Author: Abdullah Numan
The Record<> utility type in TypeScript is typically associated with a record or a collection of records returned from an API endpoint. It helps define a type with property names such as id and map the values to the type of the data.
Introduction
The Record<> type is a TypeScript object transformation type that is often used to derive stable object types for API data in an application. This is possible mainly because the property identifiers of the Record<> type are themselves types and the mapped value is also a type. As such, the Record<> type helps write error-free, maintainable and efficient code, and becomes increasingly useful when an application's API endpoints and versions start growing.
This post explores the TypeScript Record type with a series of examples ranging from simple strings and numbers based types to more common ones involving API data and React components. With the examples, we see how to derive Record<> types by assigning types for Keys and Value, discuss how a Record<> type is advantageous over simple object types and index signatures, and try to understand the quirks of its usage.
Understanding the Record Type
Starting easy, let's begin with a simple object type that represents a user:
type TUser = {
email: string;
password: string;
};
const user: TUser = {
email: "example@example.com",
password: "12345678",
};
console.log(user.email); // "example@example.com"
The object type is fine for typing a user. And we need it to base a record type on it.
We could also use an index signature:
type TIUser = {
[s: string]: string;
};
const iUser: TIUser = {
email: "example@example.com",
password: "12345678",
};
console.log(iUser.email); // "example@example.com"
Not very insightful. This is because albeit acceptable, the index signature is a misrepresentation of the user data that has a well-defined shape in itself.
For API data like a user, a much more accurate representation is by constructing a member based on indexed table columns like id:
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
};
This is crucial, especially in backend development for building RESTful APIs that use ids in query params to fetch data from API endpoints.
Enter the Record<>
The TypeScript Record<> type helps reconstruct API data types into structured maps that are easier to handle:
type TUser = {
email: string;
password: string;
};
const user: Record<string, TUser> = {
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
}
};
console.log(user["3xamp1eUSERIdSTOR3DinAdb"].email); // "example@example.com"
Now the type is a more structured hash map whose members can be plucked with the id.
Notice the Record<Keys, Value> is a generic. The Keys passed represents the type or union of types for the member name. And the Value is the type for the value of the property.
The important thing here is that we're still using the TUser type to base the record type on the actual shape of the data.
The derived Record type actually represents a collection of data:
const users: Record<string, TUser> = {
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
},
"another3xamp1eUSERIdSTOR3DinAdb": {
email: "another_example@example.com",
password: "12345678",
},
};
console.log(users["another3xamp1eUSERIdSTOR3DinAdb"].email); // "another_example@example.com"
Union of Key Types
The transformed type above is unrestricted with respect to the number of items because we can add as many members as possible.
However, if we want to restrict the collection to a number of ids, Keys can have a union of types:
type TUser = {
email: string;
password: string;
};
type ActiveUserIds = "3xamp1eUSERIdSTOR3DinAdb" | "another3xamp1eUSERIdSTOR3DinAdb" | "yetAnother3xamp1eUSERIdSTOR3DinAdb";
const activeUsers: Record<ActiveUserIds, TUser> = {
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
},
"another3xamp1eUSERIdSTOR3DinAdb": {
email: "another_example@example.com",
password: "12345678",
},
"yetAnother3xamp1eUSERIdSTOR3DinAdb": {
email: "yet_another_example@example.com",
password: "12345678",
},
};
console.log(activeUsers[ "3xamp1eUSERIdSTOR3DinAdb"].email); // example@example.com
console.log(activeUsers["amongOther3xamp1eUSERIdsSTOR3DinAdb"].email);
/*
Property 'amonganother3xamp1eUSERIdSTOR3DinAdb' does not exist on type 'Record<activeUserIds, TUSer>'. Did you mean 'another3xamp1eUSERIdSTOR3DinAdb'?(2551)
*/
Notice the Keys argument is now a union of id strings, which helps us restrict the members to only activeUserIds. This means that accessing users with ids that are not included, like amongOther3xamp1eUSERIdsSTOR3DinAdb, throws the above TypeScript 2551 error.
