TypeScript: Check for object properties and narrow down type

February 3, 2020

TypeScript’s control flow analysis lets you narrow down from a broader type to a more narrow type:

function print(msg: any) {
  if(typeof msg === 'string') {
    // We know msg is a string
    console.log(msg.toUpperCase()) // 👍
  } else if (typeof msg === 'number') {
    // I know msg is a number
    console.log(msg.toFixed(2)) // 👍
  }
}

This is a type-safety check in JavaScript, and TypeScript benefits from that. However, there are some cases where TypeScript at the time of this writing needs a little bit more assistance from us.

Let’s assume you have a JavaScript object where you don’t know if a certain property exists. The object might be any or unknown. In JavaScript, you would check for properties like that:

if(typeof obj === 'object' && 'prop' in obj) {
  //it's safe to access obj.prop
  console.assert(typeof obj.prop !== 'undefined')
  // But TS doesn't know :-(
}

if(typeof obj === 'object' && obj.hasOwnProperty('prop')) {
  //it's safe to access obj.prop
  console.assert(typeof obj.prop !== 'undefined') 
  // But TS doesn't know :-(
}

At the moment, TypeScript isn’t able to extend the type of obj with a prop. Even though this works with JavaScript.

We can, however, write a little helper function to get correct typings:

function hasOwnProperty<X extends {}, Y extends PropertyKey>
  (obj: X, prop: Y): obj is X & Record<Y, unknown> {
  return obj.hasOwnProperty(prop)
}

If you don’t want to know how this works, copy it and be happy. If you want to know more, let’s check out what’s happening:

Our hasOwnProperty function has two generics:
1. X extends {} makes sure we use this method only on objects
2. Y extends PropertyKey makes sure that the key is either string | number | symbol. PropertyKey is a builtin type.
There’s no need to explicitly define the generics, they’re getting inferred by usage.
(obj: X, prop: Y): We want to check if prop is a property key of obj
The return type is a type predicate. If the method returns true, we can retype any of our parameters. In this case, we say our obj is the original object, with an intersection type of Record<Y, unknown>, the last piece adds the newly found property to obj and sets it to unknown.

In use, hasOwnProperty works like that:

// person is an object
if(typeof person === 'object' 
  // person = { } & Record<'name', unknown>
  // = { } & { name: 'unknown'}
  && hasOwnProperty(person, 'name') 
  // yes! name now exists in person 👍
  && typeof person.name === 'string' 
  ) {
    // do something with person.name, which is a string
  }

That’s it! A lovely little helper to make TypeScript understand your code better. Here’s a playground for you to fiddle around.

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TypeScript: Check for object properties and narrow down type

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