⋅ 4 min read

Understanding infer in TypeScript

Simohamed Marhraoui Vue and React developer | Linux enthusiast | Interested in FOSS

Recent posts:

8 Tips To Reduce Unused Javascript

8 tips to reduce unused JavaScript

Knowing how to reduce unused JavaScript can help you save time, optimize performance, and improve efficiency.

Abhinav Anshul
Nov 27, 2023 ⋅ 5 min read
Guide to mobile emulator testing with MiniSim

Guide to mobile emulator testing with MiniSim

MiniSim makes virtual emulator testing easy — learn how to use it to test both Android and iOS apps on macOS in this detailed post.

Emmanuel John
Nov 22, 2023 ⋅ 5 min read
Implementing safe, dynamic localization in TypeScript apps

Implementing safe, dynamic localization in TypeScript apps

After internationalization, you can further localize your apps to make user experiences more friendly. Learn how to do this in TypeScript.

Yan Sun
Nov 21, 2023 ⋅ 6 min read
Using Dev Containers In Vs Code For An Easier Dev Setup Process

Using dev containers with VS Code for an easier dev setup

You can leverage containers to streamline the process of setting up a dev environment. Let’s see how using VS Code and Docker.

Yashodhan Joshi
Nov 21, 2023 ⋅ 11 min read
View all posts

6 Replies to "Understanding <code>infer</code> in TypeScript"

  1. A literal [ ‘hello’ , ‘world’ ] in Typescript code is by default typed as a mutable array not a readonly tuple, but you can resolve this with `as const`.

    Although it was a two-arg string array when you created it, Typescript models it as a mutable array, because you could push(), pop() and so on. One way to defeat this type-widening, alex should be declared `as const` which prevents it from being considered mutable and makes push(), pop() a compiler error so it can never vary from being a two-value tuple.

    I really liked the learning associated with infer, (for when you can’t edit the function), but for the case where you can edit the function, I think a better fix is for the person type to be asserted readonly in the first place and to use `as const` when composing person objects, which allows the original code to compile…

    function describePerson(person: Readonly<{
    name: string;
    age: number;
    hobbies: Readonly; // tuple
    }>) {
    return `${person.name} is ${person.age} years old and love ${person.hobbies.join(” and “)}.`;

    const alex = {
    name: ‘Alex’,
    age: 20,
    hobbies: [‘walking’, ‘cooking’] // type is [string, string]
    } as const;


    Getting this right means that you haven’t type-widened the alex object, to turn e.g. hobbies into [string,string] by declaring it as a Person. When you use `as const` the hobbies property can still be inferred by the editor as being the narrower [‘walking’,’cooking’]. This has saved me a million times where compiler and editor awareness of the values is needed to guard sensitive logic. For example, some other type might be {hobby:’cooking’|’walking’, favouriteOutdoorMeals:string[]} and the compiler can know that both values of alex.hobbies fulfil the hobby value. This is not possible after type-widening them to string.

    See also https://learntypescript.dev/10/l4-readonly-function-parameters and https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-readonly-parameter-types.md

    You can see the above approach in the playground https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABAEwKYGcICcYCNUAKqW6CAFAA7GlgBciASqgIbIIA2AngDwDeAsAChEiMMwC2qeuig4wAcwDcQkc3lTRIcfizLhiABZxcuGBnpNWHHgG0Zc+QBpE9mAoC6APkWIA9L8QoEAp2VCEAX08ASkQBfSxUIKwkAAMAEl4qEgQAOjFJcMQYdEQMrJoctVRCzhYSRDh2ZERmMGb2OAA3VFLM6lyjEzN0HIArODcyACIWtpEpqPCclL1woSEIBBkW0IAPRABeWJVRCQ0AcgBBPfPHE6r6ACYABjv9QdNzRBtzgHdmdgAazc8luiHOmzgwIU53cfgCUE4VCKJTsshBzlcHgiLRKmzAMj0QjQmBw+CI2TAZABqF2USAA

  2. Thank you so much for this great article. I didn’t get a sense of “infer” from official TS guide. But here it described perfectly

  3. This is so COOL! This article let me understand the concept of ‘infer’. Thanks a lot, Marhraoui 🙂

  4. For you who need to infer Function return Promise,

    type PromiseReturnType = T extends Promise ? Return : T
    type FunctionReturnType = T extends (…args: any[]) => infer R
    ? PromiseReturnType
    : any

Leave a Reply