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Calling .then within a new Promise() javascript

I’ve learned javascript async/await before learning promise.then() syntax, and I am now attempting to go back and learn promise.then().

I currently have the following code

let getDatabaseData = async () => {
    let res1 = await dbQuery()
    let res2 = await dbQuery2(res1)
    return res1 + res2
}

let dbData = await getDatabaseData()
console.log(dbData)

I want to translate this to promises, and have done so below

let getDatabaseData = () => {
    return new Promise(resolve => {
        dbQuery().then(res1 => {
            dbQuery2(res1).then(res2 => {
                resolve(res1 + res2)
            })
        })
    })
}

getDatabaseData().then(dbData => {
    console.log(dbData)
})

Is this the most effective way to do so? I was looking into promise chaining, but it doesn’t exactly get me what I want because I want getDatabaseData to act as a helper method to combine the results from dbQuery and dbQuery2

Tags:

2

Answers


  1. 0

    Your code most directly translates (to me) to:

    let getDatabaseData = () => {
    // we can store intermediate values in the closure if we want
    let val1;
    return dbQuery().then(val => {
        val1 = val;
        return dbQuery2(val);
    }).then(val2 => val1 + val2)
}

getDatabaseData().then((data) => console.log(data))

    Another option without using the enclosing fn’s scope could look like this:

    let getDatabaseData = () => dbQuery()
      // here, we use Promise.all to proxy values forward
      // in the resultant promise
      .then(val => Promise.all([val, dbQuery2(val)]))
      .then(([val1, val2]) => val1 + val2)

    Remember, any time you’re using the new Promise(...) syntax, and calling proimse-generating functions inside that promise, you’re probably doing something that could be better accomplished with promise chaining.

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  2. 0

    You can do it without the need for creating an extra variable or using Promise.all() if you simply nest the dbQueries using the .then() method:

    // I am setting up a testing scenario here:
const url="https://jsonplaceholder.typicode.com/",
  dbQuery=id=>fetch(`${url}posts/${id??1}`).then(r=>r.json()),
  dbQuery2=id=>fetch(`${url}users/${id??1}`).then(r=>r.json()),


// This is the actual function:
  getDatabaseData = pid =>
   dbQuery(pid).then(p =>
     dbQuery2(p.userId).then(u =>
       `Post ${p.id}:"${p.title}" by ${u.name}`
     )
   );

// Now, test the function in an async IIFE:
(async ()=>
  console.log(await getDatabaseData(1+Math.floor(100*Math.random())))
)();

    This gives me full access to the results from both queries in a local scope under the objects p (post) and u (user).

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