A central solution to error handling in Angular Reactive Forms

error.handler.ts

import {AbstractControl, FormArray, FormControl, FormGroup, ValidationErrors} from '@angular/forms';
import {Injectable} from '@angular/core';
import {debounceTime, distinctUntilChanged} from 'rxjs/operators';


export declare interface ServerError {
    [key: string]: [];
}


@Injectable({
    providedIn: 'root'
})
export class ErrorHandler {
    private form: FormGroup;
    private serverError: any;
    private errorObject: ServerError;
    private message: string;


    private static hasError(control: AbstractControl): boolean {
        return control.invalid && (control.dirty || control.touched);
    }


    /**
     * Takes server error obj and set errors to appropriate fields at form given.
     *
     * @param serverError       Error object that is received from the server
     * @param form              Form to which errors belong to.
     */
    public organizeServerErrors(serverError: ServerError, form: FormGroup) {
        if (serverError && typeof serverError === 'object') {
            this.form = form;
            this.serverError = serverError;
            this.setErrorToFormFields();
            this.findErrors(form.controls);
        }
    }

    /**
     * Listen's for invalid status of the form given and find's it's errors.
     *
     * @param form              Form to be listened
     * @param errorObject       Error object which to set errors.
     */
    public handleErrors(form: FormGroup, errorObject: any) {
        this.form = form;
        this.errorObject = errorObject;
        form.valueChanges.pipe(
            debounceTime(500),
            distinctUntilChanged(),
        ).subscribe(() => {
            if (form.invalid) {
                this.findErrors(form.controls);
            }
        });
    }

    /** Used to call when an invalid form has been submit
     *  By setting error we emit a status changed event for the form.
     */
    public invalidFormHasSubmit() {
        // fixme: changed the listener to value changes so setErrors() wouldn't trigger it
        // fixme: look up for another method to trigger value changes
        this.form?.markAllAsTouched();
        this.form?.setErrors({});
    }

    /** Finds appropriate fields on form and set's the server error. */
    private setErrorToFormFields() {
        Object.keys(this.serverError).forEach(field => {
            if (!Array.isArray(this.serverError[field])) {
                this.setErrorsToNestedFields(field);
            } else {
                const errorMessages: any[] = this.serverError[field];
                this.form.get(field)?.setErrors({serverError: errorMessages[0].message});
            }
        });
    }

    /**
     * Takes nested field name and set's server error to appropriate nested field.
     * @param nestedFieldName       Usually formControlName of a FormArray.
     */
    private setErrorsToNestedFields(nestedFieldName: string) {
        const nestedFormErrors = this.serverError[nestedFieldName];
        Object.keys(nestedFormErrors).forEach(field => {
            const formControl = this.form.get(field);
            if (formControl) {
                formControl.setErrors({serverError: nestedFormErrors[field]});
            } else {
                const nestedForm = this.form.get(nestedFieldName);
                const nestedField = nestedForm.get(field);
                if (nestedField) {
                    nestedField.setErrors({serverError: nestedFormErrors[field]});
                }
            }
        });
    }

    /**
     * Find which control contains the error and set required { control -> error message } combination
     * into the errorObject given previously.
     *
     * @param controls      Abstract Controls of the form which contains errors.
     */
    private findErrors(controls: { [key: string]: AbstractControl }) {
        Object.keys(controls).forEach((control: string) => {
            if (controls[control] instanceof FormArray) {
                Object.defineProperty(this.errorObject, control, {value: [], writable: true});
                this.findErrorsOnFormArrays(controls[control] as FormArray, control);

            } else if (controls[control] instanceof FormControl) {
                this.findErrorsOnFormControls(controls, control);
            }
        });
    }

    private findErrorsOnFormArrays(formArray: FormArray, formArrayName: string) {
        let i = 0;
        for (const formGroup of formArray.controls as FormGroup[]) {
            const controls = formGroup.controls;
            const formArrayErrors: any[] = this.errorObject[formArrayName];
            formArrayErrors.push({});
            Object.keys(controls).forEach(control => {
                if (ErrorHandler.hasError(controls[control])) {
                    this.setErrorMessage(controls[control].errors);
                    Object.defineProperty(formArrayErrors[i], control, {value: this.message, writable: true});
                }
            });
            i++;
        }
    }

    private findErrorsOnFormControls(controls: { [key: string]: AbstractControl }, control: string) {
        if (ErrorHandler.hasError(controls[control])) {
            this.setErrorMessage(controls[control].errors);
            this.setErrorToErrorObject(control);
        }
    }

    /**
     * Find's error type and set's a message value for this type.
     *
     * @param errors        Validation Error obj.
     */
    private setErrorMessage(errors: ValidationErrors) {
        if (errors.serverError) {
            this.message = errors.serverError;
        } else if (errors.required) {
            this.message = 'Required field';
        } else if (errors.minlength) {
            this.message = `Max length is ${errors.minlength.actualLength}/${errors.minlength.requiredLength}`;
        } else if (errors.maxlength) {
            this.message = `Min length is ${errors.maxlength.actualLength}/${errors.maxlength.requiredLength}`;
        } else if (errors.email) {
            this.message = 'Email is not valid';
        } else if (errors.min) {
            this.message = `Min value is ${errors.min.min}, actual value is ${errors.min.actual}`;
        } else if (errors.max) {
            this.message = `Max value is ${errors.max.max}, actual value is ${errors.max.actual}`;
        } else if (errors.pattern) {
            this.message = 'Invalid value';
        } else if (errors.passwordMismatch) {
            this.message = 'Passwords do not match';
        } else {
            this.message = '';
        }
    }

    /**
     * Set's a new property to errorObject with key from the field's name and error message as a value.
     * @param field         Field which contains error.
     */
    private setErrorToErrorObject(field: string) {
        Object.defineProperty(this.errorObject, field, {value: this.message, writable: true});
    }
}

To simplify error handler, we could also apply a filter pipe on valuesChanges:

/**
 * Listen's for invalid status of the form given and find's it's errors.
 *
 * @param form              Form to be listened
 * @param errorObject       Error object which to set errors.
 */
public handleErrors(form: FormGroup, errorObject: any) {
  this.form = form;
  this.errorObject = errorObject;
  form.valueChanges
  .pipe(
    debounceTime(400),
    distinctUntilChanged(),
    filter(() => this.form.invalid),
  )
  .subscribe(() => this._handleErrors());
}

And then simplify the _handleErrors method presented by @d0lb33 :

/**
 * Performs necessary steps for finding errors.
 */
private _handleErrors() {
  this.clearErrors();
  this.findErrors(this.form.controls);
}

But I could be wrong, since I'm a new developer, but it works fine for me.

Also, the setErrorMessage could be more generic by selecting the first (and default) message from the validation error. Quite useful when you wrote custom error messages or, on fallback, display the default validator message:

/**
 * Find's error type and set's a message value for this type.
 *
 * @param errors        Validation Error obj.
 */
private setErrorMessage(errors: ValidationErrors) {
  ...
  } else {
    this.message = Object.values(errors)[0].message || 'Invalid field';
  }
}

Thoughts

Should we not unsubscribe from the valueChanges subscription in order to save memory, or it's unnecessary ? I was thinking about adding a private subscription property and a public unsubscribe method. This way, on our component, we could simply call it in our ngOnDestroy.

Thanks for your work, really helpful ! ❤️