What is the significance of var-less variables in TypeScript class definitions?

Why is it that when creating a component class in Angular2, we don't need to use var when declaring a new variable? For example:

@Component({
  selector: 'my-app',
  template: `
    <h1>{{title}}</h1>
    `
})
export class AppComponent {
  title = 'My title';
}

Instead of var title = 'My title';, why do we simply write title = 'My title';?

EDIT: This question arose while I was reading through this. The original code from the tutorial looked like this:

import { Component } from '@angular/core';
export class Hero {
  id: number;
  name: string;
}
@Component({
  selector: 'my-app',
  template: `
    <h1>{{title}}</h1>
    <h2>{{hero.name}} details!</h2>
    <div><label>id: </label>{{hero.id}}</div>
    <div>
      <label>name: </label>
      <input [(ngModel)]="hero.name" placeholder="name">
    </div>
    `
})
export class AppComponent {
  title = 'Tour of Heroes';
  
  hero: Hero = {
    id: 1,
    name: 'Windstorm'
  };
}

Answer №1

That snippet showcases a shorthand syntax in TypeScript (and proposed for ES2015) used to set instance variables for objects of the AppComponent type. It's important to note that these differ from regular scoped variables in JavaScript.

While this feature is still in the proposal stage for ES2015, TypeScript, being a strict superset of ES, already implements most of the upcoming ES standards. To use this feature with Babel, you'll need to include the stage-1 plugin or opt for a different transpiler that supports this specific ES proposition. However, keep in mind that since it's just a proposal, it's not yet standard outside of TypeScript.

The equivalent vanilla JavaScript code for this example can be seen here:

function AppComponent() {
  this.title = 'Tour of Heroes';
  this.hero = 'Windstorm';
}

var appComponent = new AppComponent();
console.log(appComponent);
console.log(appComponent.title);
console.log(appComponent.hero);

Note: Given that Angular2 typically utilizes TypeScript, having access to an online transpiler can be helpful if you decide to go with TypeScript over other alternatives.

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