Bitwise test in JavaScript

Question

I hava a following bit string:

var input = '11100001';

I want to check if eight bit is present, so I have tried with:

input & '10000000'

but it gives me 8918528. How should I test it ?

@Bergi I get it from API. Looks like, parsing to int is a solution then. — hsz
– hsz, Commented Aug 7, 2014 at 11:21

pawel · Accepted Answer · 2014-08-07 11:32:31Z

7

You're trying to do a bitwise AND on two strings, which implicitly converts them to integers assuming base 10. You should parse them to integers explicitly supplying the base:

parseInt(input,2) & parseInt('10000000',2)
// 128

Edit: to check if n-th bit is set you could just shift to right by n-1 bits and check if it's 1

parseInt(input,2) >> 7 & 1 === 1

edited Aug 7, 2014 at 11:32

answered Aug 7, 2014 at 11:19

pawel

37.1k7 gold badges59 silver badges54 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

sampathsris · Accepted Answer · 2014-08-07 11:42:20Z

4

Adding to @pawel's excellent answer, let me explain why you get 8918528:

As soon as JavaScript sees this:

'11100001' & '10000000'

it will try to cast the two strings to integers; Integers in base 10 precisely. So we get:

11100001 (base10) & 10000000 (base10)

which is,

101010010101111101100001 (base2) & 100110001001011010000000 (base2)

Applying bit-wise AND,

101010010101111101100001 &
100110001001011010000000
------------------------
100010000001011000000000 (base2)

Which is,

8918528 (base10)

answered Aug 7, 2014 at 11:42

sampathsris

22.4k12 gold badges74 silver badges102 bronze badges

1 Comment

pawel Over a year ago

Great work with this detailed explanation, thanks :)

Collectives™ on Stack Overflow

Bitwise test in JavaScript

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related