AWK replace string in file with javascript code - regex

Question

I want to replace a constant javascript line in awk with some code I read from a file .
Interactively running this at the command line in bash works:
CUSTOM_CODE=`cat custom_code.txt`
awk -v r=$CUSTOM_CODE '{gsub(/export default function\(\) \{/,r)}1' main.js > main-patched.js
The problem is that if I place this 2 commands in a bash file it doesn't work anymore with the following awk error :awk: cannot open myobj (No such file or directory).
Sample files :
main.js
import asdas form "asdasdsa"
export default function() {
let a ="asdasd"
}
custom_code.txt
const myobj = {
aaaa: bbb.asdasd("12345"),
aaa2: 1
}
function myfunction(params){
//1
// comment
let var1 ="a"

Answer 1

Copy/paste your script into http://shellcheck.net and it'll explain the error message you're getting (hint - always quote your shell variables) and more. It is impossible for the code you posted to work on the command line or in a script.
The right way to do this is read the new code into an awk variable (not a shell variable) and then do a string match and replace (not *sub() as that'd fail given & in the new code and would need you to escape the regexp metachars) in the main file:
$ awk '
NR==FNR { new=new sep $0; sep=ORS; next }
$0=="export default function() {" { $0=new }
{ print }
' custom_code.txt main.js > main-patched.js
$ cat main-patched.js
import asdas form "asdasdsa"
const myobj = {
aaaa: bbb.asdasd("12345"),
aaa2: 1
}
function myfunction(params){
//1
// comment
let var1 ="a"
let a ="asdasd"
}