How to set, clear, toggle and check a bit in JavaScript?
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@user JavaScript and C/C++ are different languages. Very helpful to have different answers from them both.cmac– cmac2017-03-09 20:00:33 +00:00Commented Mar 9, 2017 at 20:00
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4 Answers
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.
Comments
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);
}
5 Comments
Raoul
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.
unloco
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)
Teepeemm
I think "impair" should be "odd" (1, 3, 5, 7, ...).
unloco
Yes, I have a french background. I used the french word for "odd" (couldn't edit to correct my comment)
birwin
Do you have similar functions for testing numbers beyond the 32-bit range?
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.
1 Comment
Mister_CK
Thanks, the descriptive variable names really helped, why do you do
res === 0 ? 0 : 1; in the get bit though, instead of just returning the bits?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);
}
