Read binary data from Buffer

Question

I am trying to examine the binary data given a Buffer like so:

<Buffer 08 30 66 6d 41 64 69 76 66>

TMK that is 8 bytes of data.

What I am looking to do is examine the first byte, and look at the most significant bit (leftmost bit) in that first byte.

How can I find out if that first bit is a zero or one?

nadavvadan · Accepted Answer · 2017-01-18 13:44:11Z

3

The simplest solution is this:

Number(buf[0] > 127)

Works because:

JS treats bytes as unsigned, AKA always positive; a byte's value is thus in the range [0..255], as opposed to [-128..127] for signed bytes.
Number(true) == 1, Number(false) == 0
the most significant bit is 1 only when byte is in range [128..255]

(Bonus) A harder to read but sexier version:

((buf[0] & 0xff) >> 7)

answered Jan 18, 2017 at 13:08

nadavvadan

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