Understanding Types in Typescript

Updated on August 14, 2019     Kisan Patel

TypeScript has some pre-defined data-types:-

// string

let myName: string = 'Kisan Patel';
myName = 28; // Type 'number' is not assignable to type 'string'

// number

let myAge: number = 29;
myAge = 'Kisan Patel'; // Type 'string' is not assignable to type 'number'

// boolean

let hasSkills: boolean = false;
hasSkills = 1; // Type 'number' is not assignable to type 'boolean'

// assign types

let myRealAge: number;
myRealAge = 27;
myRealAge = '27';

// array

let hobbies: any[] = ["Music", "Sports"];
hobbies = [100];
// hobbies = 100;

// tuples

let address: [string, number] = ["Hello World", 99];

// enum

enum Color {
  Gray, // 0
  Green = 99, // 100
  Blue = 2// 2
}
let myColor: Color = Color.Blue;
console.log(myColor);

// any

let car: any = "Suzuki";
console.log(car);
car = { brand: "Suzuki", series: 3};
console.log(car);

// functions

function returnMyName(): string {
   return myName;
}
console.log(returnMyName());

// void

function sayHello(): void {
   console.log("Hello!");
}

// argument types

function multiply(value1: number, value2: number): number {
    return value1 * value2;
}
// console.log(multiply(2, 'Max'));
console.log(multiply(10, 2));

// function types

let myMultiply: (a: number, b: number) => number;
// myMultiply = sayHello;
// myMultiply();
myMultiply = multiply;
console.log(myMultiply(5, 2));

// objects

let userData: { name: string, age: number } = {
    name: "Kisan Patel",
    age: 27
};
// userData = {
//    a: "Hello",
//    b: 22
// };

// complex object

let complex: {data: number[], output: (all: boolean) => number[]} = {
   data: [100, 3.99, 10],

   output: function (all: boolean): number[] {
       return this.data;
   }
};
// complex = {};

// type alias

type Complex = {data: number[], output: (all: boolean) => number[]};

let complex2: Complex = {
    data: [100, 3.99, 10],

    output: function (all: boolean): number[] {
          return this.data;
    }
};

// union types

let myRealRealAge: number | string = 27;
myRealRealAge = "27";
// myRealRealAge = true;

// check types

let finalValue = 30;
if (typeof finalValue == "number") {
    console.log("Final value is a number");
}