TypeScript: how to declare array of fixed size for type checking at Compile Time

Question

Update: These checks are meant for compile time, not at runtime. In my example, the failed cases are all caught at compile

Answer 1

From Typescript: Can I define an n-length tuple type?, programmatically, with dynamic length:

type Tuple<TItem, TLength extends number> = [TItem, ...TItem[]] & { length: TLength };

type Tuple9<T> = Tuple<T, 9>;

Answer 2

Here is a simple example of a class to control the length of its internal array. It isn't fool-proof (when getting/setting you may want to consider whether you are shallow/deep cloning etc:

https://jsfiddle.net/904d9jhc/

class ControlledArray {

  constructor(num) {
    this.a = Array(num).fill(0); // Creates new array and fills it with zeros
  }

  set(arr) {
    if (!(arr instanceof Array) || arr.length != this.a.length) {
      return false;
    }
    this.a = arr.slice();
    return true;
  }

  get() {
    return this.a.slice();
  }

}

$( document ).ready(function($) {

  var m = new ControlledArray(3);

  alert(m.set('vera')); // fail
  alert(m.set(['vera', 'chuck', 'dave'])); // pass

  alert(m.get()); // gets copy of controlled array

});

Answer 3

Update 2: From version 3.4, what the OP asked for is now fully possible with a succinct syntax (Playground link):

class MyClass {
  tableHead: readonly [string, string, string]
  tableCells: readonly [number, number, number]
}

Update 1: From version 2.7, TypeScript can now distinguish between lists of different sizes.

I don't think it's possible to type-check the length of a tuple. Here's the opinion of TypeScript's author on this subject.

I'd argue that what you're asking for is not necessary. Suppose you define this type

type StringTriplet = [string, string, string]

and define a variable of that type:

const a: StringTriplet = ['a', 'b', 'c']

You can't get more variables out of that triplet e.g.

const [one, two, three, four] = a;

will give an error whereas this doesn't as expected:

const [one, two, three] = a;

The only situation where I think the lack of ability to constrain the length becomes a problem is e.g. when you map over the triplet

const result = a.map(/* some pure function */)

and expect that result have 3 elements when in fact it can have more than 3. However, in this case, you are treating a as a collection instead of a tuple anyway so that's not a correct use case for the tuple syntax.