numbers?

JavaScript's Modulus Operator Behavior for Negative Numbers

In JavaScript, the modulus operator (%) calculates the remainder of the division operation. However, it behaves differently for negative dividend values than mathematical conventions and other programming languages.

When performing a modulus operation on a negative dividend, JavaScript returns a negative remainder, while mathematical calculations and most other programming languages yield a positive one. This can be demonstrated with the following example:

• Google Calculator: (-13) % 64 = 51 (expected result)
• JavaScript: console.log(-13 % 64) = -13 (incorrect result)

Resolving the Issue

To obtain the correct result for modulus operations involving negative numbers in JavaScript, we can use the following custom function:

Number.prototype.mod = function (n) {
  "use strict";
  return ((this % n) + n) % n;
};

This function, when applied to negative dividends, ensures that the returned remainder is positive. For instance:

console.log((-13).mod(64)); // Output: 51 (correct result)

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