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TypeScript Learnings 002: Type System

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This is part of my series on TypeScript Learning, where I share micro-posts about everything I'm learning related to TypeScript.

One of the main TypeScript features is the Type System. We are going to play around with the type system and see how it works.

Basic Types

The basic types are:

We define these type by using this nomenclature: variableName: typeName, where the variableName you choose and the typeName can be a number, a string, or a boolean. Let's try it out.

const age: number = 25;
const name: string = 'TK';
const isLearning: boolean = true;

What is the power of the TypeScript type system? We define a type for a given variable or constant, it doesn't change across the whole system. A string will be a string forever. A number too. And so on.

let age: number = 25;
age = 'TK'; // Type '"TK"' is not assignable to type 'number'.

It fails on compile-time and raises a "Type Error". This helps us with immutable types and prevent possible bugs.

Collections

Here we have Arrays, Tuples, and Objects. They are collection, data structures to store, well, data.

There are two ways to define a Array type. The type followed by a []:

const bookshelf: string[] = [
  'The Compound Effect',
  'Leonardo Da Vinci',
  'Enlightenment Now'
];

The string is the type followed by [].

Another way is to use the Array type:

const bookshelf: Array<string> = [
  'The Compound Effect',
  'Leonardo Da Vinci',
  'Enlightenment Now'
];

Tuples are a little different from the array. It has a fixed number of elements, so we define the type for each index.

const thoughts: [string, number] = ['Programming', 42];

By the way, the order of the elements and types is relevant:

let thoughts: [string, number];

thoughts = ['Programming', 42]; // Succeeds!
thoughts = [42, 'Programming']; // Fails!

For objects, we can use the object type that represents the non-primitive type (number, string, boolean, bigint, symbol, null, or undefined).

const printObj = (obj: object): void => {
  console.log(obj);
};

printObj({ name: 'TK' }); // it compiles and prints!

printObj(42); // Argument of type '42' is not assignable to parameter of type 'object'.
printObj('TK'); // Argument of type '"TK"' is not assignable to parameter of type 'object'.
printObj(null); // Argument of type 'null' is not assignable to parameter of type 'object'.
printObj(undefined); // Argument of type 'undefined' is not assignable to parameter of type 'object'.
printObj(true); // Argument of type 'true' is not assignable to parameter of type 'object'.

Another way to define types for the object is to use Interfaces and Types.

Interfaces give the shape of the object. We define the types for the object structure. For the printObj function we defined before, we just need an interface to handle the object structure.

interface Person {
  name: string;
};

const printObj = (obj: Person): void => {
  console.log(obj);
};

printObj({ name: 'TK' }); // it compiles and prints!
The Type looks the same way:

type Person = {
  name: string;
};

const printObj = (obj: Person): void => {
  console.log(obj);
}

printObj({ name: 'TK' }); // it compiles and prints!

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