Misusing TypeScript Assertion Functions for Fun and Profit

A horrible (but very useful) hack I came up with yesterday for adding types to some old code.

Yesterday, a colleague working on adding types to one of our core libraries at LinkedIn asked me how to deal with an old (and no longer recommended!) pattern, and one thing we tried was misusing an assertion function. That didn’t do quite what we needed in the end,1 but I still think the pattern is interesting, so I’m writing it up!

The motivation is: let’s say you have an old JavaScript API which depends on mutating an object passed in. In idiomatic TypeScript, I would recommend creating a new object entirely, using some form of composition — decoration, delegation, etc. — , or otherwise to implement this kind of thing. However, in some cases you cannot change without breaking lots of consumers, and need to provide a useful TS API for it anyway (possibly while providing a better API to migrate to). In that case, you can use an asserts function to model this behavior in the type system.

Assertion functions

Assertions functions allow TS to model functions which do some kind of validation on their arguments and throw an error if those validations don’t pass. Node’s assert is the canonical example:

assert(someCondition, "Message if it fails");

In this case, if someCondition isn’t truthy, the function will throw an error instead of returning. TS lets us model this by saying that the function asserts the condition represented here by someCondition:

declare function assert(value: unknown, error: string | Error): asserts value;

That is, it asserts that the value argument is true”, and won’t return otherwise. Combined with TypeScript’s flow control analysis, after calling assert, TS knows whether the predicate you passed in is true. You can use this with all sorts of predicates to get more information about the types you’re dealing with:

function rejectNonStrings(value: unknown) {
  assert(typeof value === 'string', "It wasn't a string!");
  // Now this type checks because TS knows `value` is a `string` here:

This basic outline is enough for the purposes of this post: we now have enough information to see how to misuse asserts to solve a totally different problem. If you want to take a deeper dive, though, check out the release blog post and Marius Schutlz’s deep dive.


For a simplified example, I will use a base Person class and a function which mutates it to add an address. In JS:

class Person {
  constructor(age, name) {
    this.age = age;
    this.name = name;

function addAddress(person, address) {
  person.address = address;

let me = new Person(34, 'Chris');
addAddress(me, '1234 Some St., Example City, CO 00000');

When we initially convert this to TypeScript, the compiler will let us know that the addAddress implementation is unsafe.

class Person {
  age: number;
  name?: string | undefined;

  constructor(age: number, name?: string | undefined) {
    this.age = age;
    this.name = name;

function addAddress(person: Person, address: string): void {
  person.address = address;
  //     ^^^^^^^ Property 'address' does not exist on type 'Person'.

let me = new Person(34, 'Chris');
addAddress(me, '1234 Some St., Example City, CO 00000');
//             ^^^^^^^ Property 'address' does not exist on type 'Person'.

We can introduce an interface which represents a Person with the address added and do a safe widening” type cast:

class Person {
  // sample implementation

interface PersonWithAddress extends Person {
  address: string;

function addAddress(person: Person, address: string) {
  // SAFETY: TS only allows this if `person` *can* be narrowed or widened to
  // this type. Narrowing would be unsafe; widening is actually strictly safe,
  // just not in a way that TS supports. This only remains safe because we fully
  // initialize the new fields immediately, though!
  (person as PersonWithAddress).address = address;

This works! …but only works within the body of the function. On the calling side, we still don’t have any visibility into the fact that the Person item now has an address field:

//             ^^^^^^^ Property 'address' does not exist on type 'Person'.

This is where we get to the asserts trick which motivates this post. We can update addAddress to assert that the person passed in is actually the PersonWithAddress type:

function addAddress(
  person: Person,
  address: string
): asserts person is PersonWithAddress {
  (person as PersonWithAddress).address = address;

Now, when we call addAddress, TS recognizes that the address field exists:

addAddress(me, '1234 Some St., Example City, CO 00000');

That is because we asserted that calling addAddress means me has an address field. Note that this isn’t exactly true… but this does actually convey the correct semantics. You can check this example out live in this TS playground if you’d like to play with it yourself.


