Why Typescript Interface Not Render In Javascript

Question

I have been working with TypeScript for a week now. I am stuck on interfaces; here I am unable to figure out how TypeScript converts an interface into JavaScript, as it does not appear in the JavaScript code after compiling.

TypeScript

interface IPerson { 
  firstName: string;
  lastName: string;
  sayHi: () => string; 
}

var customer: IPerson = { 
  firstName: "Tom",
  lastName: "Hanks", 
  sayHi: (): string => { return "Hi there" } 
} 

console.log("Customer Object ") 
console.log(customer.firstName) 
console.log(customer.lastName) 
console.log(customer.sayHi())  

JavaScript

var customer = {
firstName: "Tom",
lastName: "Hanks",
sayHi: function () { return "Hi there"; }
};
console.log("Customer Object ");
console.log(customer.firstName);
console.log(customer.lastName);
console.log(customer.sayHi());

Answer 1

Concepts such as interfaces don't exist in plain JavaScript. Interfaces only exist in TypeScript where they are used to guarantee that usages and definitions adhere to the contract.

interface Post {
  id: string;
  title: string;
}

The above definition is used by the compiler to guarantee that the contract is respected:

const goingToNewYork: Post = {
  id: '123-455-222-111',
}

Once the job of type checking is done, typescript simply compiles ts to plain javascript (which omits declarations).

• Declarations can be outputted to separate files, normally known as d.ts: https://basarat.gitbooks.io/typescript/docs/types/ambient/d.ts.html