'return res.json' isn't stopping my request as expected?-CodePudding

I created some sample code to demonstrate my issue on a smaller scale. I would like a solution that doesn't involve adding 'unique: true' to my model, if possible, because I seem to run into similar problems in many different scenarios:

const express = require('express')
const mongoose = require('mongoose')
const app = express()
const PORT = 6000
app.use(express.json())


// Initializing mongoose model and connection
const SampleSchema = mongoose.Schema({
    username: String,
    password: String
})
const Sample = mongoose.model('sample', SampleSchema)
mongoose.connect('mongodb://localhost/testdatabase', {
    useNewUrlParser: true,
    useUnifiedTopology: true
})


// Running my post request
app.post('/api', async (req, res) => {
    await Sample.findOne({
        username: req.body.username
    }).then(data => {
        if(data) {
            console.log(data)
            return res.json('This user already exists in my database')
        }
    })
    await Sample.create({
        username: req.body.username,
        password: req.body.password
    })
    return res.json('User created')
})


app.listen(PORT, () => {
    console.log('Server running on 6000')
})

Here is my request and database the first time I send a request:

This is as intended. However, if I send the same request a second time:

I want the code to stop on the first 'res.json' if that line of code is executed - basically, in this example I don't want to create a new Sample if one with the same username already exists. I do realize that in this case I can approach the issue differently to solve the problem, but I want to know why my 'Sample.create' line runs, and if there's a way to prevent it from running aside from the aforementioned method.

CodePudding user response：

This is because the .then callback executes after the enclosing function has already finished. In this code here:

    await Sample.findOne({
        username: req.body.username
    }).then(data => {
        if(data) {
            console.log(data)
            return res.json('This user already exists in my database')
        }
    })

The function being returned from is the data => ... arrow function passed to .then, not the enclosing request handler, so it doesn't prevent subsequent code from executing.

You want to rewrite that bit to use async/await syntax as well:

const data = await Sample.findOne({
    username: req.body.username
})

if(data) {
    console.log(data)
    return res.json('This user already exists in my database')
}

You might want to read up a bit on async/await and Promises in general-- asynchronous code can be quite confusing at first! https://developers.google.com/web/fundamentals/primers/async-functions