Running node sqlite3 code synchronously

Question

I'm having trouble adjusting to the async-first nature of node / js / typescript. This intent of this little function should be pretty clear: it takes a database and returns an array of courses that are listed in that database.

The problem is that the return statement gets run before any of the database operations have run, and I get an empty list. When I set a breakpoint inside the database each loop, I can see that the rows are being found and that courses are being put into ret one by one, but these courses never become visible in the scope where courseList() was called.

const courseList = (database: sqlite3.Database): Course[] => {
    let ret = new Array<Course>();

    database.serialize();
    database.each("select ID, Title from Course", (err: Error, row: Object) => {
        ret.push(new Course(
            row.ID,
            row.Title
        ))
    })

    return ret;
}

Suggestions?

The calling code just wants to print information about courses. For example:

let courses = courseList(db);
console.log(courses.length); // logs 0, even though the db contains courses

Nothing special about NodeJS here, just higher order functions, specifically callbacks. Can you show the calling code? — Aluan Haddad
– Aluan Haddad, Commented Apr 17, 2017 at 21:07
@NiloCK you have a bug somewhere else or code you are omitting. The function indeed works. See here where I have your code but with a mocked database jsbin.com/rixivofeke/1/edit?js,console Yes, there is a race condition, but the function returns a value. It does not behave as you describe. — Aluan Haddad
– Aluan Haddad, Commented Apr 17, 2017 at 23:34
@AluanHaddad I think I muddled that. If you're VSCode handy, this repo: www.github.com/nilock/lynda-copy-course can be built / launched to demonstrate the behavior. See lines 9-12 of /bin/lynda-copy-course.ts, and lines 20-35, and 66-86 of /src/index.ts — NiloCK
– NiloCK, Commented Apr 18, 2017 at 0:05

basarat · Accepted Answer · 2017-04-18 00:58:52Z

2

database.each takes a complete callback. Use that to resume e.g.

const courseList = (database: sqlite3.Database, complete): Course[] => {
    let ret = new Array<Course>();

    database.serialize();
    database.each("select ID, Title from Course", (err: Error, row: Object) => {
        ret.push(new Course(
            row.ID,
            row.Title
        ))
    }, complete);

    return ret;
}
let courses = courseList(db, () => {
    console.log(courses.length); 
});

More

There are better ways to write this. Use promises https://basarat.gitbooks.io/typescript/content/docs/promise.html
The documentation is horrible : https://github.com/mapbox/node-sqlite3/wiki I would be tempted to look elsewhere (TS First) for a database solution. Its not worth the pain for me personally. YMMV.

answered Apr 18, 2017 at 0:58

basarat

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