Types

For programs to be useful, we need to be able to work with some of the simplest units of data: numbers, strings, structures, boolean values, and the like.

Type Inference

In TypeScript, there are several places where type inference is used to provide type information when there is no explicit type annotation. For example, in this code

let x = 3;
let y = x + 3

The type of the x variable is inferred to be number. Similarly, the type of y variable also is inferred to be number. This kind of inference takes place when initializing variables and members, setting parameter default values, and determining function return types.

All the examples below give an example type annotation, but will work just the same without the annotation.

Boolean

The most basic datatype is the simple true/false value, which is called a boolean value.

let isDone: boolean = false;

Number

Integer values can be specified via decimal, hexadecimal and octal notation:

let decimal: number = 42;
let hex: number = 0xf00d;
let binary: number = 0b1010;
let octal: number = 0o744;

String

As in other languages, we use the type string to refer to textual data. Use double quotes (") or single quotes (') to surround string data.

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

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 fullName: string = `Bob Bobbington`;
let age: number = 37;
let sentence: string = `Hello, my name is ${ fullName }.

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

This is equivalent to declaring sentence like so:

let fullName: string = `Bob Bobbington`;
let age: number = 37;
let sentence: string = "Hello, my name is " + fullName + ".\n\n" +
    "I'll be " + (age + 1) + " years old next month."

Array

Arrays allow you to work with an expandable sequence of values, addressed by an integer-valued index. Array types can be written in one of two ways. In the first, you use the type of the elements followed by [] to denote an array of that element type:

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

The second way uses a generic array type, Array<elemType>:

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

Enum

A helpful addition to the standard set of datatypes from JavaScript is the enum. 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 c: Color = Color.Green;

Or, even manually set all the values in the enum:

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

Any

The TypeScript type any is not supported in the micro:bit.

Void

void is the absence of having any type at all. You may commonly see this as the return type of functions that do not return a value:

function warnUser(): void {
    basic.showString("This is my warning message");
}

Declaring variables of type void is not useful.