TypeScriptLesson 2 of 16

Basic types and annotations

The three main primitives are string, number and boolean. Often you can skip the annotation: TypeScript infers the type from the value.

Key points

string is text, number is any number, whole or decimal, boolean is true or false.
Inference: let city = "Dubai" already has the type string.
any switches checking off, so use it as rarely as possible.
unknown is the safe version of any: check the value before using it.
In strict mode null and undefined are separate types and cannot stand in for a string.

Example

let city = "Dubai"; // inferred as stringlet temperature: number = 31;const isSunny: boolean = true; temperature = 29; // fine: still a number// city = 42;     // error: number is not assignable to string let input: unknown = "42";if (typeof input === "string") {  input = Number(input); // inside the check TypeScript knows it is a string} console.log(`${city}: ${temperature}°C, sunny: ${isSunny}, input: ${input}`);

city has no annotation, but TypeScript inferred string, so assigning 42 is an error. The unknown value input can only be used after a typeof check.

Task

How to use the editor⌘ Enter or Ctrl Enter runs the code, ⇧ ⌘ Enter checks the task.Tab and Shift Tab indent the line or the selection; Esc, then Tab leaves the editor.The check compares what the program printed with the task; spaces at line ends do not matter.After two failed checks you may open the solution. Your code is kept in this browser.

Annotate the four constants, calculate total = price * quantity and print two lines: "Notebook x3 = 13.5" and "In stock: true".

// TODO: annotate every constant with its typeconst title = "Notebook";const price = 4.5;const quantity = 3;const inStock = true; // TODO: create a constant total (a number) = price * quantity// and print two lines:// Notebook x3 = 13.5// In stock: trueconsole.log(title);

// TODO: annotate every constant with its type
const title = "Notebook";
const price = 4.5;
const quantity = 3;
const inStock = true;

// TODO: create a constant total (a number) = price * quantity
// and print two lines:
// Notebook x3 = 13.5
// In stock: true
console.log(title);

Output

What your program prints appears here.