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Custom Extensions

Extensions are a versatile formly feature that allows you to implement cross-cutting functionality that can be applied to all of your fields.

Internally, a lot of formly's logic is built on extensions, so understanding them can greatly increase the things you can do with formly. If you're looking for more resources and examples, check out these links:

Creating a Custom Extension

Creating a custom extension is easy. The following example creates an extension that defines a default label if the FormlyFieldConfig doesn't define one itself.

See live demo: demo

1. Define the custom extension:

There are two options when writing an extension:

  • Create an object of type FormlyExtension.

  • Create a class implementing the interface FormlyExtension. Do this if your extension comes with any dependencies.

    For this simple example, we will choose the first option.

    // default-label-extension.ts
    import { FormlyExtension } from '@ngx-formly/core';

    export const defaultLabelExtension: FormlyExtension = {
    prePopulate(field): void {
    if (field.props?.label) {
    field.props = {
    label: 'Default Label'

FormlyExtension allows you to define up to three methods that will be called in this order during the form construction process:

  1. prePopulate
  2. onPopulate
  3. postPopulate

2. Register the custom extension in NgModule declaration:

// app.module.ts
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { ReactiveFormsModule } from '@angular/forms';

import { FormlyModule } from '@ngx-formly/core';
import { FormlyBootstrapModule } from '@ngx-formly/bootstrap';

import { AppComponent } from './app.component';
import { defaultLabelExtension } from './default-label-extension';

imports: [
extensions: [
name: 'default-label',
extension: defaultLabelExtension
declarations: [AppComponent],
bootstrap: [AppComponent]
export class AppModule {}

extensions: [ ... ] is where we define which extensions should be active.

3. Create a custom FormlyFieldConfig and see the magic happen:

fields: FormlyFieldConfig[] = [
key: 'input',
type: 'input',
props: {
label: 'Basic Input'
key: 'input2',
type: 'input'

Look at the demo to see that the default label has been automatically added to the field.

Extension priority

When registering an extension with the FormlyModule, you can provide an additional priority property of type number. This will be used to change the order in which extensions are executed. If you have multiple extensions that change the same properties of a FormlyFieldConfig, you can use this to ensure they are executed in the correct order. Extensions with higher priority values will be executed later.

If you don't provide a priority option, a default value of 1 will be used.
Formly's own internal extensions have a priority of <= -1.

extensions: [
name: 'default-label',
extension: defaultLabelExtension,
priority: 3