node.js：发送后无法设置标头

Question

I am learning Node.js at the moment, and i am trying to create a shopping list application, and i am trying to implement a search route, which checks if the query matches the val, here is the code

const express = require('express');
const bodyParser = require('body-parser');

const app = express();
const port = 3000;

//Array List 
let list = ['Fish', 'Lettuce', 'Chicken'];

//Set view engine to pug
app.set('view engine', 'pug');
//use bodyParser


app.get('/', function(request, response){
    response.render('list', {list});
});

app.get('/search', function(request, response){
   return list.map(function(val){
      if(request.query.search === val){
          return response.send('Yup you got ' + val);
        }
     response.send('Not found')
    });
});

app.get('/new-item', function(request, response){
    response.render('new');
});

app.post('/add-item', function(request, response){
   let add = response.send(request.body);
   list.push(add);
});

app.listen(port, function(){
    console.log('Listening on Port 3000');
});

Now the problem is with the /search route's if conditional, i know the reason i am getting the error is because i can't use the response.send twice, i am looking for a way to send either response, based on if the condition is met. Any help is appreciated Thanks

Answer 1

Move response.send('Not found') outside of the loop. Also, you shouldn't use Array.map here, use Array#find instead:

app.get('/search', function(request, response) {
   let foundVal = list.find(function(val) {
     if (request.query.search === val) {
       return val;
     }
   });
   if (foundVal) {
     return response.send('Yup you got: ' + foundVal);
   }
   response.send('Not found');
});

Answer 2

Build your structure with callbacks.

app.get('/search', function(request, response){
    checkValue(list,request.query.search,function (result) {
        response.send({
            data : result
        });
    });

    function checkValue(list, value, callback) {
        var isHere = false;
        list.map(function(val){
            if(request.query.search === val){
                isHere = true;
            }
        });
        callback(isHere);
    }
});

Answer 3

In this piece of code:

app.get('/search', function(request, response){
   return list.map(function(val){
      if(request.query.search === val){
          return response.send('Yup you got ' + val);
        }
        response.send('Not found')
    });
});

You are doing response.send() inside your .map() callback which means you can easily call it more than once and the error you asked about indicates that you are calling it more than once. Keep in mind that the return inside your .map() does not break out of the .map() . It only returns from that iteration of the callback function and then the next iteration of the .map() continues right after you return .

If you want to break out of the iteration, then switch to a regular for loop to do your iteration (which does not use a callback) and then your return will do what you want like this:

app.get('/search', function(request, response){
    for (let val of list) {
        if (request.query.search === val){
            return response.send('Yup you got ' + val);
        }
    }
    response.send('Not found')
});