Lazy is hard

Implementing Lazy access in TS with Key Mapping

Published On: 06 Aug 2022

TL; DR ->

In my last article Lazy is easy, I implemented lazy loading the properties of a JS object, with full type-safety. There was one caveat, though — we named the properties as variable names (name), when they were the getters. Ideally, we could have named them getName, and Proxy convert the access to the name to call to getName. This, however, adds couple of interesting challenges from the TS domain, which we shall go through in this article.

Enforcing the Property Names as get<Something>

This is one of the first requirements of the problem. Let’s go ahead and modify the LazyInput type from the previous article to support this.

 type LazyInput = {
-  [key: string]: () => any
+  [key: GetterName]: () => any

The GetterName type enforces that the string should start with get, followed by an uppercase character and any string.

type GetterName = `get${UppercaseLetters}${string}`

We can define UppercaseLetters as an union of all uppercase letter literals, but I’m lazy to do that 😅. Instead, let’s define a constant string containing all lowercase characters, and split it somehow to a union of uppercase characters.

const letters = "abcdefghijklmnopqrstuvwxyz"

type Split<T extends string> =
  T extends `${infer First}${infer Rest}`
    ? First | Split<Rest>
    : never

type LowercaseLetters = Split<typeof letters>

type UppercaseLetters = Uppercase<LowercaseLetters>

Here, type Split is recursively splitting input to union until it is empty. We then use TS-inbuilt type Uppercase to get the desired union.

Deriving the property name from GetterName

In the Proxied lazy object, we want to access properties, not call getters. Let’s create a type to convert GetterName to property names. For example, this type would convert getPropertyName to propertyName. Here, we are

  • removing the get from the begining of the string
  • converting the next letter to lowercase
  • keeping the reamining part as it is.
type PropertyName<T extends GetterName> =
  T extends `get${infer FirstLetter}${infer Name}`
    ? `${Lowercase<FirstLetter>}${Name}`
    : never

Let’s try it in action.

type t1 = PropertyName<"getPropertyName"> // = propertyName
type t2 = PropertyName<"setPropertyName"> // Error
type t3 = PropertyName<"getpropertyName"> // Error 

Now that we get it working, let’s implement the return type of our new lazy function.

The new Lazy

Armed with the type PropertyName, we can now define the Lazy type as:

type Lazy<T extends LazyInput> = {
  [K in keyof T as PropertyName<
    K & GetterName
  >]: ReturnType<T[K & GetterName]>;

There are several differences here from previous article:

  • We are renaming the keys here using as operator to PropertyName
  • While doing so, we are intersecting the key type K with GetterName to ensure we pass in the right types.

Let’s check if that works:

type t1 = Lazy<{}> // = {}
type t2 = Lazy<{
  getX: () => number,
  getLongProperty: () => string
}> // = {x: number, longProperty: string}
type t3 = Lazy<{
  hello: number
}> // = {}

Cool, our type seems to work, except on t3, where I thought TS should give us an error. Let’s dig deep and understand why it is not happening.

Enter the TS Rabbithole

Let’s look closely at the definition of Lazy<T>, where we defined a constraint on T that it should extend LazyInput. Let’s check if the type {hello: number} satisfies that constraint.

type test = {hello: number} extends LazyInput ? true: false // = true

Woah, seems like TS is doing something wrong here 😮, although we specify the key format in LazyInput, TS is not respecting that at all! To investigate this further, let’s supply a GetterName key, but with an invalid value (say number)

type test = {getHello: number} extends LazyInput ? true: false // = false

Ah, now it is false and TS seems back alive. Does it mean that TS is validating the type of the values, but not the keys:thinking:? Btw, what does the extends is checking, exactly?

The extends Keyword in Type Expressions

Let’s think of extending a class in object oriented languages. The extended class (a.k.a subtype) has all the properties (and methods) of its parent class (a.k.a supertype) along with some of its own (also overrides). In TS also, inheritance works in the same way. Let’s go back to some examples now. Here, I’m using interface to make use of extends keyword; we can achieve the same things with type and intersection using &.

interface Person {
  name: string,
  age: number

interface PersonWithPhone extends Person {
  phoneNumber: string

interface PersonWithHealthInfo extends Person {
  bloodGroup: string

type t1 = PersonWithHealthInfo extends Person ? true: false // = true
type t2 = PersonWithPhone extends Person? true: false // = true
type t3 = {} extends Person? true: false // = false
type t4 = Person extends Person? true: false // = true
type t5 = {name: string} extends Person? true: false // = false

Here, the extends keyword is both used to create subtypes and check if a type is subtype of another, and the results are as what we expect — if a type contains all properties of Person, the extends check returns true. Keeping this model in mind, let’s go beyond pre-defined properties to explore what happens in case of keyed properties.

