CODEWITHSUNDEEP
javascriptecmascript-6promisereturn
Habebit
Habebit

Reputation: 975

Return value within promise then is always undefined

I have a function that makes a database call. I want to return a boolean value if this operation is successful, but I always get undefined. As I've read the return value should be a promise as well, but if I want to then/catch the "promise" my function returns an undefined.

const foobar = function(data) {
    db.call(data).then(function(res) {
        console.log(res); // returns the expected data, e.g.["foobar", "foo", "bar"]
        return res.includes("bar");
    }).catch(function(err) {
        // if an error occures, the function should ...
        // ... return a false value as well
        return false;
    });
};

My function call looks like this: foobar({1: 'a'}). How can I fetch the boolean return value in the correct way? I've tried it with the following two options:

First I thought that the return value would be a normal boolean value:

const return_value = foorbar({1: 'a'}); // returns undefined

After I've read that the return value within a then function would be a promise as well, I've tried to then/catch it:

const return_value = foobar({1: 'a'}
).then(function(res) {
    console.log(res); 
}).catch(function(err) {
    console.log(err); 
});

Uncaught TypeError: Cannot read property 'then' of undefined

The output clearly indicates that the result is not a promise, it's just undefined. I appreciate any help.

Upvotes: 1

Views: 131

Answers (1)

Jack Bashford
Jack Bashford

Reputation: 44087

You're not returning anything from the foobar function itself - just the calls to .then and .catch.

return db.call(data).then(...).catch(...);

You could also change foobar to an ES6 arrow function and take advantage of the implicit return:

const foobar = data => db.call(data).then(...).catch(...);

Note that ES6 functions won't be accessible before they're defined if you use const or let - so make sure if you're using arrow functions, they're all defined in one area at the top. ES5 functions have no such restriction because they are all located before any of the code runs.

Upvotes: 4

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