Is it advisable to prevent Set and Map from having unspecified generics in TypeScript?

Question

Is it advisable to prevent Set and Map from having unspecified generics in TypeScript?

Upon reviewing some old code that I wrote, I realized that I had neglected to specify a generic type for a Set. The type was set as Set<unknown>. Strangely, despite this, I found that I could still utilize all the methods of the Set without encountering any errors. This stands in contrast to untyped arrays, which would result in errors. For example:

const set = new Set();
set.has(1);

const map = new Map();
map.has(1);

const arr = []; // Variable 'arr' implicitly has type 'any[]' in some locations where its type cannot be determined.
arr.includes(1); // Variable 'arr' implicitly has an 'any[]' type.

I am now wondering how I can modify the behavior of Set and Map to also throw errors when their types are unknown.

javascript typescript

Answer 1

Answer №1

In TypeScript, one way to restrict calling new Map() and new Set() without arguments is by merging your own declarations to produce unusable values like Map<never, never> and Set<never>:

// declare global { // if in module scope

interface SetConstructor {
  new(): Set<never>
}

interface MapConstructor {
  new(): Map<never, never>;
}

// } // if in module scope

The never type in TypeScript signifies that no valid value can be assigned. This will result in errors if you attempt to use such Maps and Sets:

const map = new Map();
map.has(1); // error!
// ---> ~
// Argument of type 'number' is not assignable to parameter of type 'never'.

const set = new Set();
set.has(1); // error! 
// ---> ~
// Argument of type 'number' is not assignable to parameter of type 'never'.

Check out this Playground link for the code

Answer 2

In TypeScript, one way to restrict calling new Map() and new Set() without arguments is by merging your own declarations to produce unusable values like Map<never, never> and Set<never>:

// declare global { // if in module scope

interface SetConstructor {
  new(): Set<never>
}

interface MapConstructor {
  new(): Map<never, never>;
}

// } // if in module scope

The never type in TypeScript signifies that no valid value can be assigned. This will result in errors if you attempt to use such Maps and Sets:

const map = new Map();
map.has(1); // error!
// ---> ~
// Argument of type 'number' is not assignable to parameter of type 'never'.

const set = new Set();
set.has(1); // error! 
// ---> ~
// Argument of type 'number' is not assignable to parameter of type 'never'.

Check out this Playground link for the code

Is it advisable to prevent Set and Map from having unspecified generics in TypeScript?

Answer №1

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