By Kimserey Lam with

# Angular Reactive Form

Jun 15th, 2017 - written by Kimserey with .

I have been following the reactive movement for a long time now, especially from UI perspective from push design with websockets to in browser reactivity with WebSharper.UI.Next. In term of form design, I always felt like there was a lack. A form concept is simple, but it always escalates to an extremely over designed code with inline validation, server validation, incorrect validation at time, multisteps, asynchronous selection, etc… Something was missing, how the validation was done and how dirty it was to code a proper post back form.

WebSharper was the real first innovation. It came out with a set of combinator that I explained in a [previous blog post] removing the “dirtiness” of form handling. It just felt right. And when things in programming start to feel right, it means you are on the right path.

Since its evolution from 1.x.x, Angular have supports for reactive form with a form builder. Today I will explain the concept and how to use it.

1. Create a reactice form with form builder
2. Setting values and submitting
3. Validation

## 1. Create a reactice form with form builder

All the directives are provided by the ReactiveFormsModule module which can be imported from @angular/forms. In order to create a form, we have access to three main elements; FormGroup, FormControl and FormArray.

• FormGroup is used to group single elements or a set of elements together.
• FormControl is used to represent a single element control.
• FormArray is used to represent a dynamic set of FormGroup, therefore a dynamic set of fields or set of set of field.

To build and manipulate a form, Angular provides us a FormBuilder which is provided by the ReactiveFormsModule and can be injected in constructor.

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constructor(private fb: FormBuilder) {
const ingredients = [ 'carrot', 'beans' ];
const ingredientFromGroups = ingredients.map(i => fb.group({ ingredientName: i }));

this.recipeForm = fb.group({
profile: fb.group({
name: ['', Validators.required ],
timeEstimation: ''
}),
ingredients: fb.array(ingredientFromGroups)
});
}


Then in our template we can have the following:

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<form [formGroup]="recipeForm">
<div formGroupName="profile">
<label>Recipe name</label>
<input type="text" formControlName="name" />
<label>Estimation</label>
<input type="text" formControlName="timeEstimation" />
</div>

<div formArrayName="ingredients">
<div *ngFor="let ingredient of ingredients.controls; let i=index" [formGroupName]="i">
<label># Ingredient</label>
<input type="text" formControlName="ingredientName" />
</div>
</div>
</form>


[fromGroup] directive is meant to be used for the top level form. [formControl] is meant to be used for a top level single control element. formControlName is used to indicate a dot notation. xxxName is used to provide dot notation. There is also formGroupName and formArrayName.

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[formGroup]="myForm"
- formControlName="something"


This will find myForm.something. Similarly if we have nested form groups, formGroupName can be used.

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[formGroup]="myForm"
- formGroupName="something"
- formControlName="else"


This will find the value for the control myForm.something.else.

The FormArray is slightly different. It is a array of group or control. It is there to cater for dynamic fields or group of fields. For example here we have a dynamic list of ingredients.

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const ingredients = [ 'carrot', 'beans' ];
const ingredientFromGroups = ingredients.map(i => fb.group({ ingredientName: i }));


We are creating a list of groups with a single ingredientName control. To display that we need to provide the dot notation by using the formArrayName="ingredients". Then under it we can use *ngFor and iterate over the ingredients.controls.

Also the index is needed to target a control in the form array, so we need to use let i=index" and [formGroupName]="i". [formGroupName] is special case where [] is needed as it is contained within the array. Under it, we use formGroupName and formControlName.

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<div formArrayName="ingredients">
<div *ngFor="let ingredient of ingredients.controls; let i=index" [formGroupName]="i">
<label>#{{ i + 1 }} Ingredient</label>
<input type="text" formControlName="ingredientName" />
</div>
</div>


The benefit of the form array is its dynamic nature. We can push and remove control from the form. For example we could add a add button and remove button:

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<div formArrayName="ingredients">
<div *ngFor="let ingredient of ingredients.controls; let i=index" [formGroupName]="i">
<label>#{{ i + 1 }} Ingredient</label>
<input type="text" formControlName="ingredientName" />
<button (click)="removeIngredientAtIndex(i)">-</button>
</div>
</div>


And add addIngredient and removeIngredientAtIndex.

