[ngx-formly] Customize Angular Formly validation messages

When you display error messages you want to make them as understandable as possible such that the user is able to figure out the problem. Example: the error message of a min validator should ideally contain the actual number and the min number allowed. So the message could be like "You have to enter a number bigger than X. You entered Y". Let's see how Formly allows us to display such highly customizable messages.

// app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { ReactiveFormsModule } from '@angular/forms';
import { FormlyModule, FormlyFieldConfig } from '@ngx-formly/core';
import { FormlyMaterialModule } from '@ngx-formly/material';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

import { SharedModule } from './shared/shared.module';
import { DebugComponent } from './shared/debug.component';

// global min error message, you can override by validation.messages.min in field
export function minValidationMessage(err, field: FormlyFieldConfig) {
  return `Please provide a value bigger than ${err.min}. You provided ${err.actual}`;
}

@NgModule({
  declarations: [AppComponent],
  imports: [
    BrowserModule,
    SharedModule,
    ReactiveFormsModule,
    FormlyModule.forRoot({
      validationMessages: [
        {
          name: 'required',
          message: 'This field is required',
        },
        {
          name: 'min',
          message: minValidationMessage,
        },
      ],
    }),
    FormlyMaterialModule,
    BrowserAnimationsModule,
  ],
  providers: [],
  bootstrap: [AppComponent],
})
export class AppModule {}

    {
      key: 'age',
      type: 'input',
      templateOptions: {
        type: 'number',
        label: 'Age',
        min: 18, // use global min error message
      },
      // override the global min error message
      validation: {
        messages: {
          min: 'Sorry, you have to be older than 18',
        },
      },
    },