Union of keys types in Record<> is much more restrictive - in the sense that TypeScript considers the union strictly as a set. If we miss one item from the union as the map key, we get a 2741 error:
type TUser = {
email: string;
password: string;
};
type ActiveUserIds = "3xamp1eUSERIdSTOR3DinAdb" | "another3xamp1eUSERIdSTOR3DinAdb" | "yetAnother3xamp1eUSERIdSTOR3DinAdb";
const activeUsers: Record<ActiveUserIds, TUser> = {
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
},
"yetAnother3xamp1eUSERIdSTOR3DinAdb": {
email: "yet_another_example@example.com",
password: "12345678",
},
};
/*
Property 'another3xamp1eUSERIdSTOR3DinAdb' is missing in type '{ "3xamp1eUSERIdSTOR3DinAdb": { email: string; password: string; }; yetAnother3xamp1eUSERIdSTOR3DinAdb: { email: string; password: string; }; }' but required in type 'Record<activeUserIds, TUSer>'.(2741)
*/
This is not the case with the values though. For example, in the below code, the values include TProjectManager type and there is no such member in the map, and TypeScript acts as usual and does not comlain:
// No error with missing a type in values.
type TUser = {
email: string;
password: string;
};
type TProjectManager = {
phone: string;
email: string;
password: string;
}
type ActiveUserIds = "3xamp1eUSERIdSTOR3DinAdb" | "another3xamp1eUSERIdSTOR3DinAdb" | "yetAnother3xamp1eUSERIdSTOR3DinAdb";
const user: Record<ActiveUserIds, TUser | TProjectManager> = {
"3xamp1eUSERIdSTOR3DinAdb": {
email: "example@example.com",
password: "12345678",
},
"another3xamp1eUSERIdSTOR3DinAdb": {
email: "another_example@example.com",
password: "12345678",
},
"yetAnother3xamp1eUSERIdSTOR3DinAdb": {
email: "yetAnother_example@example.com",
password: "12345678",
},
};
Other Quirks
Among other quirks are the types allowed for keys and values.
Allowed Types for Keys
The types for Keys can only be number, string and symbol. Types other than these throw 2344 error at definition:
type numberedUser = Record<number, TUser>;
type stringUser = Record<string, TUser>;
type symbolUser = Record<symbol, TUser>;
type booleanUser = Record<boolean, TUser>; // Type 'boolean' does not satisfy the constraint 'string | number | symbol'.(2344)
type booleanUser = Record<object, TUser>; // Type 'object' does not satisfy the constraint 'string | number | symbol'.(2344)
Allowed Types for Values
Types for values can be of any type. objects and function types are common. This means, they can also be React components. Below, we see a React example.
Using TypeScript Record<> with React Components
Now, we'll consider a more elaborate usage of the Record<> type using React components.
Let's say our user has permission to a few types of accounts, namely: ProjectManager, Recruiter, Employer. Each of these memberships has a dashboard page and we want to render a preview of each dashboard page somewhere inside a main dash page.
We can type the permissions and then define a Record<> type with values as JSX.Element like below:
type TPermissions = "ProjectManager" | "Recruiter" | "Employer";
type TDashBoardPreview = Record<TPermissions, JSX.Element>;
const dashboardPreviews: TDashboardPreview = {
"ProjectManager": <DashboardPreview type="ProjectManager" size="thumbnail" />,
"Recruiter": <DashboardPreview type="Recruiter" size="thumbnail" />,
"Employer": <DashboardPreview type="Employer" size="thumbnail" />,
};
We can then use the map inside main dash page.
Summary
In this post we explored how to use the Record<> type in TypeScript to construct stable types that are error-prone and more maintainable. We saw how the derived type is a hash map based on a type that represents the actual shape of the data. It also accepts and assigns types to member keys of the map, which can be restricted by using a union type. We have seen an example of using Record<> to type users data for an API endpoint as well as one example that uses Record<> type for rendering React components.
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