01 - Basic Types

TypeScript Basic Types

Boolean

let isDone: boolean = false;

Number (can also use hex, binary, octal)

• can also use hex, binary, octal

let decimal: number = 6;

String

• can use single or double quotes and template strings

let color: string = 'blue';

Array

First way of declaring array type

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

Second way of declaring array type

• uses a generic array type, Array<elemType>

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

Tuple

• allows you to express an array where the type of a fixed number of elements is known, but need not be the same
• when accessing an element outside the set of known indices, a union type is used instead, this means that the third element can be assigned to a string or number

let x: [string, number];
x = ['hello', 10];  // ok
x = [10, 'hello'];  // error

Enum

• a way of giving more friendly names to sets of numeric values
• by default numbering begins at 0, can be changed with {Red = 1}

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

Any

• a way of describing the type of variables that we don't know the values of, may come from dynamic content i.e. user-input

let notSure: any = 4;

Void

• the absence of having any type at all, could be a return type of functions
• declaring variables of type void is not useful because you can only assign undefined or null to them

Null

let n: null = null;

Undefined

let u: undefined = undefined;

Never

• represents the type of values that never occur e.g. never is the return type for a function expression or arrow function that always throws an exception

function error(message: string) : never {
  throw new Error(message);
}

Object

• represents the non-primitive type

declare function create (o: object | null): void;

Type Assertions

• a way to tell the compiler "trust me, I know what I'm doing"
• when using TypeScript with JSX, only as-style assertions are allowed

First form:

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

Second form:

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