First, and most important: this is unsafe! The compiler will not check your work. This is always true of assertion functions (as well as type guard functions), but it is worth being explicit about here. We are establishing a norm at LinkedIn that we annotate these kinds of things with // SAFETY: ... comments — an idea borrowed from the Rust community’s approach to unsafe blocks. (You can see this in the code above.) The rule is: if it involves a cast, it needs a good explanation of why that cast is legitimate so that future maintainers can uphold those invariants. And of course, if you can avoid casts in other ways, do so — but at minimum, isolate them and comment the heck out of them.

Second, this only helps if the function which asserts is part of your ordinary control flow. Type-level mutations like this don’t stick around” across the life of the object the way the runtime values do. For example, if you have two class methods and one of them uses an assertion function to update this, the other method will not know anything about that:

class Person {
  // existing implementation...

  addAddress(address: string): this is PersonWithAddress {
    this.address = address;

  addHobbies(hobbies: string[]): this is PersonWithHobbies {
    this.hobbies = hobbies;

  describe(): string {
    let base = `${this.name} is a ${this.age}-year-old`;
    let location = `living in ${this.address}`;
    //                               ^^^^^^^ does not exist!

    let listFormatter =
      new Intl.ListFormat('en', { style: 'long', type: 'conjunction' });
    let hobbies = listFormatter.format(this.hobbies);
    //                                      ^^^^^^^ does not exist!

    return `${base} ${location}, who likes to do ${hobbies}`;

Third, mutating objects like this tends not to be great for performance: the JavaScript VMs are all best able to optimize objects with consistent shapes, and this is very much making the object not have a consistent shape.

In sum, the only reason to reach for this is to model existing APIs you have which behave this way and which you cannot change for some reason.

Bonus: more misuse?

We can actually generalize this to a utility representing these kinds of mutation-based extension operations:

function extend<T extends object, U extends object>(
  value: T,
  extension: U
): asserts value is T & U {
  Object.assign(value, extension);

This lets us work with any object types this way:

let person = {
  name: 'Chris',
  age: 34,

// This works! 🎉
extend(person, { hobbies: ['running', 'composing', 'writing'] });

Seems nice, right? Wellllll there are some problems (see this in a TS playground):

// This type checks too! 😬
extend(person, { age: "potato" });
// until we try to use it, now `age` is `never`

// ...and this type checks too!
extend(person, { hobbies: 123 })
// but gives us the type `string[] & number`, which is nonsense
person.hobbies + 2
person.hobbies.find((s) => s === 'wat');

// and this "works"... but adds the array values at their numeric indices
extend(person, ['a', 'b', 'c'])
console.log(person[0]); // 'a' 🙃

Net: while this general extend pattern is tempting, you shouldn’t do it. It’ll seem nice… right up until you’re trying to figure out why age is never or any number of other weird results that TS will blithely ignore!

Thoughts, comments, or questions? Discuss on LinkedIn, Hacker News, lobste.rs, or Twitter!


  1. In our case, this was our web tracking library — not the creepy kind of tracking, but the kind that lets us analyze how features are being used, run A/B tests, etc. — which was written against versions of Ember from half a decade ago. It worked by mutating an instance of Ember’s legacy Component API during setup. You inject the service, then during init() (Ember Classic’s post-constructor initialization hook), call the service’s setupComponent method with the component instance as its argument:

    import Component from '@ember/component';
    import { service } from '@ember/service';
    export default class SomeComponent extends Component {
      @service tracking;
      init() {

    Then the tracking service method sets event listeners and adds or mutates a bunch of fields to the component:

    import Service from '@ember/service';
    import { set } from '@ember/object';
    export default class TrackingService extends Service {
      // lots of other stuff
      setupComponent(componentInstance) {
        const attributeBindings = component.attributeBindings || [];
          attributeBindings.concat(['data-control-name', 'data-control-id'])
        component.on('didInsertElement', () => {
          // ...

    In this case, the design I show in the rest of the article doesn’t actually work or help here, because it does not participate in the control flow in the way we would need it to. (This is one of many reasons not to design APIs that require mutating objects to work!) ↩︎