What is this syntax in Node.js and PostGres

Question

Hi came across this syntax in a node.js application, in which we are making a SELECT query to our postgres database.

app.get("/monsters/:id", (req, res, next) => {
    let id = req.params.id;
    pool.query(`SELECT * FROM monsters WHERE id = $1`, [id], (err, response) => {
        if (err) return next(err);
        console.log(response.rows);
        res.send(response.rows);
    })
});

I don't understand the follow the line: pool.query(SELECT * FROM monsters WHERE id = $1, [id], (err, response) => {

how does this kind of string literal work, where we have used $1 and passing an array?

Thank you

Also $1 is not connected to the string literal - it's just a normal part of the text inside. It's used by the SQL engine to be replaced with an appropriate parameter. — VLAZ
– VLAZ, Commented Apr 21, 2020 at 8:00
it is adding the first element of the [id] array. So what does $1 represent? — pi2018
– pi2018, Commented Apr 21, 2020 at 8:02
It's the parameter in the array. It's just a parametarised query. — VLAZ
– VLAZ, Commented Apr 21, 2020 at 8:04
Thank you, so again this has to do with SQL syntax and not JS ? — pi2018
– pi2018, Commented Apr 21, 2020 at 8:06

slebetman · Accepted Answer · 2020-04-21 08:08:08Z

3

This has nothing to do with javascript or node.js syntax. The string

`SELECT * FROM monsters WHERE id = $1`

is the same as:

"SELECT * FROM monsters WHERE id = $1"

because there is no interpolation done within the string.

From the point of javascript, $1 literally represents $1 and nothing else.

I don't quite know the library you are using for accessing postgres but it looks fairly obvious to me that $1 is mapped to the first element of [id] (which is just the value of id). It looks like the postgres library used maps $1 to ? in SQL syntax and moves the values in the second argument to the appropriate part of the generated sql query.

I would look at the documentation for the library used to access postgres for more info on the API.

answered Apr 21, 2020 at 8:08

slebetman

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1 Comment

pi2018 Over a year ago

Thank you so much! I was so confused by that syntax. But this clears it up!

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