JavaScript: How To Return Data From A Function In Sqlite3

#programming

Table Of Contents

The Problem
1. Why Undefined?
The Solution
1. Modified Code
2. Explanation

Whiles trying to implement a read counter for my website this morning, I run into a bug. The function which returned the query from the database always returned undefined

The Problem

const getData = (slug)=>{
    let data;
    db.serialize(()=>{
        db.get('SELECT * FROM blog WHERE slug = ?',slug, (err, rows)=>{
            data = rows;
        })
    });
    return data;
}

let data = getData("return-data-from-sqlite3-nodejs") // => undefined

This was my code snippet, the function getData takes a slug as parameter, then it searches the database to retrieve all the data for the slug. It then assigns the data to the data variable, then at the end of the function, it returns the data variable. The function without a doubt works perfectly but when you run it, it will always return undefined.

Why Undefined?

The reason why the function returns undefined is that, db.get is an asynchronous function, so the return statement will always run before the function has even completed it’s work.

The Solution

Since the db.get function is asynchronous, we need to find a way to make it return it’s value once it has finished it’s work and the solution is by using Promises. So we need to wrap the whole code in a Promise and when the function finishes executing, the promise will return the results.

Modified Code

const getData = (slug)=>{
    return new Promise((resolve, reject)=>{
    db.serialize(()=>{
        db.get('SELECT * FROM blog WHERE slug = ?',slug, (err, rows)=>{
            if (err)
                reject(err)
            resolve(rows)
        })
    });
    })
}

let promise = getData("return-data-from-sqlite3-nodejs") // => Promise { <pending> }
.then(results=>{
console.log(results) // => { slug: 'adding-matomo-website', read_times: 1, shares: 0, likes: 0 }
})

Explanation

So basically, what we did was wrap the whole function in a Promise and pass the rows to resolve if our code run successfully, else we pass the err to reject if there were any errors.

Oh, by the way, you can get the json of how many times this blog post has been read from this url /blog/return-data-from-sqlite3-nodejs.json