Promise resolution

Question

I am new to javascript promises, I am using below code from NativeScript to query sqlite database and just want to return true if row exists and false otherwise:

function hasBookmark(url) {
    (new Sqlite("pakjobs.db")).then(db => {
        db.all("SELECT url FROM bookmarks WHERE url=?", [url]).then(rows => {
            return rows.length ? true : false;
        }, error => {
            console.log("hasBookmark ERROR", error);
            return false;
        });
    }, error => {
        console.log("hasBookmark ERROR", error);
        return false;
    });

    return false;
}

However function ALWAYS returns false.

Can anybody tell how do I return true if row exists and false otherwise ?

Thanks for the help

Possible duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — ASDFGerte, Aug 25 '18 at 17:24

Axnyff · Answer 1 · 2018-08-25T18:27:49.870

2

You are not going to be able to return true or false, that's pretty much the point of Promises, you can't return something asynchronously in javascript.

What you can do, however, is directly returning the Promise you're creating and then calling .then on it.

function hasBookmark(url) {
    return (new Sqlite("pakjobs.db")).then(db => {
        return db.all("SELECT url FROM bookmarks WHERE url=?", [url]).then(rows => {
            return rows.length ? true : false;
        }, error => {
            console.log("hasBookmark ERROR", error);
            return false;
        });
    }, error => {
        console.log("hasBookmark ERROR", error);
        return false;
    });

}

const hasBookmarkPromise = hasBookmark('someurl');

hasBookmarPromise.then(value => {
    if (value === true) {
        console.log('has bookmarks');
    }
});

edited Aug 25 '18 at 18:27

answered Aug 25 '18 at 16:51

Axnyff

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I did how you told, it still returns `false` always :( – dev02 Aug 25 '18 at 17:06
1

You will also need to `return` the inner promise from the first `then` handler – Bergi Aug 25 '18 at 17:09
Yep, forgot about the inner promise. Thanks! – Axnyff Aug 25 '18 at 18:28

score -2 · Accepted Answer · edited Aug 25 '18 at 19:22

-2

function hasBookmark(url) {
    return new Promise((resolve, reject) => {
        (new Sqlite("pakjobs.db")).then(db => {
            db.all("SELECT url FROM          bookmarks WHERE url=?", [url]).then(rows => {
                resolve(rows.length ? true : false)
            }, error => {
                console.log("hasBookmark ERROR", error);
                resolve(false);
            });
        }, error => {
            console.log("hasBookmark ERROR", error);
            resolve(false);
        });
    });
}

hasBookmark(url).then(function(isValid){
   If(isValid){
     // Do something
   }
});

edited Aug 25 '18 at 19:22

dev02

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answered Aug 25 '18 at 16:53

Krishna Satwaji Khandagale

2,235
16
25

Apologies, was trying from iphone 5s , a bit difficult to do formatting from phones , but yeah have edited the answer. – Krishna Satwaji Khandagale Aug 25 '18 at 16:59
2

Avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it)! – Bergi Aug 25 '18 at 17:09

Promise resolution

2 Answers2