The 'validate' property within the 'MappingService' class cannot be assigned to the 'validate' property in the base class 'IMappingService' in typescript version 2.8.0

Currently, I am utilizing the AngularJS framework (version 1.5.8) in tandem with the latest TypeScript files (2.8.0). However, upon updating to the newest version of TypeScript, the code below is failing to compile.

The IMappingService interface:

export interface IMappingService {

/**
 * This interface deals with validating the mapping of a T object from one type to another and returning the new mapped object
 * @param obj The object to validate
 * @param type The type of the object
 * @returns {T} The newly validated and mapped object 
 */
validate<T>(obj: any, type: string, fields: string): T;    
}

The implementation of the interface:

export default class MappingService implements IMappingService {

    public validate<T>(obj: any, type: string, fields: string): T | T[] {
    let parsedFields = null;

    const replacedFields = fields.replace(/'/g, '"');
    parsedFields = JSON.parse(replacedFields);

    let tempobject = obj;

    if (obj instanceof Array) {
        tempobject = obj[0];
        if (!tempobject) {
            return [];
        }
    }
    ....
    return obj;
}

An error message that I am encountering states:

 Property 'validate' in type 'MappingService' is not assignable to the same property in base type 'IMappingService'.
Type '<T>(obj: any, type: string, fields: string) => T | T[]' is not assignable to type '<T>(obj: any, type: string, fields: string) => T'

Answer №1

The code is not functioning in TypeScript 2.7 either. It appears that this code may have worked in a previous version of Typescript, but it is not considered type-safe. Here's an alternative approach to consider:

let m : IMappingService = new MappingService();
let r = m.validate<{ name: string }>({}, "", "");
r.name // Although the return type could be T or T[], I can still access the 'name' property

To address this issue, you have two options - add support for arrays through an overload, or align the interface with the implementation signature:

export default class MappingService implements IMappingService {

    public validate<T>(obj: any[], type: string, fields: string): T[]
    public validate<T>(obj: any, type: string, fields: string): T
    // Implementation signature
    public validate<T>(obj: any, type: string, fields: string): T | T[] { ... }
}

let m = new MappingService();
let r = m.validate<{ name: string }>({}, "", "");  
var rr = m.validate<{ name: string }>([], "", ""); 
// 'r' will be a single item, while 'rr' will be an array

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