You don't need TypeScript's index types (probably)

Originally posted on JosNun.com

TypeScript's index types seem super useful when you first find out about them. If you aren't familiar with index types, here's a crash course example:

interface StringArray {
  [key: number]: string;
}

const arr: StringArray = {
  0: 0,
  1: 'one',
  2: 'two',
  [Math.PI]: Math.PI,
  // This isn't valid
  'three': 'three'
};

As you can see, StringArray in our example allows any number to be a property name (key), and any string to be a value.

Index types are really handy when you have an object that could have unknown keys. They're also handy when using an object as a dictionary or associative array. They do have some downsides, though. You can't specify what keys can be used, and the syntax is also a bit verbose, in my opinion. TypeScript provides a solution, however; the Record utility.

In it's simplest form, Record is very similar to an index type. Let's give it a look:

type StringArray = Record<number, string>;

const arr: StringArray = {
  0: 0,
  1: 'one',
  2: 'two',
  [Math.PI]: Math.PI,
  // This isn't valid
  'three': 'three'
};

Like with the index type, arr can have any number as a key, and any string as a value. For a simple example like this, Record merely does some cleanup. There are some things that Record can do, that index types can't very easily, however. One such example is specifying which keys are allowed with a union type.

// Ain't gonna work
interface TypedKeys {
    // An index signature parameter type must be 'string' or 'number'
    [key: 0 | 1]: string;
}

// Works, but is verbose
interface TypedKeys {
    0: string;
    1: string;
}

// Works, and is nice and concise
type TypedKeys = Record<0 | 1, string>;

const arr: TypedKeys = {
  0: '0',
  1: 'one',
  // Object literal may only specify known properties, and '2' does not exist in type 'Record<0 | 1, string>'
  2: 'two',
};

You could also use a more advanced type for the value of a Record as well. At first glance, there may be some downsides. It isn't immediately obvious how you could have some predefined keys, like with an index type. Nevertheless, it's still doable, though maybe not as elegant as with an index type.

interface TypedKeys {
    0: 'zero'
    [key: number]: string;
}

// Same as above
type TypedKeys = Record<number, string> & {
    0: 'zero';
};

const arr: TypedKeys = {
  0: '0',
  1: 'one',
  2: 'two',
  3: 'three'
};

So there you have it, that's TypeScript's Record utility! Hopefully you'll find new ways to use it to make your code more readable and concise.

Key takeaways

Maybe a bit of pun intended

• Record is, in it's simplest form, an easier to read version of index types
• It makes specifying the keys and values much easier when you have complex types
• Fewer characters = quicker to type for most people
• Record works well with intersection and union types

Thoughts? Questions? Let me know in the comments!

Also, I've been thinking about doing regular posts on handy TypeScript utilities or features. If you think you'd find that useful or interesting, let me know in the comments as well!