Let’s consider back our LazyInput type, with some simplifications.

type LazyInputSimplified = {
  [key: GetterName]: number

Let’s look at some valid and invalid values for this type:

const e1: LazyInputSimplified = {} // valid
const e2: LazyInputSimplified = {"hello": 1, "world": 2} // invalid
const e3: LazyInputSimplified = {2: 5} // invalid
const e4: LazyInputSimplified = {"getA": 3} // valid

This is also as expected. Note that the empty object is also a valid value. Let’s think what values a subtype (say T) of LazyInputSimplified can have of. By definition of subtyping,

  • It can have all the values that LazyInput can have, along with
  • It can define its own values.

The point that T can define extra properties is the key here. Since empty object is also a valid LazyInputSimplified value, we are free to add any keys to it as long as we don’t touch GetterName and provide it with an invalid value (other than number). To make it clear:

type t1 = {} extends LazyInputSimplified ? true: false // = true
type t2 = {hi: "hello"} extends LazyInputSimplified? true: false // = true
type t3 = {hi: "hello", getA: boolean} extends LazyInputSimplified? true: false // = false
type t4 = {hi: "hello", getA: number, getB: number} extends LazyInputSimplified? true: false // = true
  • t1 is true, as empty object type is a valid LazyInputSimplified
  • t2 is true, as it defined extra keys than LazyInputSimplified
  • t3 is false, as it violated LazyInputSimplified constraint in {getA: boolean}
  • t4 is true, as it defines extra keys than {getA: 3, getB: 4} , a valid LazyInputSimplified

Now we know why type test = {hello: number} extends LazyInput ? true: false // = true was true. It did extend LazyInput by defining an extra key hello. We did not define in LazyInput what extra keys should look like. Let’s look how we can disallow extra keys altogether!

Restricting the Generic Parameter

Now, we can frame our problem statement more abstractly. We have two types T and U, where T is a subtype of U (that is, T extends U). Remember that T can define extra properties than U, now we want to get rid of them. To do that, we should:

  • Loop over all keys k of T
  • If k is a key of U also, we proceed to compare the value types
    • If U[k] is a subtype of T[k], we keep it as it is
    • otherwise, we set T[k] to be never so that we disallow any values there
  • Otherwise, we set the value to never so that we disallow all the values to key k.

This way, we keep all the keys k, but set the values to never, so that any assignment to that key results in a type error. The type implementing this logic now looks as:

type Restrict<T extends U, U> = {
  [Key in keyof T]: Key extends keyof U
    ? U[Key] extends T[Key]
      ? T[Key]
      : never
    : never

With this type, let’s validate our assumptions on LazyInputSimplified:

type t1 = Restrict<{}, LazyInputSimplified>
// type t1 = {}
type t2 = Restrict<{hi: "hello"}, LazyInputSimplified>
// type t2 = {hi: never}
type t3 = Restrict<{hi: "hello", getA: false}, LazyInputSimplified>
// Error, violates generic constraint
type t4 = Restrict<{hi: "hello", getA: number, getB: number}, LazyInputSimplified>
// type t4 = { hi: never; getA: number; getB: number; }

As we see from the above examples, we get never on incompatible properties, thus preventing them from assigning. Now, we can redefine Lazy using new Restrict utility:

type Lazy<T extends LazyInput> = T extends Restrict<
  ? {
      [K in keyof T as PropertyName<
        K & GetterName
      >]: ReturnType<T[K & GetterName]>
  : never

The core part of the type is still the same. On top of that, we check if T contains any restricted keys, returning never if so. With that, our first example becomes,

type t1 = Lazy<{}> // = {}
type t2 = Lazy<{
  getX: () => number,
  getLongProperty: () => string
}> // = {x: number, longProperty: string}
type t3 = Lazy<{
  hello: number
}> // = never

Which is now cool, as it says we can’t have incompatible keys on the Lazy type. However, we still have to enforce it at the compile time, which we now do at the function definition part. Finally, we are out of the TS rabbithole 🎉.

The new lazy()

Now, let’s begin the fun part of implementing the actual lazy method using Proxies. Rememember, we need to

  • Take a property name
  • Convert it to getter name
  • Call the getter function, cache the result and return it.

