How to asynchronous call function with the result of foreach loop

Question

I have a function that loops over an array of objects and builds an object depending on some parameters. It should return the build object which represents a state object of a component that I will set. However, I would like to use the Array.foreach but I have no idea how to call the function after the foreach loop is done.

The function looks something like this:

 buildStateObject= (someArray: Array) => {
    let stateobject= {};
    someArray.foreach(item => {
      switch (item.Someparameter) {
        case 1:
          stateobject.someProp1= item.message;
          break;
        case 2:
          stateobject.someProp2= item.message;
          break;
        // code omitted
      }

    });
     return stateobject;
  };

and in my component I have something like this which obviously does not work :

if(someJson.jsonArray)
    this.setState(this.buildStateObject(someJson));

So what I am looking for is something like this:


Promise.all(buildStateObject).
  then((theResultObjectMaybe)=>{this.setState(theResultObjectMaybe);})

I tried to figure this out but I was not able to. I am also not sure if this is a good practice todo.

[Edit]

@PaulJanicki thank you for pointing the error out. There were just type errors. However one error you pointed out was also in my code which results in a Unhandled promise rejection and I assumed this was because the forEach was async and the result was not yet processed.

The problem was a type error foreach instead of the correct forEach

You are returning the stateobject in the forEach (also - camelcase), but you'd have to return it after the loop is finished, so the buildStateObject-function actually returns something — Thomas Altmann
– Thomas Altmann, Commented Oct 15, 2019 at 11:14
I think this might be helpful to you. stackoverflow.com/questions/13343340/… — Gyan
– Gyan, Commented Oct 15, 2019 at 11:16

Pawel Janicki · Accepted Answer · 2019-10-15 11:32:30Z

1

You are missing the return statement outside of the forEach method, in the buildStateObject function. Also forEach should be camelCase.

 function buildStateObject(someArray) {
  let stateobject = {};
  someArray.forEach(item => {
    switch (item.someparameter) {
      case 1:
        stateobject.someProp= item.message;
        break;
      case 2:
        stateobject.someProp= item.message;
        break;
      // code omitted
    }
  });
  
  return stateobject;
}

function setState(state) {
  console.log(state);
}

var someJson = [{
  message: 'first',
  someparameter: 1
}, {
  message: 'second',
  someparameter: 2
}]

setState(buildStateObject(someJson))

Also consider that stateobject.someProp will be overwritten with each loop execution, not sure if that's the intended behaviour.

edited Oct 15, 2019 at 11:32

answered Oct 15, 2019 at 11:19

Pawel Janicki

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2 Comments

Dan Starns Over a year ago

what does returning from the forEach do?

Pawel Janicki Over a year ago

@DanStarns, of course nothing, it was just a leftover from the original code. Thanks for pointing this out! Removed

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