Wait for forEach with a promise inside to finish

Try this,

makeTree: function (arr) {
   var promises = [];
   arr.forEach(function(resource) {
      promises.push(someModule.someFunction(resource));
   });
   Promise.all(promises).then(function(responses) {
      // responses will come as array of them
      // do something after everything finishes
   }).catch(function(reason) {
      // catch all the errors
      console.log(reason);
   });
}

You can refer this link for more on Promise.all with simple examples.

Instead of forEach() use map() to create an array of promises and then use Promise.all()

let promiseArr = arr.map(function (resource) {
    // return the promise to array
    return someModule.someFunction(resource).then(function (res) { //a Promise
        //do something with the resource that has been modified with someFunction
        return res;
    })
});

Promise.all(promiseArr).then(function(resultsArray){
   // do something after the loop finishes
}).catch(function(err){
   // do something when any of the promises in array are rejected
})