Passing parameters to a promise chain

Question

I'm having some problems with passing parameters through a promise chain I set up. Here's a basic example of what I'm trying to do.

var dummyReq = { client: null };
var dummyUser = { email: '[email protected]', password: 'admin' };

function printEmail(request, user) {
    return new Promise((resolve, reject) => {
        console.log('Email:', user.email);
        return resolve(request, user);
    });
}

function printPassword(request, user) {
    return new Promise((resolve, reject) => {
        console.log('Password:', user.password); // <-- user is undefined
        return resolve(request, user);
    });
}

printEmail(dummyReq, dummyUser)
    .then(printPassword)
    .catch(function(error) {
    console.log('Unexepected error has occured');
});

In the second promise the parameter user is undefined, how can I pass multiple parameters through a promise chain, as these promises continue more information gets added to the chain, by the end of it I'm passing 5 values through to the last promise.

Should I just use ES6 operators to combine them into an object and deconstruct them on every call? for example return resolve({request, user}) and then const { request, user } = param

Answer 1

You are on the right track. Promise objects hold a single fulfilled value which is passed to onfulfilled reaction handlers. A non promise value, or the fulfilled value of a a promise when it eventually becomes fulfilled, is used to fulfill the next promise in a chain of promises. If you want to propagate multiple values to the next promise handler you have to use a single object value to do so.

So let's assume that printEmail and printPassword actually do something asynchronous rather than the synchronous operations in the post.

printEmail becomes:

function printEmail(request, user) {
    return new Promise((resolve, reject) => {
        console.log('Email:', user.email);
        resolve( {request, user} );
    });
}

without a "return" statement inside the executor of the new Promise - the return value of an executor function is not used and will generally be undefined.

printPassword becomes

function printPassword(data) {
    return new Promise((resolve, reject) => {
        console.log('Password:', data.user.password);
        resolve(data);
    });
}}

Whether you deconstruct a data object into variables or not is a stylistic rather than technical question. Similarly whether you pass the same object down to multiple handlers in a chain of promises, deleting or adding properties if required, or create a new object with only the properties needed in following steps is entirely a matter of choice and style.