How the TypeScript Record Type Works

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TypeScript Records are a great way to ensure consistency when trying to implement more complex types of data. They enforce key values, and allow you to create custom interfaces for the values. The TypeScript Record type was implemented in TypeScript 2.1, and takes the form Record<K, T>. Let's take a look at how it works.

Utility Types

A Record is a utility type - that means it is a type especially defined by TypeScript to help with a certain problem. You can learn more about utility types here

How Typescript Record Types Work

Suppose you have a data set which looks like the one below.


const myData = {
    "123-123-123" : { firstName: "John", lastName: "Doe" },
    "124-124-124" : { firstName: "Sarah", lastName: "Doe" },
    "125-125-125" : { firstName: "Jane", lastName: "Smith" }
}

Our data set has an ID for its key, which is of type string. All of the values have the same format or type - that is, each has two properties: firstName and lastName.

For this data structure, a Record is the best utility type to use to define it's specific type. The Record type allows us to define a type for both the key (our ID), and the value (our firstName/lastName object). We can define our Record type as follows:


type User = {
    firstName: string,
    lastName: string
}

const myData:Record<string, User> = {
    "123-123-123" : { firstName: "John", lastName: "Doe" },
    "124-124-124" : { firstName: "Sarah", lastName: "Doe" },
    "125-125-125" : { firstName: "Jane", lastName: "Smith" }
}

Any Record in TypeScript takes the form Record<K, T>, where K is the type of the key, and T is the type of the values. Above, we have defined a new type User for our values, and set our keys to type string.

Record Types and Union Types

Sometimes, we can have an object with a predefined set of possible keys. This is particularly true when calling from an API. For example:


const myData = {
    "uk" : { firstName: "John", lastName: "Doe" },
    "france" : { firstName: "Sarah", lastName: "Doe" },
    "india" : { firstName: "Jane", lastName: "Smith" }
}

Let's presume that for our data set above, the key can only be three values: uk, france or india. In this case, we can define a type for User, and a union type for our key:


type User = {
    firstName: string,
    lastName: string
}
type Country = "uk" | "france" | "india";

const myData:Record<Country, User> = {
    "uk" : { firstName: "John", lastName: "Doe" },
    "france" : { firstName: "Sarah", lastName: "Doe" },
    "india" : { firstName: "Jane", lastName: "Smith" }
}

Using this method, we can enforce strict rules about the values the key is allowed to be, along with the type our values should conform to. Since this data structure is so common, the Record utility type is one of the most commonly used utility types in TypeScript.

Conclusion

The Record type is a powerful tool and one of the most useful utility types available in TypeScript. Understanding how it works is important in understanding TypeScript. For more useful content, check the links below:

TypeScript is full of other useful utility types like Record. Read about them here.
New to TypeScript? Check out Google's TypeScript Style Guidelines.

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