Typescript Access objects nested within other types

Question

I am relatively new to typescript and it's ins and outs and I am having trouble with a certain structure. it looks like this:

I have a function I am using to feed data to a timeline component. the data recieved by the function can be of type 1 or type 2. Now this is where it gets complicated type 1 is a an object. However, type 2 can be any 1 of 4 different types

type1 : {}

type2 : type3 | type4 | type5 | type6

types 3-6 differ slightly in structure from one another and cannot be combined. In the function below the isConversion flag is a check for an (object: type6). This object has another object inside it of type 7.

type 6: {
   ...,  
   type7: {
       ...,
       conversions
   }
}

inside type7 is a field called conversions that has the data I need to pass to the timeline.

timelineItems = (items: type1 | type2): PropsData => {
    const { dataType, isConversion } = this.state

    if(isConversion){
      const {comments, type7.conversions } = items as type6
      return {
        comments
        type7.conversions
      }
    }

I have a work around where I fetch type7 when I get the data and set it to the state. and use that value but I would like to to know if there is a way to get the conversions object as above.

Thank You.

Answer 1

So you want to know if you can determine if items type is Type6 ?

Typescript types are lost after compilation so you have to write your own typeguard which will return true if object is Type6 . It's pure javascript check, for example if you know that only Type6 has certain field you can check if it is present in this object.

interface Type6 {
  comments: any[];
  type6: Type7;
}

interface Type7 {
  conversions: any[];
}

function isType6(obj): obj is Type6 {
  return obj.type6 !== undefined;
} 

Then you can use this typeguard like this:

if(isType6(items)) {
  const {comments, type6: {conversions}} = items; // no cast needed, ts know it is Type6
  return {comments, conversions };
}