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Understanding TypeScript generics through lodash functions

Learning how to use TypeScript generics by re-implementing common functions from lodash

Sunday, August 22, 2021 ยท 8 min read

JavaScript is a highly dynamic language, so generics are instrumental in helping us to add types to make our code type safe with TypeScript. TypeScript Generics are super powerful, but also can be pretty complicated. As a result, we find lots of blog posts, YouTube videos, workshops, and courses on how to use generics in TypeScript.

This post is a little different. I learn best by seeing real-world examples. Seeing something new in the context of something familiar helps my brain make the connections. So this post walks through how we can develop with TypeScript generics (something new) by re-implementing common lodash functions (something familiar). My hope is that these examples will help build and solidify your understanding of how to leverage generics in your TypeScript code.

Let's get started!


First, let's learn how generics work by looking at a JavaScript function that simply returns the value you give it:

const identity = (value) => value

This is called an identity utility function (_.identity in lodash). It's used as the default callback function in many of the lodash functions. It takes any type of value and returns back that value. So if we pass in a string value, we get back that same string value. Or if we pass in some object type, we get back that same object of the same type.

So how do we use TypeScript to type it? Well since it takes in any value, our first thought may be to use the any type.

// โŒ This is not what we want!
const identity = (value: any): any => value

const origName = 'Ben'
const newName = identity(origName)
// `newName` has a type of `any` not `string`

// TypeScript won't complain that we're trying
// to call `.push()` on a `string` because the
// type is `any`. It lets us do whatever we want!

We've added types to appease TypeScript, but this is no more type safe than regular JavaScript. The value we get back is also any. So if we pass in a string value, we'll still get an any type back. That's not what we want because we've lost type safety. But we cannot make identity() take a string and return a string because then it wouldn't work for number or Array or any other types.

Because we don't know the type of value, we may think to use the newer unknown type.

// โŒ This is not what we want!
const identity = (value: unknown): unknown => value

const origName = 'Ben'
const newName = identity(origName)
// `newName` has a type of `unknown` not `string`

// This *should* work because we know `newName` is a
// string, but TypeScript fails type checking w/ error:
// "Object is of type 'unknown'."

Using unknown as the type of value makes sense because it truly is unknown. But it doesn't make sense as the return value. So now instead of going from string -> any, we've gone from string -> unknown. The unknown type is safer than any because TypeScript doesn't allow us to do anything with an unknown value. But now we'll have to use a typeof type guard in order to check to see if newName is a string. That feels like extra unnecessary work.

So any and unknown don't work, and neither does picking a specific type. Well, what about using function overloads to specify multiple type signatures?

// โŒ Doesn't work completely!
function identity(value: string): string
function identity(value: number): number
function identity(value: boolean): boolean
function identity(value: any): any {
  return value

const origName = 'Ben'
const newName = identity(origName)
// `newName` has a type of `string` ๐ŸŽ‰

const origNames = ['Ben', 'James', 'Leslie']
const newNames = identity(origNames)
// TypeScript fails type checking w/ error:
// "No overload matches this call."

NOTE: I believe function overloads only work with function declarations (function funcName() { }), and not with function expressions (const funcName = () => { }).

So we almost get there with function overloading. We can define an overload for string, number, and boolean. But what about arrays or other more complex objects? We have to create an overload for potentially every single possible type used in our application in order to get the correct return value. That's neither feasible nor maintainable.

This is where generics come in.

const identity = <ValueType>(value: ValueType): ValueType => value

const origName = 'Ben'
const newName = identity(origName)
// `newName` has a type of `string` ๐ŸŽ‰

const origNames = ['Ben', 'James', 'Leslie']
const newNames = identity(origNames)
// `newNames` has a type of `string[]` ๐ŸŽ‰

The <ValueType> syntax is the generic. It makes the function type flexible. It says that value is a generic type. We're calling it ValueType here but it can be named anything. Unfortunately, it's very common to use single letter generic names like T. So now when we pass in origName (a string) to identity(), ValueType becomes a string. And because identity() also returns ValueType, it will return a string when it's passed a string. The same thing happens for origNames which is a string[]. We get back a string[] type as well. The same would apply for number, boolean, a complex object, even a function, or anything else. ๐Ÿ™Œ๐Ÿพ

So now that we've learned the "why" of generics by looking at a rather simple function, let's see how generics apply to other lodash functions.


The first() function (_.head in lodash) gets the first element of an array, if one exists.

const first = <ItemType>(array: ItemType[]): ItemType | undefined => {
  // the return value is `undefined` if `array` is empty
  return array[0]

const firstName = first(['Ben', 'James', 'Leslie'])
// `firstName` is of type `string | undefined`

const firstScore = first([100, 47, 75, 98])
// `firstScore` is of type `number | undefined`

const firstPlayer = first([
  { name: 'LeBron James', team: 'Los Angeles Lakers' },
  { name: 'Kevin Durant', team: 'Brooklyn Nets' },
  { name: 'Giannis Antetokounmpo', team: 'Milwaukee Bucks' },
// `firstPlayer` is of type `{name: string, team: string} | undefined`

const mixedArray = [true, 'Ben', 87]
// `mixedArray` type is `(string | number | boolean)[]`
const firstItem = first(mixedArray)
// `firstItem` is of type `string | number | boolean | undefined`

const firstAnswer = first('yes')
// Error: Argument of type 'string' is not assignable to
// parameter of type 'unknown[]'.

FYI: I'm intentionally simplifying the code by not allowing the array to be null or undefined.

ItemType is the generic type in first(). So the array parameter is an array of generic ItemType types. It must be an array, but it can be an array of anything. And first() returns a single ItemType (or undefined).

