How do you set, clear and toggle a single bit in JavaScript?

Question

How to set, clear, toggle and check a bit in JavaScript?

@user JavaScript and C/C++ are different languages. Very helpful to have different answers from them both. — cmac, Mar 09 '17 at 20:00

score 268 · Accepted Answer · edited Jan 12 '14 at 14:17

268

To get a bit mask:

var mask = 1 << 5; // gets the 6th bit

To test if a bit is set:

if ((n & mask) != 0) {
  // bit is set
} else {
  // bit is not set
}

To set a bit:

n |= mask;

To clear a bit:

n &= ~mask;

To toggle a bit:

n ^= mask;

Refer to the Javascript bitwise operators.

edited Jan 12 '14 at 14:17

om-nom-nom

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answered Sep 17 '09 at 02:34

cletus

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unloco · Answer 2 · 2017-06-07T00:33:33.383

37

I want to add some things (with thanks to @cletus)

function bit_test(num, bit){
    return ((num>>bit) % 2 != 0)
}

function bit_set(num, bit){
    return num | 1<<bit;
}

function bit_clear(num, bit){
    return num & ~(1<<bit);
}

function bit_toggle(num, bit){
    return bit_test(num, bit) ? bit_clear(num, bit) : bit_set(num, bit);
}

edited Jun 07 '17 at 00:33

answered Dec 08 '11 at 19:24

unloco

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2

This is great thanks. Can you explain why bit_test works? I understand the right shift to move the interesting bit to the rightmost position, but I get lost trying to understand what the intermediate value is and why its remainder when divided by 2 is nonzero when set. – Raoul Nov 15 '14 at 14:49
2

After you get the wanted bit to rightmost position, you simply test if the new value is impair (new_value % 2 != 0). An impair number will have bit0 = 1 or else bit0 = 0 (because bit0 weight is 2 to power 0 which is 1) – unloco Nov 16 '14 at 11:19
3

I think "impair" should be "odd" (1, 3, 5, 7, ...). – Teepeemm Jun 27 '18 at 21:24
1

Yes, I have a french background. I used the french word for "odd" (couldn't edit to correct my comment) – unloco Jun 28 '18 at 12:43
1

Do you have similar functions for testing numbers beyond the 32-bit range? – birwin Oct 24 '18 at 20:43

score 24 · Answer 3 · answered Jun 27 '18 at 15:56

Get Bit

function getBit(number, bitPosition) {
  return (number & (1 << bitPosition)) === 0 ? 0 : 1;
}

Set Bit

function setBit(number, bitPosition) {
  return number | (1 << bitPosition);
}

Clear Bit

function clearBit(number, bitPosition) {
  const mask = ~(1 << bitPosition);
  return number & mask;
}

Update Bit

function updateBit(number, bitPosition, bitValue) {
  const bitValueNormalized = bitValue ? 1 : 0;
  const clearMask = ~(1 << bitPosition);
  return (number & clearMask) | (bitValueNormalized << bitPosition);
}

Examples has been taken from JavaScript Algorithms and Data Structures repository.

score 6 · Answer 4 · answered Sep 25 '17 at 11:43

I built a BitSet class with the help of @cletus information:

function BitSet() {
    this.n = 0;
}

BitSet.prototype.set = function(p) {
    this.n |= (1 << p);
}

BitSet.prototype.test = function(p) {
    return (this.n & (1 << p)) !== 0;
}

BitSet.prototype.clear = function(p) {
    this.n &= ~(1 << p);
}

BitSet.prototype.toggle = function(p) {
    this.n ^= (1 << p);
}

How do you set, clear and toggle a single bit in JavaScript?

4 Answers4

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