Type 'string' is not assignable to type 'undefined' with `suppressImplicitAnyInd-CodePudding

Recently I toggle TSConfig property suppressImplicitAnyIndexErrors from true to false to force me to write better code, but I encounter this issue that is the only one I wasn't able to fix.

It's a simple form with a checkbox and a text input that will add a true to filters in case that the checkbox is checked or delete the property if it existed in case it is false. The search string is also added if there is any text, if not is deleted as with the checkbox property.

interface UserFilters {
    searchText?: string;
    hasAction?: boolean;
}

const filters = {} as UserFilters;

function filterUsers(filterName: keyof UserFilters, value: string | boolean | undefined) {
    if (value) filters[filterName] = value; // ERROR: Type 'string' is not assignable to type 'undefined'
    else delete filters[filterName];
}

I did this TypeScript playground to show you the error.

CodePudding user response：

The problem with filterUsers is that filterName and value are not related in any way, so for example filterUsers('searchText', true) would be a valid call. That's also why you can't make the types work in the body.

You can fix it by making the function generic:

function filterUsers<K extends keyof UserFilters>(
  filterName: K,
  value: UserFilters[K],
) {
  if (value) filters[filterName] = value
  else delete filters[filterName]
}

TypeScript playground