What return type should be used when returning `_.orderBy` from a TypeScript function?

The current packages I have installed are lodash and @types/lodash.

In my code, I am using:

import _ from 'lodash';

function doSomething(): string[] {
  const letters = ['c', 'a', 'b'];

  return _.orderBy(letters, [null], ['asc']);
}

console.log(doSomething());

However, the line

return _.orderBy(letters, [null], ['asc']);
is causing an error:

Type '(string | number | (<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]) | (() => string) | (() => string) | (() => string | undefined) | ((...items: string[]) => number) | ... 25 more ... | { ...; })[]' is not assignable to type 'string[]'.
  Type 'string | number | (<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]) | (() => string) | (() => string) | (() => string | undefined) | ((...items: string[]) => number) | ... 25 more ... | { ...; }' is not assignable to type 'string'.
    Type 'number' is not assignable to type 'string'.

Even though it seems like an array of strings should be returned.

Is there a solution to this issue other than manually casting it to

return _.orderBy(letters, [null], ['asc']) as string[];
? Can we specify the type for the orderBy function based on the letters variable?

Answer №1

The issue you're facing is due to the fact that the value [null] being passed to the iteratees parameter (the second one) doesn't match the function's signature.

According to the documentation, the iteratees parameter should be of type

(Array[]|Function[]|Object[]|string[])
.

To resolve this problem, replace the second argument with [_.identity], the default value, or set the second argument to undefined, which will also use the default value. For more information on Default Parameters, refer to the MDN Docs on Default Parameters.

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