Types

“locking” a variable definition to a particular kind of variable (number, string, etc.)

TypeScript has a small number of built-in types, including

• Number

• String

• Boolean

• Array

• Enum

• Void

Basic Type Example

Types can create clearer code and prevent errors

function timesTwo( n : number ) : number { 
    return n * 2 
};

timesTwo(“6”); // compile-time error
var multiplied : string = timesTwo(5); // compile-time error
var num:number = timesTwo(5); // Correct

Number

The number type can be set to any number

var g : number;
g = 6;
g = 1.5;
g = 2 * 6;
g = 10e6;
g = Math.random();
g = "hello"; // compile-time error

String

The string type can be any string – i.e., any sequence of Unicode characters

var s : string;
s = "hello world";
s = "o" + "kay";
s = "fundamentals".slice(0,3);
s = 66; // compile-time error

Boolean

Booleans have only four valid values – true, false, undefined and null

var b : boolean;
b = true;
b = false;
b = undefined;
b = null;
b = 0; // compile-time error
b = "a"; // compile-time error
b = NaN; // compile-time error

Array Types

Array types define both that an variable is an array and the kind elements it contains

var numbers : number[] = [];
numbers[0] = 1;
numbers.push("two"); // compile-time error

var strings : string[] = [];
strings.push("hello");
strings[1] = 1337; // compile time error

var things : any[] = [];
things.push(1);
things.push("hello");

Enum

Collection of unique strings

enum OrderStatus {complete,pending,declined};
var n : OrderStatus;
n = OrderStatus.complete;
n = OrderStatus.unfinished; // compile-time error
n = "on the way"; // compile-time error

Any

• Can be anything

• Similar to * in other languages

• To be avoided

var w : any;
w = "string";
w = 8;
w = false;
w = [];

Void

• Absence of any type

• Mostly used for function return signatures

function nothing () : void  {
    var g = "I don't return anything.";
}