Can someone explain me why awk's sub() / gsub() works like this?

Question

I know awk can do text/string substitution with sub() and gsub() like:

kent$ echo "fffff"|awk '{gsub("f", "b")}1'
bbbbb

or

kent$ echo "fffff"|awk '{gsub(/f/, "b")}1'
bbbbb

However today I made a typo mistake, I wrote the line as:

kent$ echo "fffff"|awk '{gsub('f', "b")}1'

But awk didn't complain about that but generated output as usual, of course, unexpected output, it took me sometime to find out the error. The output awk gave me was:

bfbfbfbfbfb

another example:

kent$ echo "fafafafafXX"|awk '{gsub('fa', "B")}1'
BfBaBfBaBfBaBfBaBfBXBXB

example with sub() is strange too:

kent$  echo "thanks in advance"|awk '{sub('a', "B")}1'
Bthanks in advance

Could someone explain me how was the strange substitution done?

kent$  awk --version
GNU Awk 4.0.2

EDIT

thanks for the answer from Joni. maybe this example explains it better, I just add it here:

kent$  echo "thanks in advance"|awk '{f="k";sub('f', "B")}1'
thanBs in advance

kent$  echo "thanks in advance"|awk '{sub('th ank', "B")}1'
awk: cmd. line:2: {sub(th
awk: cmd. line:2:        ^ unexpected newline or end of string

Answer 1

When you write

echo "fffff"|awk '{gsub('f', "b")}1'

what awk sees is {gsub(f, "b")}1. It interprets f as a variable, with an empty value, and substitutes every empty string in the input with b.

The empty string is found between each character and after the last one, so awk inserts a b after each f.

You can substitute // or "" for the same effect, without an unused variable:

echo "fffff"|awk '{gsub(//, "b")}1'            # fbfbfbfbfb