JSON Parse error: Unrecognized token '?' in React Native - json

Question

I am working on React Native project and I try to transform datas from a server to JSON.
I have already done it on other projects, so I know how it works, but this time I have an error :
"JSON Parse error: Unrecognized token '<' ".
Here is my code :
fetch('https://app.fr', {
method: 'POST',
headers: new Headers({
'Content-Type': 'application/x-www-form-urlencoded',
}),
})
.then((response) => response.json())
When I do response.text() instead, I get a string which is a correct JSON format. So I know the datas are not the problem.
fetch('https://app.fr', {
method: 'POST',
headers: new Headers({
'Content-Type': 'application/x-www-form-urlencoded',
}),
})
.then((response) => response.text())
After looking on forums, I found that the error could be that the server send me datas whith content-type "text/html" instead of "application/json". And yes, the server send me datas with content-type "text/html".
So I tried to change the content-type in the header:
fetch('https://app.fr', {
method: 'POST',
headers: new Headers({
'Content-Type': 'application/json',
}),
})
.then((response) => response.json())
But I got that error : "JSON Parse error: Unrecognized token '?' "
So I thought it means that I need to change directly the content-type of datas on the server. But I can't do it because my client is using these datas for others projects.
Do yo know any possibility to transform to JSON datas with "text/html" content-type, without getting this kind of error?
Thanks !

Answer 1

I have found the problem !
In the datas client send to me, there was an invisible first character.
When I was rendering it on my phone I saw "{"message":"ok","testes":...} but when I log it in my console I saw that there was a strange character before the first "{".
I deleted it with response.text().substring(1) and it works !
Thanks for your answers !