The absence of an index signature is causing a validation issue in AngularJS + TypeScript when using $validators

Currently, I am in the process of developing a directive using AngularJS 1.5 and TypeScript 1.7 to enable custom form validation.

I came across this example but encountered an issue with Typescript displaying a 'Type signature is missing in type' error when extending the ng.INgModelController.

interface IOneItemRequiredValidator extends ng.INgModelController {
    $validators: {
        oneItemRequired(modelValue: any, viewValue: any) : boolean;
    };
}

Upon reviewing $validators, I observed that it is defined as type IModelValidators, with the following signature:

interface IModelValidators {
    [index: string]: (modelValue: any, viewValue: any) => boolean;
}

To address this, I attempted to align my approach with the existing structure. While TypeScript no longer throws errors, I face difficulty accessing the new property within the $validators object.

interface IOneItemRequiredValidator extends ng.INgModelController {
    $validators: {
        [oneItemRequired: string]:  (modelValue: any, viewValue: any) => boolean;
    };
}


angular
    .module('app')
    .directive('oneItemRequired', () => {
        return {
            restrict: 'A',
            require: 'ngModel',
            link: (scope: ng.IScope, element: ng.IAugmentedJQuery, attributes: ng.IAttributes, controller: IOneItemRequiredValidator) => {
                controller.$validators.oneItemRequired = (modelValue, viewValue) => {
                        return viewValue !== null && viewValue !== undefined && viewValue.length > 0;
                    }
                }
            };
        });

https://i.sstatic.net/LhGXo.png

Could there be an issue with how I have declared the new IOneItemRequiredValidator interface? Any insights would be greatly appreciated. Thanks!

Answer №1

When working with TypeScript and Angular 1, the naming of properties in interfaces doesn't have a significant impact. Essentially, you are indicating that any additional property not explicitly named in the interface is accepted.

The responsibility lies with the Angular declaration file to include this aspect within the IModelValidators declaration. Refer to this resource for more information.

The issue with the previous approach arises from the compatibility mismatch between the object type containing a single function oneItemRequired and the IModelValidators type.

To address this, there are two potential solutions:

(1) Reintroduce the indexer as follows:

interface IOneItemRequiredValidator extends INgModelController {
    $validators: {
        oneItemRequired(modelValue : any, viewValue : any) : boolean;
        [otherProps: string]:  (modelValue: any, viewValue: any) => boolean;
    };
}

Explore a non-Angular demonstration

The first option minimizes code but may be seen as a temporary workaround, while the second offers a more structured solution. The choice is yours!

If you encounter any issues or need further assistance, feel free to reach out.

Best regards,

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