JavaScript - bitwise NOT operator

The main goal of this article is to show how bitwise NOT operation (~ operator) works on the JavaScript example.

The main concept of the bitwise NOT operation is to change each bit to oposite bit.

Bitwise example:

     0101
NOT  ----
     1010   <---- bitwise NOT operation result


Hint: 0101 in base 10 has 5 value

The truth table for the NOT operation is: 

So, the above table on 1 bit numbers can be presented in JavaScript as:

// ONLINE-RUNNER:browser;

console.log(getBits(~0, 1));  // 1
console.log(getBits(~1, 1));  // 0


// Helpers:

function getBits(number, length) {
	let text = '';
  	for (let i = length - 1; i > -1; --i) {
    	text += 0x01 & (number >> i);
    }
  	return text;
}

More complex eample

JavaScript uses Two's complement to store numbers in the operating memory.

It means:

• if the number is positive, in base 2 has filling with 0 numbers on the left,
• if the number is negative, in base 2 has filling with 1 numbers on the left.

Hint: check this article to know how Two's complement works.

// ONLINE-RUNNER:browser;

const a = 5;  // 00000101

console.log(a);                   // 5        (in dec / base 10, number with Two's complement)
console.log(getBits(a, 8));       // 00000101 (in bin / base 2)

const result = ~a;  // <----- bitwise NOT operation example

console.log(result);              // -6       (in dec / base 10, number with Two's complement)
console.log(getBits(result, 8));  // 11111010 (in bin / base 2)
                                  // ^^^^^
                                  // |   |
                                  // filling with 1 (-6 is a negative number)


// Helpers:

function getBits(number, length) {
	let text = '';
  	for (let i = length - 1; i > -1; --i) {
    	text += 0x01 & (number >> i);
    }
  	return text;
}

Bitwise NOT operation limitations

Hint: in JavaScript, bitwise NOT works on 32-bit integers. If number is bigger, the most significant bits are dicarded to get 32-bit integer number - it means only bits indexed from 0 to 31 will be used.

So, the above table can be also written as:

    5 (base 10) = 00000000000000000000000000000101 (base 2)
                  --------------------------------
   ~5 (base 10) = 11111111111111111111111111111010 (base 2) = -6 (base 10, 32 bit number with Two's complement)

Features

1. bitwise NOT operation is fast - easy to do by any CPU.

See also

1. JavaScript - bitwise operators

2. JavaScript - bitwise AND example

3. JavaScript - bitwise OR example

4. JavaScript - bitwise XOR example

5. JavaScript - get supported max integer value

References

1. Bitwise NOT - Wikipedia
2. Two's complement - Wikipedia
3. Bitwise operation - Wikipedia