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this.ingredients.push(this.fb.group({
ingredientName: 'new ingredient'
}));
}

removeIngredientAtIndex(index) {
this.ingredients.removeAt(index);
}


## 2. Setting values and submitting

When our input changes it is also possible to change the content of any control using setValue or patchValue. setValue is used to set all values for the control while patchValue is used to set a part of the values. setValue will crash if not all the values are provided.

Those functions are available from the AbstractControl.

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this.recipeForm.get('profile').setValue({ lastName: '', firstName: '' });
this.recipeForm.get('profile').patchValue({ firstName: '' });


We can also observe any changes of values in the form using valueChanges and a side effect function like forEach or subscribe.

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this.recipeForm
.valueChanges
.forEach(c => console.log(JSON.stringify(c)));


Any abstract control can be observed. This makes it easy to show a live preview of the info or a show a status as we can subscribe to any changes of the overall form.

To submit the form, the same method as template form is used with a ngSubmit and a button with type submit. We can have access to all the values with the property form.value and also access the form status with form.valid/pristine/dirty/etc...

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<form [formGroup]="recipeForm" (ngSubmit)="save()">
<!-- some other controls -->
<button type="submit" [disabled]="!recipeForm.valid">Submit</button>
</form>


Be sure to make a deep copy of the objects and arrays within the form in order to not expose a reference of our form model to other components.

## 3. Validation

We saw how to construct an entire form without validation. Of course the server side will validate but it is always nice to prevent the user from even submitting the form and guide as much as possible to ease the process of filling up the form.

Validators can be applied to a single control or to a group of control as both implement the abstraction AbstractControl.

For a single control, the Validators gives access to some already built in validations:

• required
• email
• min length
• max length
• pattern
• and a no-op validator nullValidator

Those default validators can be applied any form control. Multiple validators can be given as array.

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name: [ '', [ Validators.required, Validators.pattern('carrot') ] ],


The array (or tuple) notation takes the value as first element, a validator as second argument and an async validator as last.

The source code which create the control can be found on forms.js:

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else if (Array.isArray(controlConfig)) {
const /** @type {?} */ value = controlConfig[0];
const /** @type {?} */ validator = controlConfig.length > 1 ? controlConfig[1] : null;
const /** @type {?} */ asyncValidator = controlConfig.length > 2 ? controlConfig[2] : null;
return this.control(value, validator, asyncValidator);
}


Our validator previously defined ensures that the value isn’t null and that the string must be carrot.

We return null if the control is valid, it will pass to the next validator.

If we need to apply a validation on multiple control, we need to create a custom validator and apply it to a group. To do that we can implement the function definition ValidationFn.

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function nameValidator(nameKey: string, descriptionKey: string): ValidatorFn {
return (control: AbstractControl): {[key: string]: any} => {
const fg = control as FormGroup;
const name = fg.get('name').value;
const description = fg.get('description').value;
return name.length > description.length ? { 'descriptionTooSmall': {name, description}} : null;
};
}


For example here we verify that the description is longer than the name. This check spans accross two controls. In order to use it, we need to pass it to the formbuilder group function as followed:

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fb.group({
name: '',
description: '',
timeEstimation: ''
}, {
validator: nameValidator('name', 'description')
})


The validator is passed as extra, the code can be found in the forms.js where we see that the extra can accept a validator or an async validator:

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group(controlsConfig, extra = null) {
const /** @type {?} */ controls = this._reduceControls(controlsConfig);
const /** @type {?} */ validator = extra != null ? extra['validator'] : null;
const /** @type {?} */ asyncValidator = extra != null ? extra['asyncValidator'] : null;
return new FormGroup(controls, validator, asyncValidator);
}


This sort of validation can be also useful in a password/confirm-password check.

# Conclusion

Reactive forms together with form builder is very powerful. The explicitness of its definition makes it attractive as all configuration, values setting, validations is done in a single place, in the component code. It is also very easy to handle tree structure with sections by defining groups within groups. Creating validators on control is also very easy now as we saw, it is a single interface to implement. Lastely the handling of array of controls is simplified by the form array which handles all the UI for us. Hope you enjoyed this post, if you have any question leave it here or hit me on Twitter @Kimserey_lam. See you next time!

Designed, built and maintained by Kimserey Lam.