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Prefer precise string types

Prefer precise string types

April 2, 2022

Moch Lutfi
Name
Moch Lutfi
Twitter
@kaptenupi

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:

pg_stat_statements in actionGolang SQL Database