Use overloaded function with default initializer

Question

Hi I'm trying to overload a function that change return type on parameter

isValidPropertyValue(propertyName: string, value: any,
    options?: { errors: true }): ValidationWithErrors;
  isValidPropertyValue(propertyName: string, value: any,
    options?: { errors: false }): boolean
  isValidPropertyValue(propertyName: string, value: any,
    options = { errors: false }): boolean | ValidationWithErrors {
    if (typeOf(propertyName) !== "string") throw new Error("propertyName must be a string");

    const conditions = this.properties[propertyName];
    if (!conditions) {
      return options.errors ? {
        valid: false,
        errors: [{
          property: propertyName,
          error: `Unauthorized property '${propertyName}'`
        }]
      } : false;
    }

    const errors: PropertyError[] = [];
    for (const [condition, validator] of Object.entries(propertyValidators)) {
      if (conditions[condition]) {
        try {
          validator(conditions[condition], value);
        } catch (e: any) {
          errors.push({ property: propertyName, error: e.message });
        }
      }
    }

    return options.errors ? { valid: !errors.length, errors } : !errors.length;
  }

I have set a default initializer on the option parameter like you can see but when I'm trying to call the function I got an error

TS2554: Expected 3 arguments, but got 2. 

Futhermore it doesn't change my return type and keep it on ValidationWithError even if I set the option to false

Answer 1

I'm not sure how you go the argument error. TS2554: Expected 3 arguments, but got 2. I wasn't able to reproduce that with your code. But if you remove the ? from the first signature , TS will correctly infer the return type. It makes sense because when options is undefined or { errors: false } , you want to return a boolean but you only return ValidationErrors when it's { errors: true } and not when it's undefined.

interface ValidationWithErrors {
  valid: boolean;
  errors: { property: string; error: string }[];
}

function isValidPropertyValue(
  propertyName: string,
  value: any,
  options: { errors: true }
): ValidationWithErrors;
function isValidPropertyValue(
  propertyName: string,
  value: any,
  options?: { errors: false }
): boolean;
function isValidPropertyValue(
  propertyName: string,
  value: any,
  options = { errors: false }
): boolean | ValidationWithErrors {
  if (options.errors) {
    return {
      valid: false,
      errors: [{ property: propertyName, error: 'invalid value' }],
    };
  }
  return false;
}

const isValid = isValidPropertyValue('property', 'value'); // boolean
const validationWithErrors = isValidPropertyValue('property', 'value', {
  errors: true,
}); // ValidationWithErrors