You would need to use a Javascript interface/library such as Fetch API, which allows you to send data in JSON format (example is given below). For submiting Form data instead, have a look at this answer, while for posting both Files and Form/JSON data, have a look at this answer.
For the frontend, you could use Jinja2Templates to render and return a TemplateResponse that includes your HTML/JS code, etc. You can use an HTML form to submit your data and then have the form-data converted into JSON, as described here. Otherwise, you could post your JSON data directly, as shown here—for example, body: JSON.stringify({name: "foo", roll: 1}).
app.py
from fastapi import FastAPI, Request
from fastapi.templating import Jinja2Templates
from pydantic import BaseModel
app = FastAPI()
templates = Jinja2Templates(directory="templates")
class Item(BaseModel):
name: str
roll: int
@app.post("/")
async def create_item(item: Item):
return item
@app.get("/")
async def index(request: Request):
return templates.TemplateResponse("index.html", {"request": request})
templates/index.html
<!DOCTYPE html>
<html>
<body>
<h1>Post JSON Data</h1>
<form method="post" id="myForm">
name : <input type="text" name="name" value="foo">
roll : <input type="number" name="roll" value="1">
<input type="button" value="Submit" onclick="submitForm()">
</form>
<div id="responseArea"></div>
<script>
function submitForm() {
var formElement = document.getElementById('myForm');
var data = new FormData(formElement);
fetch("https://stackoverflow.com/", {
method: 'POST',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
body: JSON.stringify(Object.fromEntries(data))
})
.then(resp => resp.text()) // or, resp.json(), etc.
.then(data => {
document.getElementById("responseArea").innerHTML = data;
})
.catch(error => {
console.error(error);
});
}
</script>
</body>
</html>