Native javascript promise loop

Question

The purpose of this is to loop through a request a few times before proceeding. github.repos.getBranch should build an object with 10 elements before I can continue and use the results.

The github api request works as expected, but the results logged at: console.log('++', res) come back with the following: ++ [ undefined, undefined, undefined, undefined, undefined ] . If I log allBranches after the loops have finished, the data is all there.

I'm obviously missing a step after the github requests. I have refactored this as many ways as I can think of, with no success.

getAllData: () => {
  let allBranches = []
  let startGetData = config.repositories.map(repo => {
    config.branches.map(branch => {
      return allBranches.push(
        github.repos.getBranch({
          owner: config.organisation,
          repo,
          branch
        })
      )
    })
  })

  return Promise.all(startGetData)
  .then(res => {
    console.log('++', res)
  })
}

Answer 1

You're not returning anything from your config.repositories.map call (it's a verbose arrow function with no return ). So you end up with an array of undefined , which is what you pass to Promise.all You need a return .

But that's not the only issue. You're passing startGetData into Promise.all , but that's not where the github promises are stored. You're storing them in allBranches . So you'd need to wait on allBranches , not startGetData .

I suspect your goal is to get an array of repositories containing an array of branches. If so, you'll need multiple Promise.all calls (waiting on the branches of a repository) and an overall Promise.all call (waiting for all the repositories to finish).

If that's your goal, here's one way it would look:

getAllData: () => Promise.all(                    // Gather up all results
  config.repositories.map(repo =>                 // Map repos to promises
    Promise.all(config.branches.map(branch =>     // Map branches to promises
      github.repos.getBranch({                    // Promise for branch
        owner: config.organisation,
        repo,
        branch
      })
    )).then(branches => ({repo, branches}))       // Wrap up branch results in...
  )                                               // ...an object identifying...
)                                                 // ...the repo

That gives you a promise for an array of objects like this:

[
    {
        repo: /*...repo...*/,
        branches: [
            /*...branch...*/,
            /*...branch...*/,
            /*...branch...*/
        ]
    },
    {
        repo: /*...repo...*/,
        branches: [
            /*...branch...*/,
            /*...branch...*/,
            /*...branch...*/
        ]
    },
    {
        repo: /*...repo...*/,
        branches: [
            /*...branch...*/,
            /*...branch...*/,
            /*...branch...*/
        ]
    }
]

If you just want a pure array of arrays:

[
    [
        /*...branch...*/,
        /*...branch...*/,
        /*...branch...*/
    ]
    [
        /*...branch...*/,
        /*...branch...*/,
        /*...branch...*/
    ],
    [
        /*...branch...*/,
        /*...branch...*/,
        /*...branch...*/
    ]
]

...then just remove the final then clause ( .then(branches => ({repo, branches})) ).