How come TypeScript doesn't retain the positions and types of array elements in memory?

I am currently working on creating an array of objects that consist of questions, associated actions to perform (functions), and arguments to supply to the functions. I am facing issues with TypeScript not recognizing the types and arguments, and I would like to learn how to tackle this problem effectively.

Although my code is functioning as expected and returning the desired results, TypeScript seems to be unhappy...

playground

type Digit = 0|1|2|3|4|5|6|7|8|9;
const Color = {
  red: 0,
  black: 1,
} as const;
type Color = typeof Color[keyof typeof Color];
type Code = {
        value: Digit,
        color: Color
}
function countOfEven(codes: Code[]): number {
  return codes.filter((code) => code.value % 2 === 0).length;
}

 function countOfColor(codes: Code[], color: Color): number {
  return codes.filter((code) => code.color === color).length;
}

let code: Code [] = [
        {
        value: 1,
        color: Color.black
        },
        {
        value: 2,
        color: Color.red      
        }
]

const questions = [
  {
    body: "How many even numbers?",
    action: countOfEven,
  },
  {
    body: "How many red numbers?",
    action: countOfColor,
    args: Color.red
  }]
  
let question1 = questions.at(0)
let question2 = questions.at(1);

console.log(question1?.body, question1?.action(code)) // error!
//                                     ~~~~~~
// Expected 2 arguments, but got 1.

console.log(question2?.body, question2?.action(code, question2.args)) // error!
//                                                 ~~~~~~~~~~~~~~
// Argument of type '0 | undefined' is not assignable to parameter of type 'Color'.
//  Type 'undefined' is not assignable to type 'Color'.

Answer №1

You are facing two main issues here: TypeScript's inability to determine the position of each action within the questions array, and TypeScript's struggle to define the return type of its `at()` method.


Starting with the first problem, let's examine the structure of the questions array:

const questions = [
  {
    body: "How many even numbers?",
    action: countOfEven,
  },
  {
    body: "How many red numbers?",
    action: countOfColor,
    args: Color.red
  }]

/* const questions: (
    { body: string; action: (codes: Code[]) => number;  args?: undefined; } | 
    { body: string; action: (codes: Code[], color: Color) => number; args: 0; }
  )[] */

The current form is in the format ({⋯} | {⋯})[]. This represents an array type which is unordered and can have any length, containing elements of a union type. TypeScript lacks the knowledge of whether position 0 contains a (codes: Code[]) => number or if position 1 holds a

(codes: Code[], color: Color) => number
.

To address this issue and enable TypeScript to accurately track the position and types of elements, you should consider utilizing a tuple type. An easy way to achieve this inference is by employing a const assertion:

const questions = [{ ⋯✂⋯ }] as const;

/* const questions: readonly [{
    readonly body: "How many even numbers?";
    readonly action: (codes: Code[]) => number;
}, {
    readonly body: "How many red numbers?";
    readonly action: (codes: Code[], color: Color) => number;
    readonly args: 0;
}] */

This approach may seem overly specific but it provides TypeScript with clarity on the element positions.


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