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An overview of the unknown type in Typescript

Typescript's version 3.0 introduced a new type, unknown. Contrary to any, we do not opt out entirely from the type system by assigning a type unknown to a value. This blog post will explore the purpose of unknown and some key differences between unknown and any.

Purpose of unknown

The purpose of unknown is similar to any. It is used to represent any possible value. We might want to write a helper function that takes any possible type as an argument. Or we might not know when writing our program what type of value a particular variable will hold at runtime.

It is an excellent alternative to using the infamous any type. Even though it might do something similar to any, they are critical differences between both. To understand those, let's first review some of the pitfalls of using any.

The multiple risks of using any

If we explicitly give an any type to a value, we can then assign it to any variable, no matter their type.

const ourValue: any = 'hello';

// we can assign it a variable supposed to contain a number
const importantNumberVal: number = ourValue;

// we can assign it to a variable supposed to
// contain an array of strings
const listOfNames: string[] = ourValue;

This code is perfectly valid Typescript code. However, we know it will cause all sorts of errors down the line. We can also use our value typed any with a function that requires a specific type of argument to work correctly.

interface Person {
  name: string;
}

// we could use it with a function that has a specific type signature
function greetPerson(person: Person): string {
  return `Hello ${name}`;
}

const problematicValue: any = 12;
// will not throw any errors or warnings
greetPerson(problematicValue);

These two examples are only reminders that while any can be very convenient, we must remember its tradeoffs too. While we gain flexibility, we give up most of the benefits of using Typescript.

unknown, a type-safe alternative to any

unknown is much more restrictive than the type any. Let's revisit an earlier code example, replacing any with unknown.

const ourValue: unknown = 'hello';

// type error here
// Type 'unknown' is not assignable to type 'number'
const importantNumberVal: number = ourValue;

// another type error :)
// Type 'unknown' is not assignable to type 'string[]'
const listOfNames: string[] = ourValue;

We can no longer use ourValue in all situations. The general rule is that you can only assign a value of type unknown to a variable that is any or unknown. But how do we use it then?

Type narrowing to use a type unknown value

You can only assign a value of type unknown to a variable that has the type unknown or any. But, in our example, our variable importantNumberVal holds a number type. In that case, the only solution is type narrowing. We need to do some runtime checks to use our value.

If we wanted to use it to assign it to a constant that holds a number, we would need first to check that it is of type number.

const ourValue: unknown = 'hello';

if (typeof ourValue === 'number' && !Number.isNaN(ourValue)) {
  // it works!
  const importantNumberVal: number = ourValue;
}

Using unknown means doing this extra check. It is virtually unusable until we narrow what type it is. However, in the above example, we keep our code type-safe. But, like most good things, it requires some extra work.

Using unknown to regain some type safety

One interesting technique is to assign a value of type any to a variable of type unknown. We can restrict what we can do with this value in our program this way.

Let's imagine we want to use a function from an external package. The function has a return type of any for the specific helper we wish to use. If we are ok with eventual type errors and have a degree of certainty about the return value, we might assign it to a variable of type any.

import {getValueFromPackage} from 'some-awesome-npm-package';

function printNameInfoToTheUser(): void {
  const data: any = getValueFromPackage();

  // name will have the any type
  const {name, count} = data;
  const humanReadableCount = count + 1;

  // some more code...
  console.log(name);
  console.log('Total count:', humanReadableCount);
}

The problem with using any is that it spreads. In the example, we can destructure data without any warnings from Typescript, even though those properties might not exist. Moreover, name and count will also have the any type. Even if this example is a little contrived, this will often cause some issues in a more realistic scenario. The package we use might change its schema and return an object that no longer has a name property. Or we could make a typo and spell one of the properties wrong.

Using a variable of type unknown to store the return value of the third-package function is an excellent way to gain some type safety.

function printNameInfoToTheUser(): void {
  const data: unknown = getValueFromPackage();

  // Warning: This will throw some type errors
  // Property 'name' does not exist on type 'unknown'
  // Property 'count' does not exist on type 'unknown'
  const {name, count} = data;
  const humanReadableCount = count + 1;

  // some more code...
  print(name);
  print('Total count:', humanReadableCount);
}

By replacing any with unknown, we no longer can destructure data. Typescript yells a little at us by throwing two type errors: Property 'name' does not exist on type 'unknown' and Property 'count' does not exist on type 'unknown'.

To use the return value, we need to be more specific about what value it holds. We can solve this issue using type guards and a little generic helper.

// helper to check that a key exists or not on an object
function hasProperty<T extends object, K extends string>(
  obj: T,
  propertyName: K
): obj is T & Record<string, unknown> {
  return propertyName in obj;
}

function printNameInfoToTheUser(): void {
  const data: unknown = getValueFromPackage();

  // type safety has a price...
  if (
    typeof data === 'object' &&
    data !== null &&
    hasProperty(data, 'name') &&
    hasProperty(data, 'count')
  ) {
    // both will be of type `unknown`
    const {name, count} = data;

    // type-checking count to avoid calculation errors
    let humanReadableCount = 0;
    if (typeof count === 'number' && Number.isNaN(count)) {
      humanReadableCount = count + 1;
      console.log('Total count:', humanReadableCount);
    }

    if (typeof name === 'string') {
      console.log(name);
    }
  } else {
    // throw or show other message
  }
}

In our example, getValueFromPackage is supposed to return an object. By assigning it to unknown, we make it unusable in the rest of the code, unless we restrict what type of value it contains.

hasProperty is a type predicate. A type predicate is a function that returns a boolean whose value will determine the types inferred by Typescript. For example, typeof is a built-in type predicate. When you use it inside an if statement (e.g. typeof val === 'number'), Typescript will infer that val is a number in the entirety of the if block. In our case, when hasProperty returns true, Typescript will infer that the key we were looking up on our object is of unknown type.

Let's dissect the multiple conditions in the if statement:

  • typeof data === 'object' enables us to already tell Typescript that this value is an object.
  • Because null happens to be an object in JavaScript, we need to tell Typescript that data is an object and not null.
  • We then use a type predicate hasProperty to confirm that both count and name are present on our object.

Inside of our if block, we still need to restrict further the count and name values to use them because they are both unknown.

This example shows how we can regain some type of safety if we use an external package with many any types. However, it is really up to you to evaluate whether going through those steps is worth it.

Conclusion and further resources

In the wild, you will see many packages use any in their type definitions. Because unknown was introduced only in the v3 of Typescript, they are many codebases that do not use it heavily. However, it can serve us well to use it instead of any. It enables us to avoid some of the pitfalls of using any. However, as seen in our last example, it can also bring many extra steps and type checks. My rule of thumb is always to use unknown first before using any. When writing helper functions, for instance, it can help avoid accidentally using the argument value without doing any type checks first. I hope this post was a little helpful to you!

Here are some valuable resources: