为什么我不能通过回调事件更改变量值？

Question

Scenario One (the question)

---

POST request To /radikpidr

headers: nothing special

body: pidr=radik

POST response From /radikpidr

headers: nothing special

body: pidr=radik

expectation(NOT MET):

Print request body content into the console.

reality(FRUSTRATING):

Varibale [body] was not changed since initialisation, as it seems.

note:

I've got the request body content in response to my request, which is imposible to achieve without changing body (variable)

---

const app = require("express")();

let body = "not as expected";

app.post("/radikloh", (req,res)=>{
    req.on("data",function(chunk){
        body = chunk.toString();
    });

    console.log(body)//"not as expected"

    req.on("end",function(){
        res.send(body);
    });

    console.log(body)//"not as expected"
});

app.listen(process.env.PORT);

---

Scenario Two (a solution)

---

POST request To /radikpidr

headers: nothing special

body: pidr=radik

POST response From /radikpidr

headers: nothing special

body: pidr=radik

expectation(MET):

Print request body content into the console.

reality(more or less Satisfying):

Just as expected

---

const app = require("express")();

let body = "not as expected";

function buff (input){
    body = input;
}

app.post("/radikloh", (req,res)=>{
    req.on("data",function(chunk){
        body = chunk.toString();
        buff(body);
    });

    console.log(body)//"pidr=radik"

    req.on("end",function(){
        res.send(body);
    });

    console.log(body)//"pidr=radik"
});

app.listen(process.env.PORT);

---

THE QUESTION IS WHY ?

I thought this is because of a callback function scope but it works fine in this expample:

function a (cb){
    cb("It worked just FINE");
}
function b (){
    let body = "not as expected";

    a(function(seter){
        body = seter;
    });

    console.log(body);//THE OUTPUT: "It worked just FINE"
}

---

And even in this:

let body = "not as expected";
function a (cb){
    cb("It worked just FINE");
}
function b (){


    a(function(seter){
        body = seter;
    });

    console.log(body);//THE OUTPUT: "It worked just FINE"
}

Answer 1

you can check this out on node site about request body

let body = [];
request.on('data', (chunk) => {
  body.push(chunk);
}).on('end', () => {
  body = Buffer.concat(body).toString();
  // at this point, `body` has the entire request body stored in it as a string
});

Are you sending data when you make a POST request ? if body remains the same then it may be not entering the req.on('data'.. function