Reading Time: 20 minutesForms are probably the most crucial aspect of your web application. While we often get events from clicking on links or moving the mouse, it’s through forms where we get the majority of our rich data input from users.

On the surface, forms seem straightforward: you make an input tag, the user fills it out, and hits submit. How hard could it be?

It turns out, forms can be very complex. Here’s a few reasons why:

• Form inputs are meant to modify data, both on the page and the server
• Changes often need to be reflected elsewhere on the page
• Users have a lot of leeway in what they enter, so you need to validate values
• The UI needs to clearly state expectations and errors, if any
• Dependent fields can have complex logic
• We want to be able to test our forms, without relying on DOM selectors

Thankfully, Angular has tools to help with all of these things.

• FormControls encapsulate the inputs in our forms and give us objects to work with them
• Validators give us the ability to validate inputs, any way we’d like
• Observers let us watch our form for changes and respond accordingly

In this chapter we’re going to walk through building forms, step by step. We’ll start with some simple forms and build up to more complicated logic.

FormControls and FormGroups

The two fundamental objects in Angular forms are FormControl and FormGroup.

FormControl

A FormControl represents a single input field - it is the smallest unit of an Angular form.

FormControls encapsulate the field’s value, and states such as being valid, dirty (changed), or has errors.

For instance, here’s how we might use a FormControl in TypeScript:

To build up forms we create FormControls (and groups of FormControls) and then attach metadata and logic to them.

Like many things in Angular, we have a class (FormControl, in this case) that we attach to the DOM with an attribute (formControl, in this case). For instance, we might have the following in our form:

This will create a new FormControl object within the context of our form. We’ll talk more about how that works below.

FormGroup

Most forms have more than one field, so we need a way to manage multiple FormControls. If we wanted to check the validity of our form, it’s cumbersome to iterate over an array of FormControls and check each FormControl for validity. FormGroups solve this issue by providing a wrapper interface around a collection of FormControls.

Here’s how you create a FormGroup:

FormGroup and FormControl have a common ancestor (AbstractControl). That means we can check the status or value of personInfo just as easily as a single FormControl:

Notice that when we tried to get the value from the FormGroup we received an object with key-value pairs. This is a really handy way to get the full set of values from our form without having to iterate over each FormControl individually.

Our First Form

There are lots of moving pieces to create a form, and several important ones we haven’t touched on. Let’s jump in to a full example and I’ll explain each piece as we go along.

Here’s a screenshot of the very first form we’re going to build:

In our imaginary application we’re creating an e-commerce-type site where we’re listing products for sale. In this app we need to store the product’s SKU, so let’s create a simple form that takes the SKU as the only input field.

Our form is super simple: we have a single input for sku (with a label) and a submit button.

Let’s turn this form into a Component. If you recall, there are three parts to defining a component:

• Configure the @Component() decorator
• Create the template
• Implement custom functionality in the component definition class

Let’s take these in turn:

Loading the FormsModule

In order to use the new forms library we need to first make sure we import the forms library in our NgModule.

There are two ways of using forms in Angular and we’ll talk about them both in this chapter: using FormsModule or using ReactiveFormsModule. Since we’ll use both, we’ll import them both into our module. To do this, we do the following in our app.ts where we bootstrap the app:

This ensures that we’re able to use the form directives in our views. At the risk of jumping ahead, the FormsModule gives us template driven directives such as:

• ngModel and
• NgForm

Whereas ReactiveFormsModule gives us reactive driven directives like

• formControl and
• ngFormGroup

… and several more. We haven’t talked about how to use these directives or what they do, but we will shortly. For now, just know that by importing FormsModule and ReactiveFormsModule into our NgModule means we can use any of the directives in that list in our view template or inject any of their respective providers into our components.

Reactive- vs. template-driven Forms

Angular allows you to define forms in two different ways: “reactive” or “template” driven. You can see a comparison of two ways here. Rather than describe how they’re different, we’re going to show you examples of different ways you can build forms - then you can decide which is right for your application.

