TypeScript is still throwing an error even after verifying with the hasOwnProperty

There exists a type similar to the following:

export type PathType =
  | LivingstoneSouthernWhiteFacedOwl
  | ArakGroundhog
  | HubsCampaigns
  | HubsCampaignsItemID
  | HubsAlgos
  | HubsAlgosItemID
  | TartuGecko
  | HammerfestPonies
  | TrapaniSnowLeopard
  | BeijingPigeon
  | Link
  | LinksMhdSwsVisordeltas
  | MetadatahandlingName
  | MetadatahandlingGetmetadataSequencesNameSequenceNameLabelNameLabelName;

These represent interfaces of open API definition paths.

My task is to create a function like this one:

import { PathType } from '../types/path.type';

const makeFormJSONSchema = (path: PathType) => {
  const schema = {};
  // 1. If there are any `parameters`, include them in the schema
  if (path.hasOwnProperty('parameters')) {
    path.parameters;
  }
  // 2. Peek at the method object using the `method` property
  // 3. Check for additional `parameters` and add them to the schema
  // 4. Check for `requestBody` and include it in the schema

  return schema;
};

export default makeFormJSONSchema;

In the code, I check if the path object has the parameters property, then attempt to access it.

However, TypeScript throws an error as shown here.

How can we inform TypeScript that we indeed have this property?

Answer №1

When dealing with PathType, which is a union type, it is important to remember that you can only access properties that are common across all the types in the union. To ensure TypeScript knows the object's type, you must create a type guard.

Assuming both BeijingPigeon and TartuGecko have the 'parameters' property:

You can create the following type guards:

function isBeijingPigeon(val: PathType): val is BeijingPigeon {
    return (val as BeijingPigeon).parameters !== undefined;
}
function isTartuGecko(val: PathType): val is TartuGecko {
    return (val as TartuGecko).parameters !== undefined;
}

Whenever you need to access the parameters property, use the guards like this:

if (isBeijingPigeon(pathObject) || isTartuGecko(pathObject)) {
   // accessing pathObject.parameters will not cause errors
}

Learn more about type guards and differentiating types here.

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