Suppose there is a function which return the following object.

 private static prepareExperienceFilter(experienceLevel: ExperienceFilterType): any {
    const aggregationObject = {
        'lt_1': { $lt: 1 },
        'between_1_3': { $gt: 0, $lt: 3 },
        'between_3_5': { $gt: 2, $lte: 5 },
        'gt_5': { $gt: 5 }
    };

    const condition = aggregationObject[experienceLevel];

    return { 'yearsOfExperience.min': condition };
}

How we can define a type or interface for such object?

I've tried union types, but not succeeded.

Thanks for any help.

CodePudding user response：

By moving the static object outside your method, you can reference its type in the function's return type. This will allow you to use a generic type parameter to constrain the function parameter (while also providing IntelliSense inference to the developer using it) and index the return type:

TS Playground

const aggregationObject = {
  'lt_1': { $lt: 1 },
  'between_1_3': { $gt: 0, $lt: 3 },
  'between_3_5': { $gt: 2, $lte: 5 },
  'gt_5': { $gt: 5 },
};

type ExperienceFilterType = keyof typeof aggregationObject;

function prepareExperienceFilter <T extends ExperienceFilterType>(experienceLevel: T): {
  'yearsOfExperience.min': typeof aggregationObject[T];
} {
  const condition = aggregationObject[experienceLevel];
  return { 'yearsOfExperience.min': condition };
}

const result_lt_1 = prepareExperienceFilter('lt_1'); // { 'yearsOfExperience.min': { $lt: number; }; }

const result_gt_5 = prepareExperienceFilter('gt_5'); // { 'yearsOfExperience.min': { $gt: number; }; }

const result_invalid = prepareExperienceFilter('another_key'); /*
                                               ~~~~~~~~~~~~~
Argument of type '"another_key"' is not assignable to parameter of type '"lt_1" | "between_1_3" | "between_3_5" | "gt_5"'.(2345) */