Simple SKU Form: @Component Decorator

First, let’s start by creating what’s called a “template driven” form. Starting with our component:

Here we define a selector of app-demo-form-sku. If you recall, selector tells Angular what elements this component will bind to. In this case we can use this component by having a app-demo-form-sku tag like so:

Simple SKU Form: template

Let’s look at our template:

form & NgForm

Now things get interesting: because we imported FormsModule, that makes NgForm available to our view. Remember that whenever we make directives available to our view, they will get attached to any element that matches their selector.

NgForm does something handy but non-obvious: it includes the form tag in its selector (instead of requiring you to explicitly add ngForm as an attribute). What this means is that if you import FormsModule, NgForm will get automatically attached to any <form> tags you have in your view. This is really useful but potentially confusing because it happens behind the scenes.

There are two important pieces of functionality that NgForm gives us:

1. A FormGroup named ngForm
2. A (ngSubmit) output

You can see that we use both of these in the <form> tag in our view:

First we have #f="ngForm". The #v="thing" syntax says that we want to create a local variable for this view.

Here we’re creating an alias to ngForm, for this view, bound to the variable #f. Where did ngForm come from in the first place? It came from the NgForm directive.

And what type of object is ngForm? It is a FormGroup. That means we can use f as a FormGroup in our view. And that’s exactly what we do in the (ngSubmit) output.

We bind to the ngSubmit action of our form by using the syntax: (ngSubmit)="onSubmit(f.value)".

• (ngSubmit) - comes from NgForm
• onSubmit() - will be implemented in our component definition class (below)
• f.value - f is the FormGroup that we specified above. And .value will return the key/value pairs of this FormGroup

Put it all together and that line says “when I submit the form, call onSubmit on my component instance and pass the value of the form as the argument”.

input & NgModel

Our input tag has a few things we should touch on before we talk about NgModel:

• class="ui form" and class="field" - these classes are totally optional. They come from the CSS framework Semantic UI. I’ve added them in some of our examples just to give them a nice coat of CSS but they’re not part of Angular.
• The label “for” attribute and the input “id” attribute are to match, as per W3C standard
• We set a placeholder of “SKU”, which is just a hint to the user for what this input should say when it is blank

The NgModel directive specifies a selector of ngModel. This means we can attach it to our input tag by adding this sort of attribute: ngModel="whatever". In this case, we specify ngModel with no attribute value.

There are a couple of different ways to specify ngModel in your templates and this is the first. When we use ngModel with no attribute value we are specifying:

1. a one-way data binding
2. we want to create a FormControl on this form with the name sku (because of the name attribute on the input tag)

NgModel creates a new FormControl that is automatically added to the parent FormGroup (in this case, on the form) and then binds a DOM element to that new FormControl. That is, it sets up an association between the input tag in our view and the FormControl and the association is matched by a name, in this case "sku".

Simple SKU Form: Component Definition Class

Now let’s look at our class definition:

Here our class defines one function: onSubmit. This is the function that is called when the form is submitted. For now, we’ll just console.log out the value that is passed in.

Try it out!

Putting it all together, here’s what our code listing looks like:

and the template:

If we try this out in our browser, here’s what it looks like:

Using FormBuilder

Building our FormControls and FormGroups implicitly using ngForm is convenient, but doesn’t give us a lot of customization options. A more flexible and common way to configure forms is to use a FormBuilder.

FormBuilder is an aptly-named helper class that helps us build forms. As you recall, forms are made up of FormControls and FormGroups and the FormBuilder helps us make them (you can think of it as a “factory” object).

Let’s add a FormBuilder to our previous example. Let’s look at:

• how to use the FormBuilder in our component definition class
• how to use our custom FormGroup on a form in the view

Reactive Forms with FormBuilder

For this component we’re going to be using the formGroup and formControl directives which means we need to import the appropriate classes. We start by importing them like so:

Using FormBuilder

We inject FormBuilder by creating an argument in the constructor of our component class:

During injection an instance of FormBuilder will be created and we assign it to the fb variable (in the constructor).

