How to add types to object properties using an interface in TypeScript?

Published September 25, 2021

To add types to the object properties using the interface declaration, we can start by first writing the keyword interface followed by the name you want to give to the interface and then the {} symbol (open and closed curly brackets).

Inside the brackets, we can type out the name of the property followed by the : symbol (colon) and then the type you want to assign to the property and finally to end the property type declaration we can use either the ; symbol (semi-colon) or the , symbol (comma).

TL;DR

// Person interface
interface Person {
  name: string;
  age: number;
}

// a simple object with properties
// with interface Person added
let person: Person = {
  name: "John Doe",
  age: 23,
};

To understand it a little better, let's say we have an object called person with properties called name and age with values of John Doe and 23 respectively like this,

// a simple object with properties
let person = {
  name: "John Doe",
  age: 23,
};

This is fine. But what if we want to have much more strictness over the types each property should hold. To do that, let's use the interface declaration in TypeScript.

First, let's declare an interface called Person (NOTE: don't get confused between the object person and interface Person) and add types for each of the properties the person object is going to hold.

In our case, the name property should have a value of string type and the age property should have a value of number type.

It can be done like this,

// Person interface
interface Person {
  name: string;
  age: number;
}

// a simple object with properties
let person = {
  name: "John Doe",
  age: 23,
};

Now, let's add interface Person to the person object. An interface is added to object after variable name followed by : symbol (colon).

It can be done like this,

// Person interface
interface Person {
  name: string;
  age: number;
}

// a simple object with properties
// with interface Person added
let person: Person = {
  name: "John Doe",
  age: 23,
};

Now we have a more strict and typed person object which is exactly what we need. Yay 🎉!

See the above code live in codesandbox.

That's all 😃!

Feel free to share if you found this useful 😃.


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