nodejs在postman上形成提交工作，但在浏览器中不起作用

Question

尝试使用nodejs中的mongoose将表单数据提交到我的数据库，但是当我在postman上运行代码时，它工作正常但不在浏览器中..并且没有数据保存到数据库中...我无法弄清楚为什么......请帮忙吗？

<div class="card bg-primary text-center card-form ">
                            <div class="card-body">
                                <h3>sign up </h3>
                                <p>fill up the form below to register</p>
                                <form action="/users" method="POST">
                                    <div class="form-group">
                                    </div>
                                    <div class="form-group">
                                        <input class="form-control form-control-lg" name="username" type="text" placeholder="Username">
                                        <input class="form-control form-control-lg" name="firstname" type="text" placeholder="FirstName">
                                    </div>
                                    <div class="form-group">
                                        <input class="form-control form-control-lg" name="lastname" type="text" placeholder="LastName">
                                    </div>
                                    <div class="form-group">
                                        <input type="submit" value="submit" class="btn btn-outline-light btn-block">
                                    </div>
                                </form>
                            </div>
                        </div>

var express = require("express");
var mongoose = require("mongoose");
mongoose.Promise = global.Promise
var bodyParser = require("body-parser");
var MongoClient = require("mongodb").MongoClient;

var app = express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended : true }));


mongoose.connect("mongodb://localhost:27017/register");
var mySchema = new mongoose.Schema({
    userName : { type: String, required: true},
    firstName : {type: String, required: true},
    lastName :  {type: String, required: true}
});

var users = mongoose.model("users", mySchema);

app.get("/", (req, res)=>{ 
    res.sendFile(__dirname + "/index.html");
});

app.post("/users", (req, res)=>{
    var myData = new users(req.body);
    myData.save().then(item =>{
        res.send(" your data has been saved in our database ");
    }).catch(err =>{
        res.status(400).send("sorry, we are unable to save your data")
    });
} );

app.listen(3000, ()=>{
    console.log("we are listenning on port 3000")
})