You must have used [(ngModel)] and its world-famous “banana in a box” syntax while working with Angular 2 and greater. Unlike AngularJS, Angular does not provide two way binding by default, which avoids all the digest cycle and watchers issues that AngularJS dealt with. But sometimes, there would be a need for a two way data binding between two components — Parent and its child.

First, let’s dissect the “banana in the box” syntax applied to ngModel. Here is the syntax that we all know about:

<input [(ngModel)]="name" type= "text" >

What’s interesting is that the [()] turns out to be syntactic sugar for the following:

<input [ngModel]="name" (ngModelChange)=" name= $event" type= "text" >

The above code is definitely more verbose but it makes perfect sense when you think about it. A two-way data-binding is really… two one-way data-bindings.

The real secret that the above code unveils is the use of the Change suffix.

Before diving into implementing our own custom two way binding, we should understand the two binding concepts in Angular:

Property Binding Event Binding

Property Binding

When you pass the data property of a component as an attribute within the square brackets [] to the target element in html, it is called Property Binding.

//parent.component.template.html <child-component [name]=”Codetonics” ></child-component>

The square bracket [] tells the angular to evaluate the expression given in the right hand side. It is a live property, which means when Parent Component updates the property name , Child Component will receive the updates. The property binding here is one-way. When Child Component updates the property name , Parent Component won’t know about it. There comes the Event binding.

Event Binding

A parent component can listen to an event raised by its child component by adding the target event within parenthesis () in the target element. This is called Event Binding. Read more about [How to use Event Emitters in Angular] here.(http://codetonics.com/angular/using-event-emitters-in-angular/)

//parent.component.template.html <child-component (nameChange)=”handleChildNameChange()” > </child-component>

Combining these two will create a two-way binding property in components. The syntax would be [()] , which is popularly called “banana in a box”.

Let’s look at the full code.

Parent Component

@ Component ({ selector : ‘ parent - component ’ , template : ` Parent Component: Available amount: <button (click)=”randomName()”>Randomize Name</button> <child [(name)]=”name”> </child> ` , }) export class ParentComponent { name : string = " Codetonics " ; constructor () { } ” randomName (){ this . name = " Codetonics " + parseInt ( Math . random () * 100 ); } }

In above code, clicking on Randomize Name button of parent component will randomize the name and child component will know it.

Child Component

@ Component ({ selector : ‘ child ’ , template : ` <div> Child component: <span>Name : </span> <button (click)=resetName()”>Reset Name</button> </div> ` , }) export class ChildComponent { @ Input () name : sring ; @ output () nameChange = new EventEmitter < string > (); resetName (){ this . name = " Codetonics " ; this . nameChange . emit ( this . name ); } }

In above code, clicking on Reset Name button will reset the name back to Codetonics and parent component will know it.