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Code Oz
Code Oz

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Learn about difference between Type & Interface in Typescript

#typescript #node #webdev #javascript

If you are using typescript, you might use interface & type but if I ask you the difference between them, are you able to answer it ?

At the end of this article, you will be able to answer it during any discussion or interview !

Type

The basic, it allow us to create a new type !

Interface

In contrary to type, interface is restricted to an object type.

With the news release, type and interface are similar but there some differences between them.

Similitude

Object typing

You can define the shape of an object with both, but the syntax is not the same

with interface:

interface A {
  a: number
}

const a: A = { a: 5 }
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with type:

type A = {
  a: number
}

const a: A = { a: 5 }
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Extend

Both can be extended, and the difference is ... yes the syntax again !

with interface:

interface A {
  a: number
}

interface AB extends A {
  b: number
}

const ab: AB = { a: 5, b: 6 }
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with type:

type A = {
  a: number
}

type AB = A & { b: number }

const a: AB = { a: 5, b: 6 }
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The difference

What type can do, and what interface cant do

Unlike interface, type can be used for creating new type with union, tuples or can be used to defined primitive type !

type A = string | number // union

type Primitive = string | boolean | number | null | interface | symbol // Create a new type from primitives type

type DataTuple = [number, string] // tuple typing
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What interface can do, and what type cant do

A class can implement an interface

Edit: Since TS 2.7, type can be implements on class, thank you to @faiwer

interface A {
  a: number
}

class Toto implements A {
   a = 55
}
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Interface can be merged in a single interface if there are defined multiple times

interface Toto {
  a: number
}

interface Toto {
  b: number
}
const toto: Toto = {
   a: 55,
   b: 66,
}
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Conclusion

As you can see type & interface are very similar but each other have his own dedicated feature !

I personally use interface when I need to type object structure, and I use type when I need to create type from primitive type or when I want to combine other types in one type !

I hope you like this reading!

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Top comments (12)

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faiwer profile image
Stepan Zubashev •

What interface can do, and what type cant do. A class can implement an interface

type A = { a: number };
class B implements A { 
  a: number = 1; 
}
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no errors

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fullstackchris profile image
Chris Frewin •

Coming from somewhere like C# a class implementing a type is kind of... weird... Though, all instances of a class automatically are that type... so it makes sense in a weird way too.

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codeoz profile image
Code Oz •

Hey ! thank you for your comment, I miss something wrong since a few time, this is available ! I edit my article thank you :)

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bravemaster619 profile image
bravemaster619 •

In a word, interface for OOP, type for FP

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peerreynders profile image
peerreynders • • Edited

FYI: A class implicitly exposes an interface that can be merged into - example:

type CompareFn = (lhs: number, rhs: number) => number;

const sortWith: (a: readonly number[], f: CompareFn) => number[] = (
  array,
  fn
) => array.slice().sort(fn);

const ascending: CompareFn = (lhs, rhs) => lhs - rhs;
const descending: CompareFn = (lhs, rhs) => rhs - lhs;

class Container {
  readonly #values: readonly number[];

  constructor(values: number[]) {
    this.#values = Object.freeze(values.slice());
  }

  get values(): readonly number[] {
    return this.#values;
  }
}

interface Container {
  ascending(): number[];
  descending(): number[];
}

function asc(this: Container): number[] {
  return sortWith(this.values, ascending);
}

function desc(this: Container): number[] {
  return sortWith(this.values, descending);
}

Container.prototype.ascending = asc;
Container.prototype.descending = desc;

const container = new Container([10, 45, 15, 39, 21, 26]);
console.log(container.values);
console.log(container.ascending());
console.log(container.descending());
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playground

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codeoz profile image
Code Oz •

Nice remark I learn something thank you bro !

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flutch profile image
flutch •

Nice Article CodeOzz !

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rayleigh93 profile image
Rayleigh-Sama •

Just follow you nice content man

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tcelestino profile image
Tiago Celestino •

Great. When I started to learn about TS, this topics confuse to me.

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codeoz profile image
Code Oz •

thank you Tiago

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king11 profile image
Lakshya Singh •

A type can have circular dependencies while interface can't it's super cool to try :)

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codeoz profile image
Code Oz •

nice remark Lakshya

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