Handling variable number of "type parameters" in TypeScript

Question

The goal is to create a completely type safe generic class such as a Table Class below which would allow to create a Table instance whose field types are given as type parameters (or any other possible means).

let userTable = new Table<number, string, string>(
  columnNames: ["id", "name", "address"],
  fields: [
    [0, "John", "225 Main"],
    [1, "Sam", "330 E Park"]
  ]);
let billsTable = new Table<number, Date, Date>(
  columnNames: ["custId", "invoiceDate", "dueDate", "balance"],
  fields: [ [1, new Date(), new Date()] ]);

The question is, with full type safety in focus, how would you define or implement such a generic type structure which could have unknown number of type parameters?

Still in discussion: github.com/Microsoft/TypeScript/issues/5453 — unional
– unional, Commented Feb 6, 2019 at 0:47

Karol Majewski · Accepted Answer · 2019-02-06 01:53:35Z

2

You can use tuples as your type parameters:

class Table<T extends string, U extends any[]> {
  constructor(columnNames: T[], fields: U[]) {
    /* Do something */
  }
}

If you provide type parameters explicitly, your arguments will be type-checked against them.

new Table<'custId' | 'invoiceDate', [string, number, Date]>(
  ['custId', 'invoiceDate'],
  [
    ['foo', 1, new Date()],
    ['bar', 2, new Date()],
  ]
)

Works with named parameters as well:

class Table<T extends string, U extends any[]> {
  constructor(configuration: { columnNames: T[], fields: U[]}) {
    /* Do something */
  }
}

new Table<'custId' | 'invoiceDate', [string, number, Date]>({
  columnNames: ['custId', 'invoiceDate'],
  fields:[
    ['foo', 1, new Date()],
    ['bar', 2, new Date()],
  ]
})

edited Feb 6, 2019 at 1:53

answered Feb 6, 2019 at 0:56

Karol Majewski

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2 Comments

user3330840 Over a year ago

This gives compilation errors such as: "An index signature parameter type must be 'string' or 'number' ". Also, try to add other methods or data members to the class and it complains!

Karol Majewski Over a year ago

That's because I left a broken index signature there. I wanted to see if keys can be established at the call site, but was reminded they cannot — A class can only implement an object type or intersection of object types with statically known members..

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