TypeScriptLesson 6 of 16

Type aliases and union types

The type keyword names any type, and | combines several types into a union. A value of that type can be any one of its members.

Key points

type ID = number | string means the value is a number or a string.
Before a check, only the operations shared by every member are allowed.
After typeof id === "string", TypeScript knows it is a string inside the if.
Both type and interface describe objects; only type can name a union.

Example

type ID = number | string;type Point = { x: number; y: number }; function showId(id: ID): string {  if (typeof id === "string") {    return `ID ${id.toUpperCase()}`; // here id is a string  }  return `ID #${id}`; // and here it is a number} const start: Point = { x: 0, y: 0 }; console.log(showId(42));console.log(showId("ab-7"));console.log(`Start: ${start.x},${start.y}`);

Inside the if, id has toUpperCase because it is definitely a string there. After that return, only number is left. Point is a plain object type with a name.

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.

Let Price be a number or a string, convert strings with parseFloat, add "12.50" to the array and print "499, 1250, 300" and "Sum: 2049".

// TODO: a price can be a number or a string, e.g. "12.50" from a formtype Price = number; function toCents(price: Price): number {  // TODO: if price is a string, turn it into a number with parseFloat  return Math.round(price * 100);} // TODO: put the string "12.50" between 4.99 and 3const prices: Price[] = [4.99, 3];const cents = prices.map(toCents); console.log(cents.join(", "));// TODO: print "Sum: ..." with the sum of all cents

// TODO: a price can be a number or a string, e.g. "12.50" from a form
type Price = number;

function toCents(price: Price): number {
  // TODO: if price is a string, turn it into a number with parseFloat
  return Math.round(price * 100);
}

// TODO: put the string "12.50" between 4.99 and 3
const prices: Price[] = [4.99, 3];
const cents = prices.map(toCents);

console.log(cents.join(", "));
// TODO: print "Sum: ..." with the sum of all cents

Output

What your program prints appears here.