Prefer precise string types

String type is big and giving variable a string type will give you a lot of possibilities input values. But with the great power comes great responsibility. Suppose you're building a book collection and want to define a type for a book. Here's an attempt to define a book type:

interface Book {
  title: string;
  writer: string;
  publishedDate: string; // YYYY-MM-DD
  bookType: string; // hardcover, paperback, ebook
}

That interface seems right but actually not. Here's what can go wrong:

const book: Book = {
  title: 'Chemistry 12',
  writer: 'Marthen Kanginan',
  publishedDate: 'May 4th, 1945', // ooops, wrong format
  bookType: 'Hardcover' // ooops again, wrong type
};

The publishedDate and bookType can be narrowed down to a more precise type. For publishedDate field it's better to use a Date object and avoid formatting issue. Then, for the bookType field, you can define a union type with just expected values. Here's the same interface with more precise types:

type BookType = 'hardcover' | 'paperback' | 'ebook';
interface Book {
  title: string;
  writer: string;
  publishedDate: Date;
  bookType: BookType;
}

With these changes Typescript is able to do a more thorough validation of the data.

const book: Book = {
  title: 'Chemistry 12',
  writer: 'Marthen Kanginan',
  publishedDate: new Date('1945-05-04'),
  bookType: 'Hardcover'
  // type Hardcover is not assignable to type BookType
};

Another example misuse of string is in function parameters. Let's say you want to create simple find by bookType function. Instead using string type, you can use a union type with just expected values.

If create function with parameters that expected to be properties on an object we can use keyOf T to narrow down the type of the parameter. For example pluck function with generic.

function pluck<T, K extends keyof T>(records: T[], key: K): T[K][] {
  return record.map(r => r[key]);
}

Wrap up

Avoid "stringly typed" code. Prefer more precise types where not every value is possible.
Use union types to define more precise types.
Prefer keyOf T to string for function parameters that are expected to be properties of an object.

Reference:

https://effectivetypescript.com/