To do this, let’s start by implementing a function to get the getter name given the property name.

const getterName = (prop: string) => {
  const [first,] = prop.split("")
  return ["get", first!.toUpperCase(),].join(
  ) as GetterName

Then, we can proceed to implement the lazy() method as,

const lazy = <T extends Restrict<T, LazyInput>>(
  object: T
) => {
  const cache = {} as any
  return new Proxy(object, {
    get(target, prop) {
      if (!(prop in cache)) {
        // Forward symbol | number to original object
        if (typeof prop !== "string")
          cache[prop] = target[prop as keyof T]
        else {
          const getter = (target as LazyInput)[getterName(prop)]
          cache[prop] = getter ? getter() : undefined
      return cache[prop]
  }) as any as Lazy<T>

Some notes here:

  • We can not return Proxy as Lazy as we did earlier, as the keys are also different. Since we are sure about this, we do as any as Lazy<T>
  • We also type cache as any where we could have used Lazy<T>. This is simply to save ourselves from lots of type castings throughout!
  • We choose to forward number | symbol accesses to original object as it is, while doing so, we have to convert prop to keyof T as we are not doing any modifications to key here.
  • To get the type-safe version of getter, we are first casting target to LazyInput (which is Restrict<T, LazyInput>)

Phew, that’s lots of TS manipulations even when we are writing the logic part. Anyway, let’s go ahead and test the lazy() method we just implemented:

const logAndReturn = <T>(arg: T) => {
  console.log("Logging", arg)
  return arg

const object = {
  getBoolean: () => logAndReturn(true),
  getString: () => logAndReturn("hello world"),
  getNumber: () => logAndReturn(21),

const lazyObject = lazy(object)
console.log(lazyObject.boolean) // Logging true, true
console.log(lazyObject.boolean) // true

console.log({ ...lazyObject }) // {getBoolean: undefined, ...}

Our results are bittersweet:

  • We got successful type inference for the type of the lazyObject
  • Caching is also working, we are seeing the logAndReturn call for only the first access
  • However, destructuring and logging the object does not seem to get hang of new property names yet…

Let’s go ahead and fix the problem for good.

Enter the JS Rabbithole

When we are overriding get method in the Proxy handler, we want to somehow tell the JS that we have new keys. Fortunately, we can also override the ownKeys method to return the new keys. To do that, we

  • First, get the key names from underlying object. Remember, they are getters.
  • Map them to property names, let’s write an utility function for that.
const propertyName = <T extends GetterName>(
  getterName: T
): PropertyName<T> => {
  const [first, ...r] = getterName.slice(3).split("")
  return [first!.toLowerCase(), ...r].join(
  ) as PropertyName<T>

Let’s override the ownKeys method also:

const lazy = <T extends Restrict<T, LazyInput>>(
  object: T
) => {
  // same as before
  return new Proxy(object, {
    // same as before
    ownKeys(target) {
      return Object.keys(target)
        .map((getter) => propertyName(getter as GetterName))

Then, in the driver,

const object = // same as before
const lazyObject = // same as before
console.log(Object.keys(lazyObject)) // []

Wow, it is still [] 😢. Without ownKeys override though, it was ["getBoolean", "getString", "getNumber"]. We are still better than that, but have to make our new keys visible to JS. Turns out that we have to make them enumerable and configurable. To do this, we can override the getOwnPropertyDescriptor of the proxy handler:

const lazy = <T extends Restrict<T, LazyInput>>(
  object: T
) => {
  // same as before
  return new Proxy(object, {
    // same as before
    getOwnPropertyDescriptor() {
      return {
        enumerable: true,
        configurable: true,

With this, let’s go to the driver again:

const object = // same as before
const lazyObject = // same as before
console.log(Object.keys(lazyObject)) // ["boolean", "string", "number"]
// Destruct to force evaluate all keys
console.log({ ...lazyObject }) // {boolean: true, string: "hello world", number: 21}

Nice, we are seeing the expected things now 😄.

Closing thoughts

With all the back and forth with the TS and JS Proxies, we were able to implement a version of lazy that does lazy evaluation and caching along with key mapping. If we pass in an non-matching key to the argument of lazy, it is an runtime error as well as compile time one.

const object = {
  getBoolean: () => logAndReturn(true),
  getString: () => logAndReturn("hello world"),
  getNumber: () => logAndReturn(21),
  x: 33,

// error here: Type 'number' is not assignable to type 'never'.
const lazyObject = lazy(object)
// Runtime error here, calling ownKeys, error in propertyName

console.log({ ...lazyObject })

The logic in type domain and the runtime is tightly coupled. This is not without limitations, though. Not passing an object literal to lazy might not give you all the benifits, and type errors are hard to debug many times. However, this shows the power of what TS can do at type level. As usual, I dread writing of PropertyName type and propertyName function having same logic — this is unavoidable as of now, until we have (can we?) a TS runtime with access to types.

Get your hands Dirty!

Try this code sample in CodeSandbox.


  • For a Record<string, string>, although we have specified string keys, we can have number keys. This is due to JS internally converting number index access to string access.

    • const x: Record<string, string> =  {2: "Hi"} // valid
  • The Restrict<> type seems incomplete, for example, a call to lazy(2) is still valid, though I didn’t understand why this works yet. Maybe a TS Bug? :thinking:

  • Anything else, you tell me!



There can be some typos in this writing, I was too lazy to spell check the article since writing it took an entire day. Hope you understand me 🙃


Sorry, Lazy to write, go check the full article, its all about how hard it is to be lazy!