So firstName is a string (or undefined) because we called first() with an array of string types. Similarly, firstScore is a number (or undefined) because we called first() with an array of numbers. And just to prove that generics work with complex objects, firstPlayer is an object with name and team properties that are both strings because we called first() with an array of those types of objects.

But guess what? All of the array items don't have to be the same type. Because we called first() with an array of strings, booleans, and numbers, firstItem is either a string, boolean or number (or undefined).

Lastly, if we don't pass an array at all, TypeScript fails to type check the code. The array parameter is not any generic type (like with identity()), it's specifically a generic array.

There's one thing to note. Because we've defined array to be an array of generic types, we can make use of all of the array methods and properties (indexing, .length, .map(), etc) within the implementation of first(). However, because ItemType is a generic, we cannot access any methods or properties of an individual item without using some sort of type narrowing.


The forEach() function (_.forEach in loadash) iterates over an array, calling the function for each array item.

const forEach = <ItemType>(
  array: ItemType[],
  callback: (item: ItemType, index: number, array: ItemType[]) => void,
): void => {
  for (let i = 0; i < array.length; i++) {
    callback(array[i], i, array)

forEach(['Ben', 'James', 'Leslie'], (name, index) => {
  // because we passed a `string[]`, TS knows that `name`
  // is a `string` because of the `ItemType` generic w/o
  // having to define its type
  console.log(name.toLocaleUpperCase(), index + 1)
// "BEN", 1
// "JAMES", 2
// "LESLIE", 3

Here forEach() works a bit differently. It still takes a similar array of generic ItemType values, but it doesn't return the type. It uses the ItemType in the definition of the callback function. That function will be called with 3 arguments: the first is a single ItemType (the array item), the second a number (the array item index), and the third a ItemType[] (the entire array).

Because of the association between the array and the callback function using the generic ItemType, we don't even need to specify the type of name (or index) in our call of forEach(). TypeScript can figure it all out through type inference (specifically contextual typing).


The map() function ( in lodash) creates a new array populated with the results of calling the function on every item in the array.

const map = <ItemType, MappedItemType>(
  array: ItemType[],
  callback: (
    item: ItemType,
    index: number,
    array: ItemType[],
  ) => MappedItemType,
): MappedItemType[] => {
  const newArray: MappedItemType[] = []

  for (let i = 0; i < array.length; i++) {
    newArray.push(callback(array[i], i, array))

  return newArray

const mappedNames = map(['Ben', 'James', 'Leslie'], (name) => name.length)
// [3, 5, 6] (type `number[]`)
// TS sees that the return type from the `callback` is a `number`
// so it knows the return value from `map()` is a `number[]`

The map() function takes things one step further by introducing a second generic type. The type of the array is once again ItemType[] and the callback function takes the same 3 arguments as before. The difference here is that the callback function returns a value of a different type that we're calling the generic MappedItemType. And we then use MappedItemType as the return type of map() because we're return a MappedItemType[] (an array of MappedItemType items).

This is the real power of generics. The map() function doesn't have to know the type of the array it receives, nor the type of the array it will return, yet it's fully type-safe. Moreover, when we call map() we don't have to specify any types. Because we pass it a string[] it knows that now ItemType is a string. And because the callback function returns a number, TypeScript knows that mappedNames can only be a number[].


The reduce() function (_.reduce in lodash) runs a function (called a "reducer") on each element of the array, resulting in a single output value that can be a new array, an object, a number, etc.

const reduce = <ItemType, ReducedType>(
  array: ItemType[],
  reducer: (
    accumulator: ReducedType,
    item: ItemType,
    index: number,
    array: ItemType[],
  ) => ReducedType,
  initialValue: ReducedType,
): ReducedType => {
  let reducedValue = initialValue

  for (let i = 0; i < array.length; i++) {
    reducedValue = reducer(reducedValue, array[i], i, array)

  return reducedValue

const maxValue = reduce(
  [4, 582, 38, -472, 1, 20],
  (maxInProgress, value) => Math.max(maxInProgress, value),
// 582 (type `number`)
// The type returned from the `reducer` function has to match
// the type of the `initialValue`, which ultimately becomes
// the type of the return value from `reduce()`

If you're still a bit shaky with how reduce() functions work in general, read Learn the Array reduce method by re-implementing lodash functions.

Things are getting pretty complex with reduce() now. We're using the ItemType and ReducedType generics all over the place. The ItemType generic is found in the usual spots: ItemType[] for the main array and the array within the reducer function as well as ItemType for the item within the reducer function.

The ReducedType is the type of the single value we ultimately receive from reduce(). It's also the type of the initialValue passed in. It's the type of the reducer's first parameter (typically called the accumulator) as well as the reducer's return value. When we call reduce() the types in all 3 places have to match in order for our code to properly type check.

Once again, TypeScript is able to figure everything when we call reduce() without us having to specify any types. The reduce() function is arguably the most power array utility function there is. We can go from an array of anything to a single value of a completely different type. The implementation is really tiny. And because of generics, the type definition is compact as well.

Hopefully that helped! It took me a long while to grasp generics. So even if you still don't feel 100% confident, that's okay. If you understand them just a little bit more I call it a win, and you should too. ๐Ÿ˜„ If you've got any questions (or comments) feel free to reach out to me on Twitter at @benmvp.

Keep learning my friends. ๐Ÿค“

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Hi, I'm Ben Ilegbodu. ๐Ÿ‘‹๐Ÿพ

I'm a Christian, husband, and father of 3, with 15+ years of professional experience developing user interfaces for the Web. I'm a Google Developer Expert Frontend Architect at Stitch Fix, and frontend development teacher. I love helping developers level up their frontend skills.

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