Use extends on a variable in typescript

Question

How can I use extends on a variable? I want to enforce typing on it, without overriding it's inferred type.

Ideally:

const permissions extends {[K: string]: {guest: boolean, user: boolean, admin: boolean}} = {
    readMessage: {guest: true, user: true, admin: true},
    writeMessage: {guest: false, user: true, admin: true},
    deleteMessage: {guest: false, user: false, admin: true},
};
type Permissions = keyof typeof permissions;
// Permissions = 'readMessage' | 'writeMessage' | 'deleteMessage'

The problem with using a type, is that it's overrides the type inference, so:

const permissions: {[K: string]: {/* ... */}} = {/* ... */};
type Permissions = keyof typeof permissions;
// Permissions = string

The only way I could think of, is wrapping the variable with an empty generic function:

const PermissionExtends = <T extends {[K: string]: {/* ... */}}>(v: T) => v;
const permissions = PermissionExtends({/* ... */});
type Permissions = keyof typeof permissions;
// Permissions = 'readMessage' | 'writeMessage' | 'deleteMessage'

Titian Cernicova-Dragomir · Accepted Answer · 2020-02-13 09:59:35Z

2

There is no such feature in typescript unfortunately. Variables either have their type fully inferred based on initializer or the type is specified as a type annotation.

The only way to do it is the function approach you have already discovered.

answered Feb 13, 2020 at 9:59

Titian Cernicova-Dragomir

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Comments

Vlad Gincher · Accepted Answer · 2022-09-25 11:50:57Z

1

Typescript 4.9 introduces the satisfies operator that does exactly this.

Here is the solution for the question with the satisfies operator:

interface Permission {
    guest: boolean;
    user: boolean;
    admin: boolean;
}

const permissions = {
    readMessage: {guest: true, user: true, admin: true},
    writeMessage: {guest: false, user: true, admin: true},
    deleteMessage: {guest: false, user: false, admin: true},
} satisfies Record<string, Permission>;

type Permissions = keyof typeof permissions;
// Permissions = 'readMessage' | 'writeMessage' | 'deleteMessage'

answered Sep 25, 2022 at 11:50

Vlad Gincher

1,10211 silver badges21 bronze badges

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