Issues with TypeScript arise when transferring arguments between functions

Question

Issues with TypeScript arise when transferring arguments between functions

Encountering a TypeScript error due to this pattern:

Error message: 'Argument of type '(string | number)[]' is not assignable to parameter of type 'string[] | number[]'

function foo(value: string | number) {
  return bar([value]); // <- TypeScript error
}

function bar(valueList: string[] | number[]) {
  ..does something...
}

It seems like the issue arises from mixing strings and numbers in an array.

Is there a more type-safe approach to solve this problem? Currently considering casting to any[], but unsure if it's the best course of action:

function foo(value: string | number) {
  const valueList: any[] = [value];
  return bar(valueList);
}

javascript typescript

Answer 1

Answer №1

If you want to achieve this, one approach is through the concept of declaration merging. You can define two distinct interfaces for your function (refer to function types) and then create a single implementation:

interface Foo {
    (value: string): void;
}

interface Foo {
    (value: number): void;
}

const foo: Foo = function (value) {
  return bar([value]);
}

By doing this, it ensures that the two types remain separate, allowing only one interface to be invoked at any given moment.

Answer 2

If you want to achieve this, one approach is through the concept of declaration merging. You can define two distinct interfaces for your function (refer to function types) and then create a single implementation:

interface Foo {
    (value: string): void;
}

interface Foo {
    (value: number): void;
}

const foo: Foo = function (value) {
  return bar([value]);
}

By doing this, it ensures that the two types remain separate, allowing only one interface to be invoked at any given moment.

Answer 3

Answer №2

After drawing inspiration from jonrsharpe's response, I decided to take a different approach by utilizing function overloading.

By implementing function overloads, we can define multiple signatures for a single implementation: https://www.typescriptlang.org/docs/handbook/functions.html#overloads

function foo(value: string);
function foo(value: number);
function foo(value: any) {
    return bar([value]);
}

function bar(valueList: number[] | string[]) {
}

Answer 4

After drawing inspiration from jonrsharpe's response, I decided to take a different approach by utilizing function overloading.

By implementing function overloads, we can define multiple signatures for a single implementation: https://www.typescriptlang.org/docs/handbook/functions.html#overloads

function foo(value: string);
function foo(value: number);
function foo(value: any) {
    return bar([value]);
}

function bar(valueList: number[] | string[]) {
}

Answer 5

Answer №3

While the previous response is commendable (I've given it a thumbs up), I personally lean towards

return bar([value] as string[] | number[]);

especially if time is of the essence. This doesn't imply that it's incorrect or overly clever; you're simply nudging TypeScript in a particular direction.

Side note: @paleo rightly points out that this approach isn't entirely foolproof in terms of type safety.

Answer 6

While the previous response is commendable (I've given it a thumbs up), I personally lean towards

return bar([value] as string[] | number[]);

especially if time is of the essence. This doesn't imply that it's incorrect or overly clever; you're simply nudging TypeScript in a particular direction.

Side note: @paleo rightly points out that this approach isn't entirely foolproof in terms of type safety.

Issues with TypeScript arise when transferring arguments between functions

Answer №1

Answer №2

Answer №3

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