There are two main functions we’ll use on FormBuilder:

• control - creates a new FormControl
• group - creates a new FormGroup

Notice that we’ve setup a new instance variable called myForm on this class. (We could have just as easily called it form, but I want to differentiate between our FormGroup and the form we had before.)

myForm is typed to be a FormGroup. We create a FormGroup by calling fb.group(). .group takes an object of key-value pairs that specify the FormControls in this group.

In this case, we’re setting up one control sku, and the value is ["ABC123"] - this says that the default value of this control is "ABC123". (You’ll notice that is an array. That’s because we’ll be adding more configuration options there later.)

Now that we have myForm we need to use that in the view (i.e. we need to bind it to our form element).

Using myForm in the view

We want to change our <form> to use myForm. If you recall, in the last section we said that ngForm is applied for us automatically when we use FormsModule. We also mentioned that ngForm creates its own FormGroup. Well, in this case, we don’t want to use an outside FormGroup. Instead we want to use our instance variable myForm, which we created with our FormBuilder. How can we do that?

Angular provides another directive that we use when we have an existing FormGroup: it’s called formGroup and we use it like this:

Here we’re telling Angular that we want to use myForm as the FormGroup for this form.

We also need to change onSubmit to use myForm instead of f, because now it is myForm that has our configuration and values.

There’s one last thing we need to do to make this work: bind our FormControl to the input tag.

When we want to bind an existing FormControl to an input we use formControl:

Here we are instructing the formControl directive to look at myForm.controls and use the existing sku FormControl for this input.

Try it out!

Here’s what it looks like all together:

and the template:

Remember:

To create a new FormGroup and FormControls implicitly use:

• ngForm and
• ngModel

But to bind to an existing FormGroup and FormControls use:

• formGroup and
• formControl

Adding Validations

Our users aren’t always going to enter data in exactly the right format. If someone enters data in the wrong format, we want to give them feedback and not allow the form to be submitted. For this we use validators.

Validators are provided by the Validators module and the simplest validator is Validators.required which simply says that the designated field is required or else the FormControl will be considered invalid.

To use validators we need to do two things:

1. Assign a validator to the FormControl object
2. Check the status of the validator in the view and take action accordingly

To assign a validator to a FormControl object we simply pass it as the second argument to our FormControl constructor:

Or in our case, because we’re using FormBuilder we will use the following syntax:

Now we need to use our validation in the view. There are two ways we can access the validation value in the view:

1. We can explicitly assign the FormControl sku to an instance variable of the class - which is more verbose, but gives us easy access to the FormControl in the view.
2. We can lookup the FormControl sku from myForm in the view. This requires less work in the component definition class, but is slightly more verbose in the view.

To make this difference clearer, let’s look at this example both ways:

Explicitly setting the sku FormControl as an instance variable

Here’s a screenshot of what our form is going to look like with validations:

The most flexible way to deal with individual FormControls in your view is to set each FormControl up as an instance variable in your component definition class. Here’s how we could setup sku in our class:

Notice that:

1. We setup sku: AbstractControl at the top of the class and
2. We assign this.sku after we’ve created myForm with the FormBuilder

This is great because it means we can reference sku anywhere in our component view. The downside is that by doing it this way, we’d have to setup an instance variable for every field in our form. For large forms, this can get pretty verbose.

Now that we have our sku being validated, I want to look at four different ways we can use it in our view:

1. Checking the validity of our whole form and displaying a message
2. Checking the validity of our individual field and displaying a message
3. Checking the validity of our individual field and coloring the field red if it’s invalid
4. Checking the validity of our individual field on a particular requirement and displaying a message

Form message

We can check the validity of our whole form by looking at myForm.valid:

Remember, myForm is a FormGroup and a FormGroup is valid if all of the children FormControls are also valid.

Field message

We can also display a message for the specific field if that field’s FormControl is invalid:

Field coloring

I’m using the Semantic UI CSS Framework’s CSS class .error, which means if I add the class error to the <div class= "field"> it will show the input tag with a red border.

