Simohamed Marhraoui
Oct 5, 2021 ⋅ 4 min read

Understanding infer in TypeScript

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

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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

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