TypeScript Notes – Primitive Types

Types

Boolean

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


let isDone: boolean = false;

Number

As in JavaScript, all numbers in TypeScript are floating point values. These floating point numbers get the type number. In addition to hexadecimal and decimal literals, TypeScript also supports binary and octal literals introduced in ECMAScript 2015.


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

String


let color: string = "blue";
color = 'red';

let fullName: string = `John Doe`;
let age: number = 27;
let sentence: string = `Hello, my name is ${ fullName }.
I'll be ${ age + 1 } years old next month.`;

Array

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

Tuple

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

console.log(x[0].substr(1)); // OK
console.log(x[1].substr(1)); // Error, 'number' does not have 'substr'<span data-mce-type="bookmark" style="display: inline-block; width: 0px; overflow: hidden; line-height: 0;" class="mce_SELRES_start"></span>

Enum

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

enum Color {Red = 1, Green, Blue}
let c: Color = Color.Green;

enum Color {Red = 1, Green = 2, Blue = 4}
let c: Color = Color.Green;

enum Color {Red = 1, Green, Blue}
let colorName: string = Color[2];

alert(colorName);

Any

let notSure: any = 4;
notSure = "maybe a string instead";
notSure = false; // okay, definitely a boolean

let list: any[] = [1, true, "free"];

list[1] = 100;

Void

void is a little like the opposite of any. void means the absence of having any type at all. void type is mainly used as the return type of functions that do not return a value:

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

Null and Undefined

void is a little like the opposite of any. void means the absence of having any type at all. void type is mainly used as the return type of functions that do not return a value:

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

#primitive-types, #typescript