To do this, we can use the property syntax to set conditional classes:

Notice here that we have two conditions for setting the .error class: We’re checking for !sku.valid and sku.touched. The idea here is that we only want to show the error state if the user has tried editing the form (“touched” it) and it’s now invalid.

To try this out, enter some data into the input tag and then delete the contents of the field.

Specific validation

A form field can be invalid for many reasons. We often want to show a different message depending on the reason for a failed validation.

To look up a specific validation failure we use the hasError method:

Note that hasError is defined on both FormControl and FormGroup. This means you can pass a second argument of path to lookup a specific field from FormGroup. For example, we could have written the previous example as:

Putting it together

Here’s the full code listing of our form with validations with the FormControl set as an instance variable:

And the template:

Removing the sku instance variable

In the example above we set sku: AbstractControl as an instance variable. We often won’t want to create an instance variable for each AbstractControl, so how would we reference this FormControl in our view without an instance variable?

Instead we can use the myForm.controls property as in:

In this way we can access the sku control without being forced to explicitly add it as an instance variable on the component class.

Custom Validations

We often are going to want to write our own custom validations. Let’s take a look at how to do that.

To see how validators are implemented, let’s look at Validators.required from the Angular core source:

A validator:
- Takes a FormControl as its input and
- Returns a StringMap<string, boolean> where the key is “error code” and the value is true if it fails

Writing the Validator

Let’s say we have specific requirements for our sku. For example, say our sku needs to begin with 123. We could write a validator like so:

This validator will return an error code invalidSku if the input (the control.value) does not begin with 123.

Assigning the Validator to the FormControl

Now we need to add the validator to our FormControl. However, there’s one small problem: we already have a validator on sku. How can we add multiple validators to a single field?

For that, we use Validators.compose:

Validators.compose wraps our two validators and lets us assign them both to the FormControl. The FormControl is not valid unless both validations are valid.

Now we can use our new validator in the view:

If you run the sample code, one neat thing you’ll notice is that if you type something in to the field, the required validation will be fulfilled, but the invalidSku validation may not. This is great - it means we can partially-validate our fields and show the appropriate messages.

Watching For Changes

So far we’ve only extracted the value from our form by calling onSubmit when the form is submitted. But often we want to watch for any value changes on a control.

Both FormGroup and FormControl have an EventEmitter that we can use to observe changes.

To watch for changes on a control we:

1. get access to the EventEmitter by calling control.valueChanges. Then we
2. add an observer using the .subscribe method

Here’s an example:

Here we’re observing two separate events: changes on the sku field and changes on the form as a whole.

The observable that we pass in is an object with a single key: next (there are other keys you can pass in, but we’re not going to worry about those now). next is the function we want to call with the new value whenever the value changes.

If we type ‘kj’ into the text box we will see in our console:

As you can see each keystroke causes the control to change, so our observable is triggered. When we observe the individual FormControl we receive a value (e.g. kj), but when we observe the whole form, we get an object of key-value pairs (e.g. {sku: "kj"}).

ngModel

NgModel is a special directive: it binds a model to a form. ngModel is special in that it mimics two-way data binding.

Two-way data binding is almost always more complicated and difficult to reason about vs. one-way data binding. Angular is built to generally have data flow one-way: top-down. However, when it comes to forms, there are times where it is easier to opt-in to a two-way bind.

Let’s change our form a little bit and say we want to input productName. We’re going to use ngModel to keep the component instance variable in sync with the view.

First, here’s our component definition class:

Notice that we’re simply storing productName: string as an instance variable.

Next, let’s use ngModel on our input tag:

Now notice something - the syntax for ngModel is funny: we are using both brackets and parentheses around the ngModel attribute! The idea this is intended to invoke is that we’re using both the input [] brackets and the output () parentheses. It’s an indication of the two-way bind.

Last, let’s display our productName value in the view:

Here’s what it looks like:

Easy!

Wrapping Up

Forms have a lot of moving pieces, but Angular makes it fairly straightforward. Once you get a handle on how to use FormGroups, FormControls, and Validations, it’s pretty easy going from there!