What Does the Double Tilde (~~) Operator Do in JavaScript?

Double Tilde Operator: A Deeper Dive into ~~

In JavaScript, you may encounter the ~~ operator, also known as the "double tilde" operator. While the single tilde ~ represents bitwise NOT, you may wonder what role the double tilde plays.

Contrary to intuition, ~~ does not perform a NOT of a NOT operation. Instead, it implicitly converts its operands to 32-bit integers and removes everything after the decimal point. This behavior applies to both numbers and strings.

Technically, ~~ behaves like the following function:

<code class="javascript">function(x) {
  if(x < 0) return Math.ceil(x);
  else return Math.floor(x);
}</code>

However, it only provides accurate results when the operand x is within the range of -(2^31) to 2^31 - 1. Beyond this range, overflow occurs, leading to "wrapping around" of the number.

It may seem tempting to use ~~ for numeric parsing of string arguments, but this is discouraged. Overflow and incorrectness for non-integers make it a poor choice. Prefer x or Number(x) instead.

Understanding Double Tilde as NOT of NOT

To understand how ~~ effectively performs a NOT of a NOT operation, consider the following example:

The number -43.2 is represented as a signed 32-bit binary number:

-43.2 = 11111111111111111111111111010101 (2)

Applying bitwise NOT:

NOT -43.2 = 00000000000000000000000000101010 (2) = 42

A second application of bitwise NOT:

NOT 42 = 11111111111111111111111111010101 (2) = -43

Notice that the ~~ operator produces the same result as two consecutive bitwise NOT operations, even though it doesn't perform the NOT of a NOT operation in a straightforward manner.

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