TypeScript Basic Data Types


The most basic datatype is the simple true/false value, which JavaScript and TypeScript call a boolean value.

let isDone: boolean = false;


function f() { 
     let message : boolean = true; 
     return message; 


As in JavaScript, all numbers in TypeScript are floating point values. These floating point numbers get the type number.

let decimal: number = 6; 
let hex: number = 0xf00d; 
let binary: number = 0b1010; 
let octal: number = 0o744;


TypeScript uses double quotes (") or single quotes (') to surround string data.

let color: string = "blue"; 
color = 'red';
let sentence: string = "Hello, my name is " + fullName + ".\n\n" + "I'll be " + (age + 1) + " years old next month.";

Template String

You can also use template strings, which can span multiple lines and have embedded expressions. These strings are surrounded by the backtick/backquote (`) character, and embedded expressions are of the form ${ expr }.

let sentence: string = `Hello, my name is ${ fullName }. 

I'll be ${ age + 1 } years old next month.`;


TypeScript, like JavaScript, allows you to work with arrays of values.Array types can be written in one of two ways.

let list: number[] = [1, 2, 3];

let list: Array<number> = [1, 2, 3];


Tuple types allow you to express an array where the type of a fixed number of elements is known, but need not be the same. For example, you may want to represent a value as a pair of a string and a number.

// Declare a tuple type 
let x: [string, number]; 
// Initialize it 
x = ["hello", 10]; // OK 
// Initialize it incorrectly 
x = [10, "hello"]; // Error


As in languages like C#, an enum is a way of giving more friendly names to sets of numeric values.

enum Color {Red, Green, Blue} 
let c: Color = Color.Green;

By default, enums begin numbering their members starting at 0. You can change this by manually setting the value of one of its members. For example, we can start the previous example at 1 instead of 0:

enum Color {Red = 1, Green, Blue} 
let colorName: string = Color[2]; 
alert(colorName); // Displays 'Green' as it's value is 2 above


The any type is a powerful way to work with existing JavaScript, allowing you to gradually opt-in and opt-out of type-checking during compilation.You might expect Object to play a similar role, as it does in other languages. But variables of type Object only allow you to assign any value to them – you can’t call arbitrary methods on them, even ones that actually exist.

let notSure: any = 4; 
notSure.ifItExists(); // okay, ifItExists might exist at runtime 
notSure.toFixed(); // okay, toFixed exists (but the compiler doesn't check) 

let prettySure: Object = 4; 
prettySure.toFixed(); // Error: Property 'toFixed' doesn't exist on type 'Object'.


void is a little like the opposite of any.You may commonly see this as the return type of functions that do not return a value.

function warnUser(): void { 
    alert("This is my warning message"); 

Null and Undefined

In TypeScript, both undefined and null actually have their own types named undefined and null respectively.

// Not much else we can assign to these variables! 
let u: undefined = undefined; 
let n: null = null;


The never type represents the type of values that never occur. For instance, never is the return type for a function expression or an arrow function expression that always throws an exception or one that never returns; Variables also acquire the type never when narrowed by any type guards that can never be true.

// Function returning never must have unreachable end point 
function error(message: string): never { 
    throw new Error(message); 
// Inferred return type is never 
function fail() { 
    return error("Something failed"); 
// Function returning never must have unreachable end point 
function infiniteLoop(): never { 
    while (true) { } 

Type assertions(Type Casting)

A type assertion is like a type cast.

Type assertions have two forms. One is the “angle-bracket” syntax:

let someValue: any = "this is a string"; 
let strLength: number = (<string>someValue).length;
let someValue: any = "this is a string"; 
let strLength: number = (someValue as string).length;

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